Newsgroups: comp.lang.java.programmer,comp.lang.java.help, comp.lang.java.gui,comp.answers,news.answers
Approved: news-answers-request@MIT.EDU
Subject: Java Programmers FAQ
Followup-To: poster
Summary: This posting answers frequently-asked questions by Java programmers
Supersedes: <6dcvct$ao9$1@shell15.ba.best.com>
Archive-name: computer-lang/java/programmers/faq
Posting-Frequency: twice-weekly
Last-modified: 1998/2/28
URL: http://www.afu.com/javafaq.html
Copyright: (c) 1997,1998 Peter van der Linden
Maintainer: Peter van der Linden
Frequently Asked Questions (with answers) for Java programmers
_____________________________________________________
________| |________
\ | Java Programmers FAQ http://www.afu.com | /
\ | Copyright (c) 1997-98 Peter van der Linden | /
/ |_____________________________________________________| \
/___________) (__________\
The Java FAQs here are intended for people who already know how to program
in some language (not necessarily Java though).
The most up-to-date copy of this FAQ is available at:
http://www.afu.com/
in both HTML and text forms.
It reflects the latest and most current FCS version of Java: JDK 1.1.5.
Please support Java Portability!
The biggest value of Java is its portability. Portability makes it easier
for companies to upgrade operating systems and platforms. Portability
makes it easier for Java programmers to transfer job skills to
different employers.
Microsoft has made unauthorized changes to Java libraries, to undermine
portability.
Microsoft's unauthorized changes are described by an
independent Java users organization at
http://www.javalobby.org/action/msjava-diffs.htm
and at
http://www.netaction.org/msoft/world/.
The contract in which Sun licensed its Java source code to Microsoft
requires Microsoft's derivative product
to pass the Java compatibility test suite. Sun charges Microsoft with
breaking this contract, and is suing Microsoft.
The contract clause that Microsoft is alleged to have broken is described at
http://www.afu.com/contract.html
Here is a link explaining
why Java portability is important.
The best way for programmers to support portable Java is to reject non-standard
tools. Please avoid Microsoft near-Java tools.
- Use development environments
from other vendors, or convert Microsoft Visual J++ to
use Sun's JDK, following the instructions at
http://www.orbiter.demon.co.uk/
- Use Netscape Communicator (not Internet Explorer)
- Use a JVM from GNU, Kaffe or Sun (not Microsoft's J++ SDK)
Free standard Java compilers can be downloaded from
http://java.sun.com.
Free standard Java Virtual Machines can be downloaded from
http://www.kaffe.org.
and
http://www.redhat.com/linux-info/jolt
Free Java AWT software can be downloaded from
http://www.biss-net.com/biss-awt.html and the
files are all at ftp.blackdown.org (the linux site) too.
Free Java software can be downloaded from
http://www.gnu.org/software/java/java.html
0. A WORD FROM OUR SPONSOR
1. LOOKING FOR INFORMATION
2. COMPILERS, ETC.
3. LANGUAGE ISSUES
4. AWT and Related Topics
5. CORE LIBRARIES
6. I/O
7. NETWORKING & DISTRIBUTED
OBJECTS
8. MULTI-MEDIA
9. SECURITY
10. Java IDIOMS
11. FOR C and C++ AFFICIONADOS
12. FURTHER RESOURCES
13. TOP TEN NOVICE PROBLEMS
-------------------------------------------------------------------------
0. A WORD
FROM OUR SPONSOR
Learning Java
This Java FAQ is maintained as a service to the Java community.
Please consider these books from the FAQ author when
you are looking for a programming text.
Sample chapters are on Sun Microsystems webpage at
http://www.sun.com/971124/cover-linden/.
1. Looking
For Information
1.0 Is
Java "Open" or "Proprietary"?
A. Java is a freely-available open
specification. Sun Microsystems publishes the Java specifications
and public interfaces as open specifications that
can be independently implemented by anyone without charge (termed a "clean-room"
implementation in industry jargon). Specifically, Sun includes a
royalty-free license in the front of the Addison-Wesley books containing
the specification.
Further, by submitting Java specifications to be ISO/IEC standards,
Sun is giving up some control in exchange for the world being able to
refer to a Java specification that is guaranteed to be open by ISO/IEC's
charter and rules.
Sun Microsystems has invested significant time and money to create
a Java implementation.
It has licensed, for a fee, the results of this work (the source code
for the implementation)
to 117 companies and organizations who have asked for it, including
competitor Microsoft. Sun has also allowed free download
and use of the Java implementation (binary executables) by anyone.
Over 2 million computer users have downloaded the JDK binaries for
free in the twelve month period to December 1997.
1.1 What
is the best way to refer someone to the FAQ when they ask question I know
is answered there?
A. The Java Programmers FAQ
(at http://www.afu.com) answers your question in section N.n. ...
This gives them the answer, and shows them where to go for future questions
It also demonstrates that the FAQ can answer their questions, supplying
an incentive to go there next time. It's regarded as elementary
politeness to look for the FAQ of a newsgroup and read it, before posting
any questions.
In general, FAQS for any newsgroup are available by looking
at past postings in a group, or
by searching Deja News (see Q 1.4), or
via anonymous FTP at directories under ftp://rtfm.mit.edu/pub/usenet/.
If you do not have anonymous FTP access, you can access the
rtfm.mit.edu archives by mail server as well. Send an E-mail
message to mail-server@rtfm.mit.edu with "help" in the body
for more information. "RTFM" stands for "Read The F*ing Manual" -- you
must expect to put in some time and effort to master a new area of study.
1.2
Where can I look at the definitive Java Language Specification?
1.3
Where can I find a full list of Java books and book reviews?
1.4
How do I search Deja News for past postings on Java topics, e.g. the FAQ?
1.5
How do I check on known bugs in JDK 1.1?
1.6 Is
there a Java port to Windows 3.1?
A. Yes, several. IBM's ADK1.02 is available at
the following locations:
Netscape Navigator for Win3.1 has Java support [anyone know the version?]
You could also consider JavaSoft's JavaPC $100 kit that converts a
PC into a thin client Java system. Details are sparse as yet, but this
is probably more for business users than personal PCs.
Check
http://java.sun.com/pr/1997/april/pr970402-07.html
1.7
I see a lot of postings from magazines soliciting Java articles. Are these
legitimate?
A. Anyone considering writing for the "Java
Developer's Journal", or any other Sys-Con publication, should be aware
that there have been several instances of programmers complaining that
JDJ has not paid them for commissioned articles.
Another programmer reported that he was not paid by Sys-Con for an article
for their Delphi magazine. There is a pattern of programmers complaining
that SYS-CON has not paid them.
The issue is that Sys-Con will only pay for articles that it actually
prints, even when it explicitly commissioned the article from you.
Programmers expect publishers to pay a cancellation or "kill" fee
when they don't run a work they have commissioned.
Programmer beware, and please don't support
magazines that are in conflict over payment with their programmer contributors.
1.8
What are the good folks at project Gnu doing with Java?
A. Guava (the Gnu Java compiler) can be
found at
Kaffe (a JVM) can be found at
1.9
Where can I find information about Java Certification?
A. Sun is sponsoring an examination which
programmers worldwide can take. Those passing can use the designation "Sun
certified Java Programmer". There is also a second level test, involving
writing a program to spec, and taking a further test. That results in the
qualification "Sun Certified Java Developer". You can find out all about
the exam at:
and then search for "sun certified java".
It costs $150 to sit the Java Programmer exam. It is not trivial to pass
the Java certification exam. It requires understanding the objectives of
the test, and the material that is tested for. These are given, along with
sample questions, at the URL mentioned above.
1.10
I'm looking for a Java style guide on naming conventions.
A. Check out the section "Naming Conventions"
in the language specification
Also take a look at Doug Lea's draft coding standard -
1.11
Where can I find information about future Java APIs?
A. JavaSoft has followed a policy
of creating new APIs in consultation with leading industry participants,
then posting the draft specification for public review and comments.
Check the JavaSoft roadmap of new APIs and products at http://Java.sun.com:80/products/api-overview/index.html
Also, some APIs that are under consideration, possibly for JDK 1.2
are at:
1.12
How can I find links to recent news about Java?
1.13
What computers have Java ports?
A.
A list of JDK ports is available from
1.14
What IDEs (Integrated Development Environments) are there?
A.
Some popular IDEs include:
1.15
Where can I find information on Java 3D?
A.
The Java 3D
FAQ may have the answers you're looking for. It contains general
information about Java 3D, as well as programming tips.
2. COMPILERS, ETC.
2.1
Why did I get an OutOfMemory error when porting working code from jdk 1.02
-> 1.1
2.2
Why do I get a "Statement not reached" error from javac for no apparent
reason?
A. JDK1.0 has a limit of 63 words of storage
for local variables in any method. longs and doubles require two words
of storage, and all other primitive types and all reference types require
one word. If you assign values to more than 63 words of local variables,
you will get a "Statement not reached" error on the statement after you
assign to the variable that contains the 64th word. In JDK 1.1, the low
limit was removed.
2.3
Is there a lex and yacc or preferably a flex and bison equivalent for Java?
A. There is a lex equivalent called JavaLex
and a yacc equivalent called CUP.
LALR(1) parser
LL(k) parser
2.4
Where can I find a byte code obfuscator?
2.5
Which program is used to create .zip files compatible with the java* programs?
(eg classes.zip, moz3_0.zip)
A. Use the jar-tool from JDK1.1(.1):
jar [ options ] [manifest] destination input-file
[input-files]
E.g.:
jar cvf myJarFile.jar *.class
creates a compressed archive
jar cvfO myJarFile.zip *.class
creates it fullsize (uncompressed) (note the 'O'-option used for JDK1.0.2)
On Unix you can also use:
zip -rn ".class" my_file.zip *
Info-ZIP home page: http://www.cdrom.com/pub/infozip/
Latest source code: ftp://ftp.uu.net/pub/archiving/zip/src/zip21.zip
Netscape's command line version of its JAR packager and signing tool
is called "zigbert". They also have a signing tool with GUI written in
Java. More info at:
If you zip your .class files for JDK 1.0.2 (for 1.1 you'll use a Jar)
1. zip your files uncompressed (can use WinZip 6.2 up);
Unix command: zip -r0 classes.zip <directories>
2. Make sure the main class has no parent directory inside the archive,
(in other words, don't build an archive with foo/bar/myMain.class,
unless your myMain is in a package called foo.bar.
Instead start it at myMain.class). Your packages
must be placed in the archive using their corresponding filesystem pathnames.
3. Put the archive in the same directory as the .html page;
4. Put something like the following tag in the .html file:
<APPLET CODEBASE="."
ARCHIVE=my_zip_file.zip,myOtherZip.zip,thirdfile.zip
CODE="my_main_class.class"
WIDTH=600 HEIGHT=250>
</APPLET>
From JDK 1.1 on, an example of the applet tag used with a jar file is
<APPLET ARCHIVE=myfile.jar
CODE=myapplet.class
WIDTH=600 HEIGHT=250>
</APPLET>
These lines will use an applet called myapplet that can be found in the
jarfile myfile.jar. You can supply several jar filenames in a comma-separated
list. Jar files are in compressed PKZIP format.
2.6
Can I compile a Java .class file to a binary executable, .exe on a PC?
A. Some companies make products that do
this. See the webpages for Symantec and SuperCede (www.symantec.com
or www.supercede.com).
But it takes away one of the most significant benefits of Java: portability
of executables. Further,
if you want your Java DLL (or .exe) to interact with C++,
you'll have to specify which specific C++ compiler and/or actually compile some
sort of linkage via the appropriate C++ compiler.
This is because C++ does not have a single ABI.
The problem is C++ interoperability. Every C++ compiler uses
a different object model, a different way of laying out class
members, and a different way of "mangling" names for the linker.
C is much simpler. The only question here
is how structures are "packed" (i.e., are integers aligned
on four-byte bounds?). All the C++ compilers can interact with C
code, thanks to 'extern "C"' declarations.
Consider carefully why you want to compile to a native executable, and whether
there is a Java way to accomplish your goal. There may be a good reason
for compiling to native code, but it needs to be thought through.
2.7
How can I performance profile my Java code?
A. java -prof MyClass
produces some basic output in a file called java.prof, showing the
number of times methods were invoked. The output lines are of the form:
| # of calls |
method called |
called by |
time spent |
On a Unix system, you can sort the file with something like
sort -r +82 <java.prof > java.sort
More and better Java tools are a third party opportunity.
One such Java profiler is OptimizeIt,
available from
http://www.optimizeit.com.
OptimizeIt has both
performance tuning, which shows which methods took how much time, and also
memory tuning, which shows what objects are in memory and how they were
allocated. Both are important things to know.
The latest version of the CodeWarrior IDE
http://www.metrowerks.com
has a time-based profiler for Java code. Java Workshop from Sun
also has a time-based profiler.
2.8 When I use javadoc and I click on any java
class included in the jdk why do I get this message?
Netscape
is unable to find the file or directory named:
/E|/Jwrkshop/JDK/bin/java.lang.Throwable.html
A. References to the JDK classes assume
that *all* generated html files are in the same directory and, in fact,
that all files for all classes referenced are generated at the same time.
There is no way to generate files incrementally and have them all reference
each other, as you would like.
As long as you have source for everything involved (including the JDK
and all third-party classes), you can list all of your packages and all
of the others on the javadoc command line and generate the whole set at
once, but it is burdensome. Of course, if you receive any libraries as
.class files, even this workaround will not suffice.
Also javadoc will not generate the image files - you need to get them
from the images directory under the JDK API documentation files. You can
just copy the entire directory into your own doc directory. javadoc is
a very nice concept, with a few implementation flaws.
2.9 I'm working on a project with lots of classes
and I use the JDK. A recompile from scratch takes forever when I do it
a class at a time.
How do I
recompile everything?
A. The first way is
Another way is
where "tip.java" is a class "at the tip of the iceberg", i.e. that depends
on (uses) all the other classes. Typically, this may be your main class.
However, "-depend" is known to be buggy and cannot be relied upon. It also
doesn't issue compile commands in parallel to make use of multi-processor
systems.
Without the "-depend" option, the standard "javac files" doesn't look
beyond the immediately adjacent dependencies to find classes lower down
the hierarchy where the source has changed.
The -depend options searches recursively for depending classes and recompiles
it. This option doesn't help when you have dynamically loaded classes whose
names cannot be determined by thecompiler from the dependency graph. E.g.
you use something like
Class.forName(argv[0]);
The author of the code using those classes should make sure that those
classes are mentioned in a Makefile.
2.10 Why do I get the
java.lang.UnsatisfiedLinkError when I run my Java program containing Native
Method invocations?
A. Your program is not able to find your
shared library or DLL.
On Windows 95/NT, make sure that the DLL exists in a path that is included
within the PATH environment variable.
On Solaris, make sure that the environment variable LD_LIBRARY_PATH
includes the path of your shared library.
2.11
An anonymous class can't seem to access a private outer method.
Why is that?
A. It's a known bug in the JDK 1.1.4.
The code is:
public class MyDialog {
void Setup() {
addWindowListener( new WindowAdapter() {
public void windowClosing(WindowEvent e) {
myCloseWindow(); } }
); // anon inner class
}
void CloseWindow() { // outer method
dispose();
}
}
This code sends javac into an infinite loop. The workaround is to
make the private method non-private, or to make the inner class
a named class.
3. LANGUAGE
ISSUES
3.1
Why
doesn't my "hello world" program work?
A. Common causes of failure are:
-
First, did you write an applet or an application?
If an applet, you must make sure that you did extend
the java.applet.Applet class.
You must declare your class that is extended from Applet as "public".
If you don't, unfortunately some browsers will still run the code, while
others won't.
-
Your class name and the file name must match exactly, even letter case.
If your class is HelloWorld, your source file must be HelloWorld.java
-
In JDK 1.0.2 the $CLASSPATH environment variable should include "." the
current dir.
In JDK 1.1, users are no longer required to set the CLASSPATH to point to
the classes.zip file.
From JDK 1.1.2 on, it is generally an ERROR if the user sets the CLASSPATH
to include classes.zip. CLASSPATH is still needed to
point to the roots of the programmers own packages, and the use of rmic
apparently still requires it, too.
3.2 How
can I program linked lists if Java doesn't have pointers?
A. Of all the misconceptions about Java,
this is the most egregious. Far from not having pointers, in Java, object-oriented
programming is conducted exclusively with pointers. In other words, objects
are only ever accessed through pointers, never directly. The pointers are
termed "references" and they are automatically dereferenced for you.
Java does not have pointer arithmetic or untyped casting. By removing
the ability for programmers to create and modify pointers in arbitrary
ways, Java makes memorymanagement more reliable, while still allowing dynamic
data structures. Also note that Java has NullPointerException, not NullReferenceException.
A linked list class in Java might start like this:
public class linkedlist {
public linkedlist head;
public linkedlist next;
public Object data;
public linkedlist advanceToNext(linkedlist current) { ...
... }
Another choice for a linked list structure is to use the built-in class
java.util.Vector which accepts and stores arbitrary amounts of Object data
(as a linked list does), and retrieves it by index number on demand (as
an array does). It grows automatically as needed to accommodate more elements.
Insertion at the front of a Vector is a slow operation compared with insertion
in a linked list, but retrieval is fast. Which is more important in the
application you have?
3.3 What
is the true story about how parameters are passed in Java? Is is by value
or by reference?
A. All parameters (values of primitive
types, & values that are references to objects) are passed by value
[JLS sect 8.4.1]. However this does not tell the whole story, as objects
are always manipulated through reference variables in Java. Thus one can
equally say that Objects are passed by reference (and the reference variable
is passed by value). This is a consequence of the fact that variables do
not take on the values of "objects" but values of "references to objects"
as described in the previous answer.
Bottom line: primitive type arguments (int, char, etc) _do not_ change
when the corresponding parameter is changed. The fields of object type
arguments _do_ change when the corresponding parameter fields are changed.
3.4 How,
then, can I pass an object to a method, and have the method change the
reference so it points to a different object back in the calling code?
A. There are two ways. The obvious way
is "just add another level of indirection" -- wrap the object in another
class, whose purpose is simply to be passed as a parameter, allowing the
nested object reference to be modified.
The second alternative is a clearer variant of this. Pass in a single
element array. Since arrays are objects, this works.
void jfoo(Object ref[]){
ref[0] = new Object();
}
...
Object kludge[] = new Object[1];
kludge[0]= myObj;
jfoo(kludge);
if (kludge[0] == myObj) ...
else ...
3.5 Why
is String a final class? I often want to override it in some way.
A. There are several reasons.
The simplest is that being final guarantees that instances of String
are immutable. (The String class implements immutable objects, but if it
were not final it would be possible to write a subclass of String which
permitted instances to be changed.) But that's an unsatisfying answer,
because the real question is "Why must Strings be immutable?"
One reason is efficiency. It's easier to optimize accesses to an object
that is known to be immutable. Strings are very commonly used, even used
behind the scenes by the Java compiler. Efficiency gains in the String
class can yield big dividends. If no one can change a String, then
you never have to worry about who else has a reference to your String.
A more compelling reason is security. Before String was changed to be
final (while Java 1.0 was still in beta) there was a race condition which
could be used to subvert security restrictions. It had to do with having
one thread change a pathname while another thread was about to open it.
There are other ways to solve these problems, but making String final
is the one that the designers chose.
3.6 What
happened to "private protected"?
A. It first appeared in JDK 1.0 FCS (it
had not been in the Betas). Then it was removed in JDK 1.0.1. It was an
ugly hack syntax-wise, and it didn't fit consistently with the other access
modifiers. It never worked properly: in the versions of the JDK before
it was removed, calls to private protected methods were not dynamically
bound, as they should have been. It added very little capability to the
language. It's always a bad idea to reuse existing keywords with a different
meaning. Using two of them together only compounds the sin.
The official story is that it was a bug. That's not the full story.
Private protected was put in because it was championed by a strong advocate.
It was pulled out when he was overruled by popular acclamation.
3.7 I extended the class called Frotz, and
the compiler is giving me an error message "No
constructor Frotz()" in the child class. Why?
A. When you define a constructor for a
class, unless you explicitly call the superclass' constructor at the start,
a call to the superclass' parameterless constructor is implicitly inserted.
The problem you're seeing is what happens when the superclass doesn't *have*
a parameterless constructor. The solution is usually to call the correct
version of the superclass' constructor with the right parameters.
3.8 Why does <unexpected> happen in Java
floating point?
A. There are several unexpected things
that seem to bite programmers with floating point. Invariably this is a
result of the programmer not being fully conversant with FP, rather than
some fault in Java.
If you seem to be having problems with floating point, your problem
probably stems from the fact that floating-point arithmetic is inherently
imprecise. You can expect up to 7 digits of precision with floats and 16
digits with doubles. However, that does not mean that a number that can
be exactly represented in 7 digits decimal, can be exactly represented
as a binary floating point number. On the contrary, that is usually not
the case.
Additionally, when Java converts floating point numbers to a String,
as is done when they are output, enough digits are printed so the number
can be read back in with no loss of precision. For this reason, you may
see more "inaccuracies" in floating point output than you are used to.
This policy actually gives you more consistent results than on a system
where floating point output is deliberately rounded to make the output
"pretty".
There is a limitation of FP in JDK 1.0 (fixed in JDK 1.1). Namely, when
you output a floating point number in Java 1.0, the result is system-dependent
and contains no more than six digits after the decimal point. This bug
is fixed in Java 1.1.
For more information and detailed specifications on how Java deals with
floating point, see the following URLs:
http://java.sun.com/products/jdk/1.1/compatibility.html#incompatibilities
http://java.sun.com/doc/language_specification/javalang.doc.html#1466
http://java.sun.com/doc/language_specification/javalang.doc.html#5899
URL test;
try { test = new URL("http://osprey.avs.dec.com");
} catch (MalformedURLException e) {
System.out.println("bad URL:" + e.getMessage());
}
System.out.println("this is url " + test);
A. If an exception is raised in the try clause,
test will not be given a value, yet you are using it in the catch clause.
The solution is to declare test with an explicit initial value of null.
3.10
Why do I get this compiler error message?
public static void main(String[] args) {
^
Statement expected.
public static final float Conversion_Factor = 39.37;
^
Type expected.
A. A. Argument declarations and variable declarations
inside methods are never given the "public", or "static" keywords. They
are not public or static because they are local to a method. (They used
not to be allowed the "final" keyword either, but there was not good reason
for that restriction and it was dropped in JDK 1.1). Move your public or
static declarations outside the method. They are usually put at the beginning
of the class.
3.11
Why do I get this compiler error message?
MyApplet.java:11: No constructor matching MyCheckbox(myApplet)
found in class MyCheckbox.
bp1 = new MyCheckbox(this);
^
1 error
A. The first thing to check is whether you
gave a return value to the constructor, like this:
public void MyCheckbox( Container parent )
If you did, the compiler will think it is an ordinary method, not a constructor.
This is a very common mistake, and quite hard to spot.
3.12
Why do I get the compiler error message
T.java:96: Can't access protected method clone in
class java.lang.Object. OtherT is not a subclass of
the current class.
class SomeObject implements Cloneable {
public Object clone()
throws CloneNotSupportedException {
return super.clone();
}
}
[i.e. override clone to make it public, and call the superclass clone].
class Foo {
Bar bar;
Foo (Bar b) {
try {bar = (Bar) b.clone();}
catch (Exception e) {}
}
...
class Bar implements Cloneable {
public Object clone()
throws java.lang.CloneNotSupportedException{
return super.clone();
}
}
3.13
How
do you transform a char into the corresponding int value, that represents
the code value of the char?
c = (char) i;
This question crops up so frequently because the BASIC language uses functions
to map characters into ints, ASC( 'A' ) => 65 causing BASIC programmers
to seek the corresponding Java functions. Pascal and Ada have similar functions,
and no doubt other languages too.
3.14
If I extend a class with a subclass, are the constructors inherited?
A. "Constructor declarations are not members.
They are never inherited and therefore are not subject to hiding or overriding."
The default constructor is not inherited, but provided (see Section 8.6.7,
JLS).
If you don't give your child class any constructors, a default no-arg
constructor that invokes the superclass' constructor is provided for you.
If the superclass doesn't have a no-arg constructor, see 3.7
3.15
How do I allocate a multidimensional array?
A. There are several ways. If you want
a rectangular array, you can allocate the space for the array all at once.
The following creates a 4x5 array:
int arr[][] = new int[4][5];
If you want each row to have a different number of columns, you can use
the fact that a two-dimensional array is actually an array of arrays. The
following code allocates a triangular array:
int arr[][] = new int[4][]; // allocate the four row arrays
for (int i = 0; i < 4; i++) // initialize each of the four rows
arr[i] = new int[i + 1]; // row i has i + 1 columns
Note that if you allocate an array of any kind of object (as opposed to
primitive type), all the references will be null by default. These null
references can result in NullPointerExceptions if you try to dereference
them.
In other words, after doing:
int arr[] = new int[4];
you can say "if (arr[2] == 0)"
But after doing
Integer Iarr[] = new Integer[4];
you must fill in the object reference before using it, with e.g.
Iarr[2] = myInt;
or
Iarr[2] = new Int(27);
before you can say "if (Iarr[2].equals(myInt))"
3.16
Does
it make a difference to the class file in any way, if I import a package,
versus use the full name, i.e.
import java.rmi.server.*;
RemoteObject ro;
versus:
java.rmi.server.RemoteObject ro;
A. No, it makes no difference to the class
files. Import is just a shorthand for quoting the full name package and
class name (as in the examples in the question). Importing a class does
not cause the class to be loaded at run time. There is no run time penalty
for using the * form of import. The class file will contain the name of
the packages it uses, and the loader will look for those classes as needed
at runtime.
The different forms of import may or may not make a difference to compile
time. Such a difference is likely to be negligible, and should not be a
factor in which form of import you use.
Some people say that stating which classes you are importing can help
program readability. In a program with many * import statements, it may
take a programmer time to find which package an obscure class is imported
from. If you explicitly list each class you import at the top of the
program, you document which package each class you use comes from.
These people suggest that you use
import java.rmi.server.RemoteObject;
in preference to:
import java.rmi.server.*;
Other people say that it is clearer still to use the full package and class
name, at the point where you use classes in other packages. These people
suggest that you use:
java.rmi.server.RemoteObject ro;
But that gets a little lengthy when you instantiate:
java.rmi.server.RemoteObject ro
= new java.rmi.server.RemoteObject();
You always have the option of stating the full package and class name,
whether you use import or not.
Another good reason not to use the * form is when you are importing
two packages that have classes of the same name and you want to use only
one of those classes. E.g.
import com.sun.*;
import com.ms.*;
where there is a class called SerialPort in both those packages. If you
use the * form of import, you import both of the SerialPort classes and
then must fully qualify the class each time you use it, to say which of
the two you mean.
In Java 1.0, if you import a class that has the same name as a class
defined in that source file, you will get an error that the class names
clash. In Java 1.1, the local class will be used when the package name
is not given; use the package name of the imported class to use it.
The best advice is to write the program so that it is as readable as
possible. Where you have a group of well known classes, as in java.awt,
there is no reason not to use "import java.awt.*;"
3.17
What are "class literals"?
A. A feature introduced in JDK 1.1. They are literals of type
"Class" that hold a value representing any class. There are even values
to represent "void" and an array, like this:
Class myCl1 = Character.class;
Class myCl2 = Void.class;
Class myCl3 = Object.class;
Class myCl4 = String[].class;
Class myCl5 = int[][].class;
You might use it like this:
Class cl = thing.getClass();
if (cl.equals(myCl1))
System.out.println("It's a Character class");
A class literal
Component.class
is the equivalent of
Class.forName("java.awt.Component")
The second can throw an exception, but the first cannot. If you don't know
the name of the class when you write the code, you cannot use the first
form.
3.18
How do I copy the contents of an array
(with primitive type contents) to another array?
A. Use the method java.lang.System.arraycopy(Object
src, int src_position, Object dst, int dst_position, int length);
Note that there is no corresponding method to clear an array to 0.0,
0, null, false, '\u0000' etc (except for allocating a new array, which
will have the default initializations). This has lead several people to
suggest the old COBOL trick of "initialize by rollup". In initialize by
roll-up, the programmer sets the first element of the array to the desired
value, and then moves elements array[0 to n-1] to array[1 to n]. Under
some old IBM COBOL compilers, code was generated to move element 0 to 1,
then 1 to 2, and so on, "rolling up" the desired value through the array.
It was always a terrible idea, as it went outside the boundaries of defined
behavior in COBOL, and was subject to being broken in any compiler release.
In any event, intialization by roll-up is defined not to work in Java.
If the src and dst arguments refer to the same array object, then the copying
is performed as if the components at positions srcOffset through srcOffset+length-1
were first copied to a temporary array with length components and then
the contents of the temporary array were copied into positions dstOffset
through dstOffset+length-1 of the argument array.
If you simply want to clear the same array to the same value many times,
create two arrays. Fill one with the reset value, then use System.arraycopy
to copy it into the work array each time you need to clear the work array.
3.19
What is a fast way to set all elements of an array to a given value without
duplicating the (possibly large) array?
A. Using a loop that does it one by one
is probably 20 to 40 times slower than good-old memset() in C.
A fast way on many VM's is to set the first byte of the array, then
use System.arraycopy repeatedly to fill the next byte, the next two bytes,
the next four bytes, the next eight bytes, etc. (Note these are not overlapping
slices, so the issue raised in Q3.18 does not
arise).
public static void bytefill(byte[] array, byte value) {
int len = array.length;
if (len > 0)
array[0] = value;
for (int i = 1; i < len; i += i)
System.arraycopy( array, 0, array, i,
((len - i) < i) ? (len - i) : i);
}
This is faster on Sun's VM than a simple loop, and probably faster even
under JITCs because it only performs log2(array.length) bounds-checks at
most. This is a clever code idiom applying the binary chop algorithm to
arrays whose size is not a power of 2.
3.20
Is
there some declaration that I can use to make "acos", "cos", "sin", etc.
(from java.lang.Math) recognizable in my own class, so I don't have to
prefix "Math." to them?
A. No. There is no good alternative. There
are several bad alternatives:
You could wrap the functions in your own class.
double sin(double x) {
return Math.sin(x);
} // etc. for each function
If your class does not extend another class, you could make it extend Math,
to bring the namespace in, but this is very poor OOP style.
If the Math class provided a public constructor --
which it does not -- you could instantiate a local
copy of Math with a shorter name, e.g. "M". There is a cheating
way to do this which works because all the methods are static.
Declare
java.lang.Math M = null;
angle = M.cos(i);
Please don't do this.
If java.lang.Math were not final and your class did
not extend another class, you could have your
class extend Math, to bring the namespace in.
However, the final prevents this and it is very poor OOP style to
use inheritance to obtain a trivial name abbreviation (rather than
to express a type hierarchy).
The import stament only imports packages, subpackages, and classes. Members
of classes are not imported, so
import java.lang.Math.*;
doesn't work.
3.21
Why does b >>>= 1 give me the same result as b >>= 1?
A. First of all, note that ">>" is a "signed"
or "arithmetic" shift, namely, it replicates the sign bit on the left as
it shifts.
The ">>>" operator is an "unsigned" or "logical" shift; it does a shift
right and zero fill. However, there are a couple of places where ">>>"
looks like it does a signed shift!
The issue occurs when you have a non-canonical type, byte, or short,
with a negative value, e.g.
byte b = -15; // 0xF1
b = (byte) b >>> 4; // why isn't b 0x0f ?
The initial expectation is that an unsigned shift right of 0xF1 would successively
be (in binary) 0111_1000 then 0011_1100 then 0001_1110 then 0000_1111
But that doesn't happen. The rules of arithmetic in Java say that all
operands are converted at least to int before the operation (and possibly
to a more capacious type). That means our byte is promoted to an int, so
instead of shifting 0xf1, we are shifting 0xfffffff1. If you shift right
unsigned 4 places, you get 0x0fffffff. When you cast that to a byte it
becomes 0xff, or -1.
The bottom line is that the final result is the same as if you'd performed
the signed shift because the unsigned shift applied to the intermediate
int, not to the original byte. This anomaly means that ">>>" is useless
for negative bytes and shorts. It is probably safer and clearer not to
use it at all, but to mask and shift like this:
// not recommended
byte b = -15;
b = (byte) (b>>>4);
System.out.println("b= "+Integer.toHexString(b) );
// recommended
b= -15;
b = (byte) ( (b & 0xFF) >> 4 );
System.out.println("b= "+Integer.toHexString(b) );
3.22
How do I compare two Strings together?
if (s1 == s2)
is giving me funny results.
A. The comparison using "==" on objects, like Strings, is asking
the question "do these two objects have the same reference?". That is,
do they have the same address, and hence are the same object? What you
really want to do is ask "do these two Strings have the same *contents*?"
You do that with any of (and there are others too)
if (s1.equals(s2) )
or if (s1.equalsIgnoreCase(s2) )
or if (s1.startsWith(s2) )
or if (s1.endsWith(s2) )
or if (s1.regionMatches(s1_offset, s2, s2_offset, length) )
or if (s1.compareTo(s2) < 0)
Note that you can do this with literals:
if ("apple".equals(s2) ) ...
If you compare these the other way round, like this:
if ( s2.equals("apple") ) ...
and s2 is null, you will get a null pointer exception. Be alert to this.
3.23
How can I be type-safe without having to create a multitide
of subclasses (IntegerLinkedList, StringLinkedList, etc.)?
A. Generic programming in java (the rough equivalent of C++'s templates)
works reasonably well since all java classes are subclasses of Object.
There is, however one potential problem - there is always a possibility
that a generic container may contain different classes of objects.
This naturally leads to the question of how to do this in a type-safe
way. If you've created a generic LinkedList class, how can you be type safe
without having to create a multitide of subclasses (IntegerLinkedList,
StringLinkedList, etc.)?
One way to handle this would be to offer up an additional constructor
in your generic class that takes a parameter of type "Class" and uses
that parameter along with Class's "isInstance" method to guarantee
that Objects added to the container are the expected type.
public class LinkedList
{
protected Class type = Object.class;
public LinkedList(Class type) { this.type = type; }
public void addElement(Object element) throws Exception
{
if(!type.isInstance( element ))
throw new Exception(
"Expected element of type (" + type +
") got element of type (" + element + ")");
...
}
}
Note that the comments in the source for isInstance() refer to
a "specified Class parameter", suggesting that you are supposed to
write something like:
public void addElement(Object element) throws Exception
{
Class c = element.getClass();
if(!type.isInstance(c))
This works, but the
documentation for isInstance is clear that
the parameter should be an Object rather than a Class.
Also, be aware of "Collections" that are coming in JDK 1.2. More
information about this is available at the Java Developer Connection
at the Java website http://java.sun.com.
3.24
Why is there a standard JNI?
A. JNI is the Java Native Interface. It defines the way that
a Java program can call C programs. The industry has agreed on, and
Sun has codified, JNI as the standard. Microsoft shuns the standard
and uses a protocol of its own called Raw Native Interface, RNI.
You might think that once a Java program uses JNI, portability is
lost, and hence it doesn't matter if vendors diverge from the JNI
standard. However, it does affect programmers and especially
end-users. When you use native code, you
lock your program into one platform (and other platforms if you port
the native code). But programs that use native code should still
be able to use a JVM, and other Java tools and libraries from any vendor.
This is possible when everyone uses the same native code interface.
Microsoft's non-standard RNI is the reason that programs using the
Microsoft JVM cannot use the standard Java jdbc-odbc library. That
library has a piece written in C. It works for all JVM's except
Microsoft's.
JNI thus has two purposes:
- a cross-platform native API, that is the same on all operating systems.
The fact that you need to compile the native classes for each target
platform does not diminish the need to have one API for the VM manufacturers
to support for "public native int foo()".
- JNI is also about independence of JVM on a single platform, so the
same native code works with everyone's JVM.
3.25
What are the naming conventions in Java?
3.26
What are the differences between an interface and an abstract class?
A. The principal difference is that an abstract class is used
when you have a type/subtype relationship. An interface
does not imply a type/subtype hierarchy; it merely forces a class to
have some methods with a given signature. That allows some other code
to rely on being able to call those methods.
You want to subclass abstract classes when you are making subtypes that
define the abstract behavior. The subclass should have a "kind of"
relationship to the superclass. A user record is a kind of record, a
dog is a kind of mammal, etc.
Inheritance provides a "kind of" relationship.
By way of contrast, an interface provides
a "plays the role of" relationship. An image loader could play the
role of an Observable thing. Just about any class could play the role of a
"sortable thing". Interfaces therefore often have a name that
is a "something-able thing". A class may only extend one other class, but it
can implement an arbitrary number of interfaces.
Abstract classes can have abstract methods and they can also have
fully defined members. Interfaces can't have any code in them at all,
though they can have final data.
Another minor difference is that calls to a subclass method are faster than
calls to an interface method (because the interface method involves
a bit more lookup at runtime about what is actually being called).
3.27
How do static methods interact with inheritance?
A. Static (per-class, rather than per-object) methods do not
participate in overriding (choosing the right method at runtime
based on the class of the object). Probably the best and simplest way
to think about this (and to write your code) is to write every invocation
of a static method using the fully qualified class name so:
class A {
public static method1() {
A.method2();
}
public static method2() {
}
}
class B extends A {
public static method3() {
A.method1();
}
public static method2() {
}
}
Now it is perfectly clear that the static method2() that is called is
A.method2(), not B.method2(). This is true regardless of whether you use
the fully-qualified class name or not, but the use makes it obvious to all.
3.28
How do I get unsigned ints in Java?
A.
Java doesn't have unsigned ints. The reason is that this is a very
murky area of C -- the rules for what type you end up with when you mix
signed and unsigned in expressions are horribly complex in C -- and they
changed between K&R and ANSI C. (you might have heard this under the
soubriquet "unsigned preserving vs. value preserving"). And, they
depended on the underlying hardware, so they *varied* from platform to
platform, causing bugs in all kinds of unexpected places. The
book "Expert C Programming" has some examples (pages 25-29).
All that horror is behind us now, with Java.
Use type char if you are ok with 16-bit unsigned quantities. Otherwise
go to the next larger type and use masking. Specifically, to
convert an int to its unsigned representation, use:
((long)i) & 0xFFFFFFFF
This promotes the signed int to long and chops off the sign-extension,
leaving it as an unsigned 32-bit quantity.
Also worth noting is that if you're going to work with unsigned bytes, int
is a more efficient larger type to use than short or char, since smaller
values have to be promoted to int to do any arithmetic or testing on them.
3.29
How do I send a variable number of arguments to a method, or get
multiple return values back?
A.
(Easy) Use method overloading to support different parameters.
This makes things easy on the caller but can get out of hand
if you want to support a wide number and variety of parameter types.
You should ask yourself if your code design is well-organized if you
need to do this.
(More complicated)
Use arrays. It's possible to declare arrays inline as shown below:
foo("A param", new Object[] {"param3", "param4", new Integer(5)} );
// ...
void foo(String param1, Object param2[]) {
System.out.println(param1);
for (int i = 0; i < param2.length; i++) {
System.out.println(param2[i].toString());
}
}
You can even pass arrays of arrays using this method. Of course,
inside the method you need to be able to decode what the arguments are
and how you use them. Alternatively you can invent a class that
just contains all the possible fields you might want to pass into a
method (plus booleans to say if each field is set or not), and make
an object of that class be a parameter to the method.
You can return multiple values from a method the same ways; either
have the method return an array or a wrapper object.
Remember the wise words of Prof Alan Perlis however, "if your procedure
has more than about half a dozen parameters, you probably forgot a few."
Perlis meant that passing large numbers of arguments into a function
suggests that your function is badly-organized.
4. AWT
and Related Topics
4.0.1 Why
do I get this when using JDK 1.1 under X Windows?
java.lang.NullPointerException
at sun.awt.motif.MFramePeer.<init>(MFramePeer.java:59)
at sun.awt.motif.MToolkit.createFrame(MToolkit.java:153)
at java.awt.Frame.addNotify(Frame.java)
at java.awt.Window.pack(Window.java)
A. There's a missing font on your system. Move
font.properties from the "lib" subdirectory aside to font.properties.bak
Then it won't look for the font and fail to find it.
The problem occurs because the Motif AWT libraries use the Font "plain
Dialog 12 point" as a fall-back default font. Unfortunately, when using
a remote X server sometimes this font isn't
available.
On an X-terminal, the diagnostic may be slightly different, a segv
% appletviewer HelloWorldApplet.html
SIGSEGV 11* segmentation violation
si_signo [11]: SIGSEGV 11* segmentation violation
si_errno [0]: Error 0
si_code [1]: SEGV_ACCERR [addr: 0x14]
To determine which fonts you have, issue a command such as
Then pick through the long list of fonts to determine which ones you want
to use. The xfd program will let you look at a font:
xfd -fn "your font name here" &
4.0.2 Why
is GridBagLayout so hard to use?
A. GridBagLayout was contributed to Javasoft
by a programmer who wanted to support the Java effort. It was intended
as a proof that the AWT offered enough features for programmers to write
their own layout managers. It wasn't designed with human factors and ease
of use in mind. If it bothers you (it bothers me) then just don't use it.
Create your GUI on several panels and use the other layout managers as
appropriate to get the exact effect you want. The official story from the
project leader of the AWT project, as explained to the Mountain View Java
Users' Group on Dec 4 1996, is:
"The case has been made and is now accepted that GridBagLayout is too
hard to use for what it offers. GBL will continue to be supported, and
something better and simpler will eventually be provided as well. This
"better GBL" can be used instead of GBL."
Bottom line: nobody has to waste any effort on GBL, there are better alternatives
available now, and it will be replaced by the SwingSet "SpringLayout" Springs
& Struts style layout manager.
SpringLayout was to be introduced as part
of the Java Foundation Classes with JDK 1.2, but it was dropped from the
beta release as the code was not ready in time. Javasoft generously made the
preliminary code available and invited programmers to hack on it and submit
the results.
4.0.3 How
do you change the font type and size of text in a TextArea?
A. myTextArea.setFont(new Font("NAME", <STYLE>,
<SIZE>));
where:
NAME is the name of the font (eg Dialog or TimesRoman).
<STYLE> is Font.PLAIN, Font.ITALIC, Font.BOLD or any additive
combination (e.g. Font.ITALIC+Font.BOLD).
<SIZE> is the size of the font, e.g. 12. e.g. new Font("TimesRoman",
Font.PLAIN, 18);
4.0.4 Can
you have different fonts for individual words in a TextArea ?
A. No. If you're trying to write a word
processor, use the Canvas class to render on.
4.0.5 How
much text can be put in a TextArea?
A. TextArea just uses the corresponding
widget of the underlying window system. It will be bounded by the limit
imposed in the native window system. In Windows95 TextAreas can hold about
28Kbytes. The native widget allows 32Kb, but there is some overhead which
reduces the amount available to the programmer.
The limit will likely disappear with JDK 1.2 which
dispenses with peer controls.
4.0.6 MyClass
works fine except when I try to set a particular font. I just can't seem
to get it to work in Win95, but I can get it to work on a MacOS and Unix.
A. You probably specified a font name that
isn't available under your Win95; this is one of those cross-platform differences
that can bite you if you over-specify for one platform, like specifying
"Arial" as a font and expecting it to work on something other than Windows.
On both Windows 95 and Solaris 2.6, these fonts
Dialog
SansSerif
Serif
Monospaced
Helvetica
TimesRoman
Courier
DialogInput
ZapfDingbats
are revealed by this code:
import java.awt.*;
class foonly {
static public void main(String s[])
{
String n[]= new Frame().getToolkit().getFontList();
for (int i=0;i<n.length; i++)
System.out.println(n[i]);
System.exit(0);
}
}
In other words, You can get a String array of the names of the fonts by
String[] fonts = Toolkit.getDefaultToolkit().getFontList()
4.0.7 Is
it possible to draw a polygon or a line more than 1 pixel wide?
A. JDK 1.1.1 doesn't have support for this.
The standard workaround for drawing a thick line is to draw a filled polygon.
The standard workaround for drawing a thick polygon is to draw several
polygons.
There is a useful class at http://www.apl.jhu.edu/~hall/java/GraphicsUtil.html
It extends the drawxxx and fillxxx methods of java.awt.Graphics. It
adds a Line Width argument to most of the drawxxx methods, a Color argument
to most of the drawxxx and fillxxx methods, and a Font argument to draw
String and drawChars.
4.0.8 I use add(Component) to add Components
to the Container. Is there any way to explicitly
set the z-order of these Components?
A. JDK 1.0 has no way to explicitly set
the z-order of components. You can try it heuristically, based on the browser
you're using, or you can use CardLayoutManager to ensure the panel you
want is at the front.
In JDK 1.1 the z-order of components ("z-order" means "front-to-back" order,
i.e. which window is in front of which) can be controlled by using the
the method add(Component comp, int index). By default, components are added
0 to N. The method paint of class Container paints its visible components
from N to 0.
4.0.9 When
I call repaint() repeatedly, half my requests get lost and don't appear
on the screen. Why is this?
A. repaint() just tells the AWT that you'd like a paint to happen.
AWT will fold several adjacent repaint requests into one, so that only
the most current paint is done. One possible workaround might be to use
a clip rectangle and only paint the different areas that have changed.
4.0.10 What is the difference between
Component's setForeground(Color
c)
and
Graphics's setColor(Color c) ?
A. First of all, these methods do the same
thing: set the foreground color to the value of the parameter. The difference
lies in where you use them. There is also a Component.setBackground that
will set the background color.
If you are in a constructor or an event handler (e.g. "click here to
turn the canvas blue") you have a Component and should use the setForeground()
method. If you are in a paint() method, that takes a Graphics context as
its argument so you will typically use g.setColor(c).
Unlike a Component, a Graphics object doesn't have a background color
and a foreground color that you can change independently. A Graphics object
arrives in the color(s) inherited from the drawing surface. From then on,
any rendering (drawLine(), drawRect(), fillOval(), etc.) will be
done in the setColor() color. Because they do different things, the
Component and Graphics methods have different names.
4.0.11 When I start a mouse
drag inside a Component, and go outside the Component, still dragging,
the mouse events still get sent to the Component, even though I am outside
it. Is this a bug?
A. No, it is the specified behavior. The
java API documentation says:
"... Mouse drag events continue to get sent to this component even
when the mouse has left the bounds of the component. The drag events continue
until a mouse up event occurs. ..."
It is done for the convenience and ease of the application programmer.
It allows you to handle all drags from the place of origin. If you don't
want this, simply look at the coordinates of the mouseDrag Event, and if
they are outside the Component, ignore them.
4.0.12 What's all this about subclassing Canvas
and overriding paint() ? Can't
I just do a getGraphics() for a component, and draw directly on that?
A. You can do that, and it might work up
to a point (or it might not). A problem arises when the window system wants
to refresh that component e.g. because it has been partially obscured and
is now revealed. It calls paint(), and that has no knowledge of the other
g.drawing() you have done.
4.0.13 But couldn't
the AWT just remember what has been drawn to a Graphics context, and replicate
that instead of calling paint()?
A. It could, but that is not how it works. In practice it is
a lot simpler to be able to look at the paint method, and see explicitly
all the things that will be done to draw that component. Bottom line: use
paint(), not g=getGraphics(); g.drawString( ...
4.0.14 How
can I get the dimensions and resolution of the screen?
A. java.awt.Toolkit.getDefaultToolkit().getScreenSize()
java.awt.Toolkit.getDefaultToolkit().getScreenResolution()
Screen resolution is in dots-per-inch.
Take a look in the Toolkit class for other useful methods.
Toolkit.getDefaultToolkit().getColorModel().getPixelSize() gets you
the color model in terms of bits per pixel.
Math.pow(2,Toolkit.getDefaultToolkit().getColorModel().getPixelSize())
gets you the color model in terms of number of colors. Or use this:
1 << Toolkit.getDefaultToolkit().getColorModel().getPixelSize()
That does a shift left to calculate the power of two.
4.0.15
How do I allow for the size of the title bar and border when I draw a Frame?
A. Use MyFrame.getInsets(). This returns a java.awt.Insets object which
has four ints: top, left, bottom, right, giving the number of pixels each
of those margins are inset from the top. You can use these value to adjust
the Dimension object returned by component.getSize().
If you are doing this in the constructor you need to ensure that the Frame's
peer object is created first. Otherwise the Insets object returned by getInsets()
will have all zero values. Make a call to Frame.addNotify() to have the peer
created.
4.0.16 When I run the Swing
demo on Win95 I get an error "Out of environment space"
A. That's because you don't have enough space for your DOS environment.
You can fix this with:
-
Right click your MS-DOS Prompt icon or window and choose Properties.
-
Choose "Memory" and on "Initial Environment" choose 4096 instead of "auto".
-
Run Swing again, you'll be OK.
4.0.17 How do I resize
a List? I had a List defined as
List tlist = new List(10);
but the Strings in the list were 80 characters long and only the first
15 were being shown. I was not able to resize the List to display the contents
without using the scroll bar.
A. A List cannot be resized in a constructor,
so add the following to the Applet (or wherever)
public void paint (Graphics g) {
tlist.setSize(200,200);
}
Then before showing panel/frame with the List:
tlist.resize(400,400);
4.0.18
How can my program tell when
a window is resized?
A. Override the setBounds() method of Component
to do what you want. Of course, have it call super.setBounds() as well.
Note that setBounds() replaces reshape() which is deprecated.
Note the new APIs call the deprecated APIs instead of the other way
round. For example, Component.setBounds calls Component.reshape, instead
of reshape calling setBounds.
4.0.19
Why doesn't my window
close when I click on the X in the title bar?
A. Here's how to make your program
do that.
JDK 1.0.2: Handle Event.WINDOW_DESTROY to do a hide() and dispose() on the
Frame.
JDK 1.1:
a) Listen for WindowEvent and do hide(); dispose(); in
windowClosing() - this really ought to be the "default" behaviour. Or
b) enable AWTEvent.WINDOW_CLOSING and do the hide() and dispose() in
processWindowEvent().
4.0.20 How
do I clear the contents of a TextArea?
A. Set it to a null String with this:
area.setText("");
4.0.21 What are those
preferredSize() and minimumSize() methods in Component?
A. Those methods allow a LayoutManager
to calculate the preferred and minimum sizes of the Components it is arranging.
You can control the values that the LayoutManager gets by creating subclasses
of the Components you are using and overriding these methods.
4.0.22 How can I force
a synchronization of the graphics state, e.g. of a cursor change, or an
animation frame to be rendered?
A. This is done by the sync() method in
the toolkit. So just use:
AnyComponent.getToolkit().sync();
4.0.23 How do I plot
a single pixel to the screen?
A. Use g.drawLine(x1,y1,x1,y1) to draw
a line one pixel in length. If you are drawing a very large number of
individual pixels, consider using a java.awt.MemoryImageSource object and
measure whether this offers better performance.
4.0.24 How can I tab
between components?
A. In JDK 1.0, you have to read the key
press, and program it explicitly. JDK 1.1 supports tab and shift-tab (previous
field) automatically. The tab order is the order that the components were
added to the container.
4.0.25
What is the difference between "low level" and "semantic" events?
A.
Low-level events are tied to a specific component (resizing a window,
dragging the mouse, striking a key, adding a Component to a Container, etc).
Semantic events are those generated
when you frob a control (move a scrollbar, click on a button, select
from a menu, etc), and the same kind of event can be generated by several
different components. A Button and a List both fire an Action event when
they are clicked on.
To the programmer, the important difference is that you can change
a low level event such as the key value in a keypress, and it will
display the new value. You can also consume low level events so
that they do not appear in the
widget. You can't do these things with semantic events - they have
already "occurred" to the widget.
Semantic events: Use the method addXListener() to add a listener object which
implement the XListener interface, to get XEvent objects delivered
(usually via the AWTEventMulticaster).
Low level events: use the method enableEvents()
and override performX() to grab those events in the object itself.
4.0.26 Is
it possible to have a Java window float above all other windows. for example
a tool palette floats in a super-layer always above all the regular document
windows on which you use the palette's tools?
A. On MS Windows, a Window object floats
above all other windows, unlike a Frame, which is layered in with ordinary
windows. This behavior yields a "floating" effect. Whether a Window object
is really supposed to float is another question entirely.
On Mac, a Window object is either layered in with other windows, just
like a Frame is, or else it is entirely modal -- depending on which VM
you use. In Java -- there appears to be no easy way to get floating behavior.
If anyone knows otherwise, please send in your comments.
4.0.27 How
do I center a dialog box?
A. You cannot currently get the applet's
absolute screen coordinates. Its location (0,0) is relative to the
browser, not the screen itself. But you can center something that it pops up
or displays centered on the screen with code like this:
Dimension screen = Toolkit.getDefaultToolkit().getScreenSize();
my_window.move(
( screen.width - window.size().width ) / 2,
( screen.height - window.size().height ) / 2 );
my_window.show().
In a related fashion, you can center something on its parent like this.
Note the intelligent use of APIs like translate() to do the work for you.
void centre(Component parent) {
pack();
Point p = parent.getLocation();
Dimension d = parent.getSize();
Dimension s = getSize();
p.translate((d.width - s.width) / 2, (d.height - s.height) / 2);
setLocation(p);
}
4.0.28 How do I get back to a normal
echo after I have used TextField.setEchoChar('*');
A. TextField.setEchoChar('\0') works on
most Windows based browsers... but for most other platforms (ie: Netscape
under UNIX), it just locks up the textfield.
There is only one good solution, and that is to make two TextFields
on top of each other, one normal, and one with .setEchoChar('*'), and switch
between them.
4.0.29 How can I get the absolute mouse coordinates?
A. You mean, if the browser is about
640x480, you want a y-coord between 0 and 480. If the browser window is
about 800x600 you want a y-coord between 0 and 600. This might be needed
for a pop-up menu, where you want to pop up at the absolute mouse
position.
The approach is to sum up the event's (x,y) and the locations of the target
and its parents until there is no parent. Though on some browsers,
it seems this is not reliable. [Better suggestions are solicited.]
4.0.30 How can iconify/deiconify a window in Java?
A.There is no way in Java today to write code
to force
a window to iconify or deiconify. You can drop into native code to do it.
The "party line" in JavaSoft is that it is because Java is an application
language, not a window manager. Everyone wishes they'd add it.
4.0.31 How can I write text at an angle?
A.Check out
http://www.nyx.net/~jbuzbee/font.html. Jim has some code to do exactly this.
A good way to do it is to draw the text to an offscreen image and write an
ImageFilter to rotate the image.
Also, in JDK 1.2 on, the Java 2D API handles arbitrary shapes, text, and
images and allows all of these to be rotated, scaled, skewed, and otherwise
transformed in a uniform manner.
4.0.32 How do I know which mouse button was pressed, and how often?
A.
To handle mouse events you have to implement the MouseListener
interface, or derive from the MouseAdapter class in order to use
one of the mouse handling methods. The MouseEvent argument
passed to the handling methods contains fields that say which
button was pressed, and the click count.
Use code like this.
public void mouseClicked(MouseEvent m) {
boolean leftButtonPush =
(m.getModifiers() & java.awt.event.InputEvent.BUTTON1_MASK) != 0;
boolean centerButtonPush =
(m.getModifiers() & java.awt.event.InputEvent.BUTTON2_MASK) != 0;
boolean rightButtonPush =
(m.getModifiers() & java.awt.event.InputEvent.BUTTON3_MASK) != 0;
int click = m.getClickCount(); // might be 1,2,3 clicks or more
You can also call at m.isPopupTrigger().
If it returns a true value, the user has asked for a pop-up menu.
On a lot of window systems,
the right mouse button is the trigger for pop-up menus.
The code above applies to JDK 1.1. There were similar methods for JDK 1.0.
4.0.33 How do I detect a resize of a Frame?
A.
Register a ComponentListener on the frame, like this:
import java.awt.*;
import java.awt.event.*;
class MyFrame extends Frame {
public MyFrame() {
addComponentListener(new CmpAdapter());
}
class CmpAdapter extends ComponentAdapter {
public void componentResized(ComponentEvent evt) {
//doSomething();
}
}
}
Alternatively, the same effect can be achieved like this:
class MyFrame extends Frame implements ComponentListener {
public MyFrame() {
addComponentListener(this);
}
public ComponentHidden(ComponentEvent evt) { }
public ComponentMoved(ComponentEvent evt) { }
public ComponentShown(ComponentEvent evt) { }
public ComponentResized(ComponentEvent evt) {
//doSomething
}
}
4.1 APPLETS
4.1.1 What is the difference
between an application, and applet and a servlet?
A. An application is a standalone program.
An applet is a downloadable program that runs in a web-browser. Typically
an applet has restricted access to the client system for reasons of security.
Other than that it is virtually no different to a regular Java program.
A servlet is a java program whose input comes from a server and whose
output goes to a server. Other than that it is virtually no different
to a regular Java program. Think of a servlet as an application, but
one that (like an applet) requires a context in which to run, namely
web-server software. Servlets are used like CGI, and they allow the
server end to be written in Java as well as the client.
The Web Server starts up a servlet when the URL is referenced, and now
your applets have something that they can talk to (via sockets) on the
server that can write files, open connections to other servers, or whatever.
There is also a software technology from IBM called an "Aglet".
An aglet is a mobile agent that can go from machine to machine,
performing tasks, serializing data collected, and "shipping itself"
(code and data) to the next machine.
It's too early to say if aglets are a flash in the pan or
or a dawning technology. Read about aglets at
http://www.trl.ibm.co.jp/aglets/
Don't confuse Sun's JWS "Java Web Server" with JWS "Java Workshop".
Java Web Server supports servlets as does the lightweight and free server
at Acme.com:
4.1.2 My applet
works on my machine, but fails when I put it on our web server. Why?
4.1.3 How do I use
an image as the background to my applet? How do I set the background color
of my applet the same as the browser?
A. You can simply do a g.drawImage(yourImage,
x,y, this) in the paint() routine of your applet. If the image isn't big
enough to fill the entire background, tile it or scale it There is some
code to tile at:
(under AWT) Alternatively, the AWT can scale your background image to the
size of
the applet. The result quality will depend on the kind of image. Inside
an applet class, you can use:
drawImage(img, 0, 0, size().width, size().height, this);
You can set the background color to match the background color of the browser
by passing the value in as a parameter, like this:
In the HTML applet tag:
param name=BrowserColor value=F1F1F1
(value should be the same hex as the HTML COLOR value).
In the Applet init() method:
String colparam = getParameter("BrowserColor");
int col = Integer.valueOf(colparam,16).intValue();
setBackground( new Color(col) );
An applet cannot override the size imposed by the HTML. If you make the
applet larger, the browser will still clip to the origional size. If you
need more room, open up a new Frame, Window or Dialog to show your output.
4.1.4 How do you make the applet's
background transparent?
4.1.5 How do you do file
I/O from an applet?
A. See answer to question 6.8.
4.1.6 How do you get a MenuBar/Menu
in an applet?
Container parent = getParent();
while (! (parent instanceof Frame) )
parent = parent.getParent();
Frame theFrame = (Frame) parent;
This second suggestion probably won't work on Macs (where would the menubar
go?) or in some browsers. In JDK 1.1, just use a popup menu, which isn't
attached to a Frame.
4.1.7 Can
I get rid of the message "Warning:Applet Window" along the bottom of my
popup windows in my Applet?
A. This is a security feature that prevents
the applet programmer from popping up a window that looks like it came
from the native OS and asking for passwords or credit card info (etc).
Users must always be aware of when they are talking to an unsigned applet.
You can get rid of it by signing the applet, if the user accepts signed
applets from you. See the Java Signing FAQ at
http://www.fastlane.net/~tlandry/javafaq.txt
In Netscape (only), using the Capabilities API to make the call
PrivilegeManager.enablePrivilege("UniversalTopLevelWindow");
before creation of the Frame eliminates the message, if the security manager
passes it.
4.1.8 When
I subclass Applet, why should I put setup code in the init() method? Why
not just a constructor for my class?
A. The browser invokes your constructor,
then setStub, then init(). Hence when your constructor is invoked, the
AppletStub (and through it the AppletContext) is not yet available. Although
in principle you can do things in the constructor that don't rely (even
indirectly) on the AppletStub or AppletContext, it is less error-prone
to simply defer all setup to the init method. That way you know that anything
that needs the stub/context will have it
available.
4.1.9 How
do I pull a non-class file, such as a .gif, out of a jar file?
A. In your class, you should be able to
do something like this:
String imageFileName = "foo.jpg"
URL imageURL = getClass().getResource(imageFileName);
Toolkit tk = Toolkit.getDefaultToolkit();
Image img = null;
try {
img = tk.createImage((java.awt.image.ImageProducer)
imageURL.getContent());
}
catch (java.io.IOException ex) {
System.out.println(ex);
}
(Like anything involving Jar files, this is from JDK 1.1 on).
getResource(String) does not work in applets
in Netscape due to Netscape security issues. You should
use getResourceAsStream(String) there instead.
See
http://developer.netscape.com/software/jdk/relnotes.htm
4.1.10 I want to know about {applets,applications}
but the
lousy book I got just talks about {applications,applets}. What can I do?
public boolean isRunningInBrowser() {
Component p = getParent();
while(p != null && !(p instanceof Frame)) {
p = p.getParent();
}
return (p == null);
}
4.1.11 How do I print a page with an applet?
A. Browsers are starting to introduce support for this.
Until they all have it,
your best bet is to print a screendump. Using the browser
to print the page may leave a blank where the applet is. Putting
print support in the applet will print the applet only, not the rest of
the browser page. See also Q5.2.
4.1.12 How can I position my dialogs
centered (not
top left)?
A. Use some code like this:
void centre(Component parent) {
pack();
Point p = parent.getLocation();
Dimension d = parent.getSize();
Dimension s = getSize();
p.translate((d.width - s.width) / 2, (d.height - s.height) / 2);
}
4.1.13 How can I get two applets on the
same page to communicate with each other?
A.(Easy way) Make them into one applet with two GUIs.
(Hard) Use the static
members of a common class to communicate information between the applets.
This is something of a hack as it relies on class-loading behavior
that may change in future.
(Harder) Use the getAppletContext().getApplets() method to
get references to other applets on the same document. Once you
have that enumeration of Applets, you can invoke methods in them.
Don't forget to use appropriate synchronization when two threads
tweak variables.
Inter-applet communication sometimes arises when you have
a multi-screen type program and you
don't want to force the user into downloading everything at once.
Try to avoid the need for applets to talk to each other. Also check the
URL
http://java.sun.com:81/products/hotjava/1.1/applet_environment.html
which explains how it can be done in HotJava 1.1. Recommendation: stay far away
from code which is browser-specific.
4.1.14 How can I resize an applet?
A. If you want resizing behavior from an applet, you should launch an
external frame that can be resized independently. Browsers don't support
resizing of embedded panels. For the extremely tricky:
have the browser reload the page with the applet when the browser
resizes using new values for width and height (probably not what you
want most of the time). You would need javascript to generate a page
dynamically using document.write("...") when the browser resizes.
Not recommended.
4.1.15 How do I read a text file
stored in a JAR?
A. The best way to do this is to use
the method Class.getResourceAsStream() which will give you an input stream
from which you can read the text file in the JAR.
This technique can be used to get parameter info from a text file
for an applet.
See www.uq.net.au/~zzcmumme for an example.
4.2 BROWSERS
4.2.1 When
will my favorite browser support Java 1.1?
A: All popular browsers now have JDK 1.1 support.
Netscape Communicator 4.04 plus patch "J" fully
supports the features of Java 1.1. It was released in December 1997,
and is only missing the JavaSoft support for applet signing (Netscape has
gone its own way on this).
Sun's HotJava browser fully supports the JDK 1.1 features.
People who are obliged to use Internet Explorer 4.0, with its
Microsoft proprietary changes to Java, can use Java Activator.
Java Activator substitutes a standard Java virtual
machine for Microsoft's in Internet Explorer. It allows you to use
RMI, JavaBeans components, and Java Foundation Classes
in Internet Explorer 3.02, 4.0, and 4.01.
You can download Java Activator from
http://java.sun.com/products/activator.
4.2.2 Is
it possible to set and retrieve cookies from Java, in a manner that is
compatible with all browsers supporting cookies?
A. Short answer: no.
Longer answer: probably no.
Ultimate answer:
A cookie is a morsel of tasty data that a server sends back to the
client, and can retrieve on demand. It allows the server to retain some
state information for each of its clients. The information is typically
something like "what pages has the user seen?" or "is this a privileged
user?".
The DevEdge site on Netscape's home page has a javascript-java example
on getting cookies. Also
http://www.geocities.com/SiliconValley/Vista/1337
has info on connecting an applet with JavaScript functions.
It's quite involved. Stick to just Java if you can.
4.2.3 I am developing an applet and testing
it in Netscape Navigator. I
find that after I recompile, I press reload, clear the caches, retype the
URL of the HTML wrapper, and I still have the old version. Why is this?
A. It is because Netscape has completely
failed to improve the defective code that does this monstrously wrong thing.
It has been like this for many successive releases.
Flushing the network cache will make no difference; that isn't where
the caching is taking place. Although applets are sometimes "pruned" and
their ClassLoaders garbage-collected, this doesn't happen predictably,
so restarting Netscape is the only reliable work-around at the moment.
A related question is "how do I make the browser reload from a URLConnection
instead of just getting the content from the local cache?" The answer is
to use
java.net.URLConnection.setUseCaches(false)
Browsers seem to vary in their conformance to this programmatic
request. Netscape caching varies depending on whether a proxy
server is in use, and which thread in the applet made the get request.
4.2.4 So, why
can't Netscape reload the applet when you press the Reload button?
A. For the applet to be reloaded, the new
version would have to be loaded in a different ClassLoader. Navigator/Communicator's
policy for assigning ClassLoaders to applets doesn't take into account
whether a reload has been done (although there is no technical reason why
it couldn't).
Some versions of Netscape reload the Applet if you hold down <Shift>
while you click on reload. Until they fix it, use the appletviewer to test
applets. And send them mail -- developers can only fix the bugs they know
about.
4.2.5 Should I use Microsoft CAB
files or Java JAR files?
A. The question contains its own answer.
CAB format is a Microsoft-only format. So do not use it as it
destroys software portability.
JAR format is the Java standard format, based on PKZIP format including
data compression. JARs were introduced with JDK 1.1
See http://www.ibm.com/java/community/viewarchive4.html
for more information.
You should use the Java standard format JAR (Java Archive) files, not
a vendor-specific format. JAR files are not just a Java standard, they
are in industry-standard PKZIP format. One reader comments that both formats
can be used with this tag:
<APPLET NAME=myapplet
ARCHIVE="myzip.zip"
CODE="com/nnnnn/nnnn/cccccccc.class"
WIDTH=n
HEIGHT=n>
<PARAM NAME="cabbase" VALUE="mycab.cab">
</APPLET>
IE3 does not support JAR
IE4 supports compressed and uncompressed JAR, but not signed JAR
IE3 and IE4 both support CAB (signed & unsigned)
5. CORE LIBRARIES
5.1 I
can't seem to change the value of an Integer object once created.
A. Correct. Integer (Float, Double, etc)
are intended as an object
wrapper for a specific value of a number, not as a general purpose
way of shipping a primitive variable around as an Object. If you need
that it's easy enough to create: class General { public int i; }
5.2 How do I print
from a Java program?
A. Use the Toolkit.getPrintJob() method
Component c = this.getParent();
while (c!=null && !(c instanceof Frame))
c=c.getParent();
PrintJob pj = getToolkit().getPrintJob((Frame) c, "test", null);
Graphics pg = pj.getGraphics();
printAll(pg);
pg.dispose();
pj.end();
This feature was introduced with JDK 1.1. A common place to put this is
in the code that handles a button press. Printing from an untrusted applet
is subject to a check from the SecurityManager.
To print in JDK 1.0.2, some people have suggested using Jentec's JENI.
(The Java Enterprise Network Interface). JENI is a set of classes that
provides applets with file, print, email, and directory services to lots
of different protocols and locally without the applet having to change
or even be aware which is being used.
JENI is free and available at http://www.jentec.com.
For an example of how to print, see http://www.jentec.com/live/jeni/PrintingAFile.html.
To save a file, see
http://www.jentec.com/live/jeni/CreatingAFile.html.
5.3 What
are the properties that can be used in a PrintJob?
A. The properties are
awt.print.destination - can be "printer" or "file"
awt.print.printer - printer name
awt.print.fileName - name of the file to print
awt.print.numCopies - obvious
awt.print.options - options to pass to the print command
awt.print.orientation - can be "portrait" or "landscape"
awt.print.paperSize - can be "letter","legal","executive" or "a4"
The defaults are destination=printer, orientation=portrait,
paperSize=letter, and numCopies=1
You can search for info like this by joining the Java Developer Connection
(it's free)
http://developer.javasoft.com/developer/index.html
Then do a search for "PrintJob".
5.4 Is there any package
in Java to handle HTML?
A. See the answer to Question 8.14
5.5 Why
don't Dialogs work the way I want them to?
A. Modal dialogs (dialog windows that stay
up until you click on them) are buggy in many browsers and in the 1.0.2
JDK. One bug is that the dialog is not necessarily put on top when it is
displayed. Most of the modal dialog bugs are fixed in JDK 1.1.
5.6 Where can I find information
about the sun.* classes in the JDK?
A. You're not supposed to. Those classes
are only to support functions in the java.* hierarchy. They are not part
of the API, and won't be present in Java systems from non-Sun vendors.
Some people have reverse engineered the code and published an API for these
classes but you use it at your own risk, and it may change without warning.
Worst of all, those programs will not have the portability of true Java
but will only run on Sun JDKs. For the same reason you shouldn't use classes
outside the java.* packages when using JDKs from other vendors.
If you still insist on going ahead, check these URLs:
http://java.sun.com/products/api-overview/index.html
http://www.parmly.luc.edu/javaudio/
http://www.users.interport.net/~mash/javamidi.html
5.7 How do you read
environment variables from with a Java program?
A. Environment variables are not used in
Java, as they are not platform portable. The Mac doesn't have environment
variables for example. A Windows 95 application not started from a DOS window
does not have environment variables. Use properties instead.
Create your own properties file (see java.util.Properties) or specify
them with the -D option when you invoke the interpreter or JRE.
Additionally, on some systems
you can set a property from the command invocation line like this:
java -Dfoo=$foo MyClass (Unix)
or
java -Dfoo=%foo% MyClass (Win95/NT)
This sets the "foo" property to the value of the environment variable foo,
and makes it available in the System properties.
Make sure you do not leave any spaces after the -D or around the = sign.
Inside the program you get the value with:
String env = System.getProperty("foo");
More simply, just put the environment variable on the commandline
and read it as arg[0]
java MyClass %FOO% ; Win32
java MyClass $FOO ; Unix
5.8
How do you use the Date class to display the current
time in my timezone? Date.toString() always uses PST. [jdk 1.1]
(Pacific Standard Time -- the zone covering California).
A.
First of all, let's review the date related classes.
If you want to see the properly formated version of your date, you shouldn't
use getTime().toString(). That routine just returns the date info with
respect to the timezone preference of your computer. To see the correct output,
one possibility is to declare a GregorianCalendar
and pull the fields out of that, as in this example:
import java.util.*;
...
GregorianCalendar g = new GregorianCalendar();
int year = g.get(Calendar.YEAR);
int mon = g.get(Calendar.MONTH);
int date = g.get(Calendar.DATE);
System.out.println(mon +"/" +date +"/" +year);
If you want fancy formats for printing/expressing the date,
you should use the new internationalization routines.
You will need to use the DateFormat and SimpeDataFormat classes in the
java.text package. There is some sample code in the Java API documentation.
As an example of how
the new method should work jdk1.1/src/java/util/Date.java contains the
method:
DateFormat formatter = new SimpleDateFormat (
"d MMM yyyy HH:mm:ss 'GMT'", Locale.US);
formatter.setTimeZone(TimeZone.getTimeZone("GMT"));
or even:
formatter.setTimeZone(TimeZone.getDefault());
It should be reasonably straight forward to adapt this code for your preferred
format (e.g. change string to "hh:mm" or other) and timezone, e.g. change
"GMT" to "ECT", "JST" or other timezone.
5.9 How
are dates represented in Java?
A. java.util.Date stores dates as long
integers representing the number of milliseconds since 00:00:00 UTC Jan
1, 1970 (the birth of Unix, a date known as the "Epoch"). This is sufficient
to represent dates from roughly 29,227,000,000 B.C. to 29,227,000,000 A.D.
Dates earlier than the Epoch are represented as negative numbers, counting
away from 1/1/1970.
In JDK 1.1, Date was augmented by Calendar. Things didn't get any better.
Instead of being ill-conceived and simple, it is now ill-conceived and
complicated.
The code was all licensed from
Taligent. It didn't really improve matters.
Dates are the lemon of Java, as Roedy Green truly notes.
5.10 That may be, but how
do I extract day, month, year from a Date?
Date now = new Date();
Then construct a Calendar to do the translation:
Calendar mycal = Calendar.getInstance();
mycal.setTime(now);
// Now read the Calendar's fields using get().
System.out.println("Year = " + mycal.get(Calendar.YEAR));
We hope all this junk will be fixed once and for all in a future release.
The Calendar, DateFormat, and even TimeZone classes will give you the
timezone of your system (wherever you are).
You can create a TimeZone object with the GMT offset of your choice
like this:
bst_tz = new SimpleTimeZone( 1 * 60 * 60 * 1000, // 1 hour offset from GMT
"BST");
TimeZone.setDefault(bst_tz);
You can then apply that TimeZone to create a Calendar object like so:
GregorianCalendar BritishSummerTime =
new GregorianCalendar(bst_tz);
5.11 What
kind of different date formats are allowed for the Date constructor with
the string parameter.
A. The Language Specification (which also
covers the java.lang, java.util, and java.io packages) is the best reference
for answering questions like this. It has much more detail than the API
Documentation has, and it is available as a book or online at
The Date(String) constructor is deprecated in Java 1.1, by the way.
5.12
How do I get Java talking to a Microsoft Access database?
A. Use the JDBC-ODBC bridge. It is not
especially challenging to set up, but it does require painstaking attention
to detail. There is a step-by-step example in the van der Linden text "Just
Java 2nd Ed." mentioned in the sponsorship section of this document.
Note that the Microsoft version of the Java kit does not support
JDBC-ODBC access because it uses a non-standard native code interface.
Also check the JDBC FAQ listed at the end of this document.
5.13 I
can't seem to change the current working directory.
A. Correct. This missing functionality
is an oversight that we hope will be corrected in future. Changing the
user.dir property merely changes the text property, not the underlying
reality that it is supposed to reflect.
5.14 How
do I create a Vector of ints?
A. ints are primitive types and hence can't
be stored by the Vector class which stores objects. You'll need to wrap
the ints. Try this:
int i =7;
Vector holdsInts = new Vector(5,1);
holdsInts.addElement(new Integer(i));
int j = ((Integer)holdsInts.elementAt(0)).intValue();
5.15
I have several worker threads. I want my main thread to wait
for any of them to complete, and take action as soon as any of them completes.
I don't know which will complete soonest, so I can't just call Thread.join
on that one. How do I do it?
A. You need to use the wait/notify mechanism
to allow any of the worker threads to wake up your main thread when the
worker has completed.
5.16 How
do I get random numbers?
A. If you just need a quick random double
between 0.0 and just less than 1.0
double myrandom = Math.random(); // [0,1)
The notation "[0,1)" is common math notation for "zero to .9999999 etc"
The Sun documents say this returns 0.0 to 1.0, but inspection of the source
shows they are wrong. However, due to the inherent inaccuracies of floating
point arithmetic, multiplying N by 0.999999 etc can result in an answer
of N, not N * .999999 . So watch out when N is big.
Where things get even trickier is in the case where you want an int
within a certain range, say 1 to 6 to simulate the throw of a die or 1
to 52 to represent a playing card. Class Random has a nextInt method that
will return any integer:
Random r = new Random();
int i = r.nextInt();
However, that has an (almost) 50% chance of being negative, and it doesn't
come from the right range. So you just take the abs() value and then mod
it into the right range:
int dice_throw = 1 + Math.abs(i) % 6;
Except, the abs() method fails gracefully in the presence of the Integer.MIN_VALUE
(it returns the same, negative, result!). So it is better to AND to get
the non-negative value: In general, to get a random int between high and
low limits (incl.):
Random r = new Random();
int j = (Integer.MAX_VALUE & r.nextInt()) % (high-low+1) + low;
The sentence states "(almost) 50% chance" because there is one more value
in the negative integers than in the positive integers in two's complement
arithmetic as used by Java. For most purposes, the bias introduced will
be insignificant.
A worse problem is that with the algorithm used, the low order bits
are significantly less random than the higher order bits. And the low order
bits are precisely the ones you get when you do the remainder operation.
5.17
What does "deprecated" mean? I got this in a compiler error message.
A. The compiler will flag a now-obsolete
API as "deprecated". The word means "officially disapproved of". Compile
again with the "-deprecation" option to see what is deprecated. In almost
all cases, the old API has been replaced by a new one. Update your code
to use the new one.
An example of using a deprecated API is calling component.size(). That
has been replaced by component.getSize().
5.18 Where/why
should I use the Enumeration interface?
ZipFile z = new ZipFile("foo.zip");
for (Enumeration e=z.entries(); e.hasMoreElements(); ) {
ZipEntry ze = (ZipEntry)e.nextElement();
System.out.println("got " + ze.getName() );
}
You should look for opportunities in your own data structures to implement
Enumeration, anywhere where the structure has repeated elements.
5.19
Which version of WinZip is compatible with java.util.zip?
A. You need WinZip version 6.2 or later.
Version 6.1 or earlier is not good enough. WinZip can be downloaded
from
http://www.winzip.com/download.cgi.
The pkzip software works fine.
5.20
What is the Model/View/Controller paradigm?
A. Model/View/Controller is a design pattern
used extensively in JDK 1.2.
Basically, the "model" contains your data, the "view" is the
graphical representation, and the "controller" is responsible for
telling the "model" what happens in the "view".
6. I/O
6.1 How
do I read a String/int/boolean/etc from the keyboard?
A. The easiest way is to pick up the source
for the 100% pure Java class
EasyIn from http://www.afu.com/
(same place as this FAQ) Compile it with your code and use it like this:
EasyIn easy = new EasyIn();
int i = easy.readInt(); // gets an int from System.in
boolean b = easy.readBoolean(); // gets a boolean from System.in
double d = easy.readDouble(); // gets a double from System.in
... etc.
EasyIn is free, comes with source, and you can do what you like with
it, including improve it, and send me back the results.
If, instead, you want to "roll your own" code (why?!) In JDK 1.0.2
java.io.DataInputStream in = new java.io.DataInputStream(System.in);
String s = in.readLine();
One way in JDK 1.1
java.io.BufferedReader in =
new java.io.BufferedReader( new InputStreamReader(System.in));
String s = in.readLine();
Once you have the token in a String, it is easy to parse it into one of
the other types, as shown earlier in the FAQ. Yes, it is bone-headed, as
it makes the simplest case of keyboard I/O unnecessarily complicated.
A bug was filed with Javasoft to record this problem,
but don't count on this being fixed any time soon.
6.2 Why
do I have trouble with System.out.println()?
A. Check the spelling. The last two characters
are the letters "ell enn" not "one enn".
The name of the method stands for "print line", since it prints a String
and goes to the next line, rather than staying on the same line as System.out.print()
does. Yes, the name is yet another Java naming inconsistency, since the
input equivalent is readLine(), not readln().
6.3 How
do I write to the serial port on my PC using Java?
A.
There is a platform-independent serial port API introduced in JDK 1.2.
You can download the documentation by registering with the Java Developer
Connection (it's free, http://java.sun.com) and browsing
http://java.sun.com/jdc/earlyAccess/communications.html.
For systems prior to JDK 1.2, read on.
If the port exists as a pathname in
the filesystem, you can open it as a file and read/write. You can also
print text this way by writing to "prn" or "lpt1" on a pc, and "/dev/something"
on Unix.
Writing a formfeed at the end of the file is essential on Windows95.
//class that opens the printer as a file and writes "Hello World" to it
import java.io.*;
public class lpt {
public static void main (String[] argv) {
try {
FileOutputStream os = new FileOutputStream("LPT1");
//wrap stream in "friendly" PrintStream
PrintStream ps = new PrintStream(os);
//print text here
ps.println("Hello world!");
//form feed -- this is important
//Without the form feed, the text
//will simply sit in the print
//buffer until something else
//gets printed.
ps.print("\f");
//flush buffer and close
ps.close();
} catch (Exception e) {
System.out.println("Exception occurred: " + e);
}
}
}
The bigger problem is if you wish to change the characteristics of the
port (e.g. baud rate, parity, etc). Java currently offers no portable way
to do this. You will need to use a native method, or execute a system command.
At least two companies have written a library to drive the port. See
http://www.sc-systems.com has
a library for Windows95, WindowsNT, OS/2, Mac PPC, Solaris Sparc, Linux
x86, FreeBSD x86, HP/UX PA-RISC, and possibly others too.
http://www.cd.com/portio
In addition, there is a Unix serial port utility available with source
at:
http://jarvi.ezlink.com/rxtx/
and http://www.blackdown.org/Products.html.
It's still free under the LGPL, and works on linux, irix, SunOS...
While not helpful to typical home users, there is an alternative portable
COM port solution for Java 1.1 and even 1.0. Buy your COM ports
in the form of "terminal servers". Using a COM port is now as easy
as connecting to it with a Socket. Port parameters can be changed
programatically using SNMP for most terminal servers (but this is never
necessary when a modern modem or other fixed rate equipment is attached).
Any networked box can serve as a terminal server - even Win95 - with a simple
native server application for that box, but buying an actual firmware based
hardware box is much easier.
Furthermore, your Win95 native applications can now share the COM ports
(and any attached modems) via a Win95 product called "Dial-out IP":
http://www.tactical-sw.com/
6.4 How
do I append to a file?
A. There are two ways. JDK 1.1 introduced
new constructors for two of the output classes, that allowed you to set
a boolean flag:
public FileWriter(String fileName, boolean append) throws IOException
public FileOutputStream(String name, boolean append) throws IOException
Another way is to do this:
RandomAccessFile fd = new RandomAccessFile(file,"rw");
fd.seek(fd.length());
Then write using fd.
Note that the latter method does not take advantage of the
"append" mode present in many operating systems (such as all Unixes).
Such a difference may make a difference with
multiple processes or threads appending to the same
output file. This can happen frequently, even if not intended by the
programmer, e.g. with logfiles in multitasking environments.
With the lack of file-locking mechanisms in Java the issue
becomes even more significant.
6.5 Is
it possible to lock a file using Java ?
A. Java does not feature an API to lock
a file or regions within a file. Code that needs to do this must take one
of four approaches:
1. implement an advisory locking scheme using features that Java does
have (synchronized methods). This allows you to lock files against use
by other Java code running in the same JVM.
2. Use an atomic operation like file "renameTo()" and have all processes
(Java and non-Java) follow the same protocol: if the operation succeeds,
you have the lock, and you change the file back to give up the lock.
The FAQ previously recommended delete() or mkdir() as the primitive, but
there has been a report that these operations are idempotent (can be
repeated without error) in some JVMs. The suggestion of renameTo has been
proven to work in practice.
See
http://www.camb.opengroup.org/~sanfilip/FileLock/
for example locking class docs and source.
3. make calls to native code to issue the locking ioctls. This approach
is not portable, but gives you a shot at having your locks respected by
other programs using standard locking ioctls outside Java.
4. Push the work to a central server. Since socket connection requests
arrive in a single queue on the server, this can be used to serialize
lock requests. There might be some
merit in copying the NFS lockd protocol for a general approach. Rolling
your own simple version for a specific application is pretty easy.
A database would be better off locking records or fields, not byte offsets.
In theory, the server socket approach would make it
easier to perform automatic cleanup of a lock on abrupt VM process
failure, e.g. by asking "are you still alive?" to the lock holder
occasionally.
6.6 How
do I make the keyboard beep in Java?
A. In JDK 1.1, java.awt.Toolkit has the
method beep(). It does not work on NT 4.0 (bug).
System.out.print("\07");
System.out.flush();
should work, and works in JDK 1.0.2, too. That's the ASCII BEL character
(Java doesn't support the C abstraction of '\a' for an alert character).
6.7
How do I execute a command from Java? How do I do I/O redirection in Java
using exec() ?
A. See answer to question 10.5
6.8 How
do you do file I/O from an applet?
A. For security reasons, untrusted applets
accessed across the network are restricted from doing certain operations,
including I/O. This prevents rogue applets from sending out your private
data, or deleting it. A trusted (signed) applet can perform these operations
(JDK 1.1 on).
The following suggestion is for server-side input.
- You can read a file on the server if you can create a URL referencing
the file. Then open a stream, then use any of the stream-based methods
to read.
This allows reading but not writing. It requires an http demon running
on the server, which will usually be the case.
try{
URL url = new URL("http://somewhere.com/test.txt");
// or URL url = new URL( getDocumentBase(), filename);
DataInputStream dis = new DataInputStream(url.openStream());
String s = dis.readLine(); //read till you get a null line.
} catch(MalformedURLException e){System.out.println("URLException:"+e);}
catch(IOException e){System.out.println("IOException:"+e);}
You cannot write a file on the server this way.
The following suggestions are for server-side output.
It absolutely requires the cooperation of the server to allow an applet
to write a file to the server. This cooperation may take any of several
forms:
-
ftp server process
-
file server (custom written)
-
listening on a socket for data from applets
-
CGI script
-
Java RMI (remote method invocation)
In particular:
1. Open a socket back to the server and read/write the data. Have
a process on the server that listens for socket connections from applets
and does the requisite I/O. This does I/O on the server.
2. Or use a CGI script or servlet on the server to write when browsed.
There is some source at ftp://ftp.oyster.co.uk/pub/java/fileIO/
The following suggestions are for client-side I/O.
3. Use a trusted applet (see section on security). This will permit
local I/O without any of the restraints mentioned above. In this regard,
the appletviewer and many other browsers regard applets loaded from a local
filesystem (rather than across the net) as being more trustworthy, and
perhaps even allowed to do I/O.
4. Or use a browser that has a security policy that is configured to
allow file I/O (such as Sun's appletviewer).
Also see the answer to question 5.2 regarding the JENI library.
6.9 I
used a C program to write a binary file. When I instatiate a DataInputStream
on the file in Java, and try to readInt, I do not get the correct numbers.
Why is this?
public int swap(int i) {
int byte0 = i & 0xff;
int byte1 = (i>>8) & 0xff;
int byte2 = (i>>16) & 0xff;
int byte3 = (i>>24) & 0xff;
// swap the byte order
return (byte0<<24) | (byte1<<16) | (byte2<<8) | byte3;
}
Alternatively, the following code assembles bytes from
a byte array in big-endian order (as used by Java) into an int:
byte[] bytes = ... // whatever
int start_index = ... // wherever
int value = 0;
for ( int i = start_index; i < start_index+4; ++i ) {
value = ( value << 8 ) | ( bytes[i] & 0xFF );
}
If the bytes are in little-endian order, just change the "for":
for ( int i = start_index+3; i >= start_index; --i )
6.10 How
do I make I/O faster? My file copy program is slow.
A. This is the purpose of BufferedInputStream.
It is a flaw in Java that buffered I/O is not the default, with a flag
or different constructor to turn it off. I/O is the second worst designed
package in Java, after the Date class.
6.11 How
do I do formatted I/O of floating point numbers?
6.12 How
do I read numbers in exponential format in Java?
A: The program below (written by Steve
Chapel) uses StreamTokenizer to read data from the standard input and recognizes
doubles in exponential format (e.g. -1.23e-45).
import java.io.*;
public class ReadExponential {
public static void main(String argv[]) {
DataInputStream in = new DataInputStream(System.in);
StreamTokenizer st = new StreamTokenizer(in);
try {
while (st.nextToken() != StreamTokenizer.TT_EOF) {
switch (st.ttype) {
case StreamTokenizer.TT_NUMBER:
double num = st.nval;
int exp = 0;
st.ordinaryChars('\0', ' ');
st.nextToken();
st.whitespaceChars('\0', ' ');
if (st.ttype == StreamTokenizer.TT_WORD &&
Character.toUpperCase(st.sval.charAt(0)) == 'E') {
try {
exp = Integer.parseInt(st.sval.substring(1));
} catch (NumberFormatException e) {
st.pushBack();
}
} else if (st.ttype < 0 || st.ttype > ' ')
st.pushBack();
System.out.println("Num " + num * Math.pow(10, exp));
break;
case StreamTokenizer.TT_WORD:
System.out.println("Word " + st.sval);
break;
default:
System.out.println("Char '" + (char) st.ttype + "'");
break;
}
}
} catch (IOException e) {
System.out.println("IOException: " + e);
}
}
}
6.13 I'm
trying to read in a character from a text file using the DataInputStream's
readChar() method. However, when I print it out, I get ?'s.
A. Remember that Java characters are 16-bit
Unicode characters, while many hosts systems store characters as 8-bit
ASCII characters. Therefore, to read individual chacters from a text file,
you need to ensure the proper conversion. The proper way to do this is
to use an InputStreamReader, which converts from 8 to 16 bit streams:
FileInputStream fis = new FileInputStream("myfile.txt");
InputStreamReader isr = new InputStreamReader(fis);
char c3 = (char) isr.read();
The less-favored way (because it is not so portable, as the encodings translation
is not done) is just to read a byte and cast it into a character:
FileInputStream fis = new FileInputStream("myfile.txt");
DataInputStream dis = new DataInputStream(fis);
char c1 = (char) dis.readByte();
6.14 How
do I delete a directory in Java?
A. JDK 1.0 did not support directory removal.
JDK 1.1 supports directory removal with the method:
public boolean delete() in class java.io.File
Make sure you don't have any open streams in the directory you're trying
to remove. Do a close() on all streams, even if the underlying file is
gone.
6.15 How
do I tell how much disk space is free in Java?
A. There currently aren't any good Java
APIs for system introspection. There is no Java way to control processes,
or look at system resources. You have to Runtime.getRuntime().exec() to do
"df" on unix or "dir" on Windows right now.
6.16 What is the difference
between the various ZIP formats: ZIP, GZIP, and PKZIP?
A. GZIP files (the Gnu format) are not
archives and compress only one file. GZIP is essentially a one file subset
of the Zip format. Zip is an archive file format, popularized on
PCs, that contains multiple compressed files. PKZIP is a set of
commercially-available programs that create Zip files. All three PKZIP,
GZIP and Zip use the deflate compression format, which is based on the
LZ77 algorithm. This compression is also used by the zlib library and the
PNG graphics file format. The format is specified in RFCs 1950,
1951, and 1952,
and is unencumbered by licenses or patents.
An alternative compression technology, LZW compression, is encumbered
by Unisys's patent. LZW is used in GIF files and by the Unix compress command.
Luckily, as well as being free from patent restrictions, LZ77 also
gives better compression than LZW.
LZW is the initial letters of the last names of the three
computer scientists who developed the algorithm (Lempel, Ziv, Welch).
The basic classes (all in java.util.zip) that read LZ77 Zip format are
Deflater and Inflater. These are used by the stream classes DeflaterOutputStream
and InflaterInputStream. The java.util.zip classes GZIPInputStream and
ZipInputStream inherit from InflaterInputStream.
PKZIP is a commercial program for DOS, Windows, and OS/2, sold by PKWARE
Their FAQ (http://www.pkware.com/zipgfaq.html)
specifically says
"Because PKWARE has dedicated the .ZIP file format to the public domain,
it is possible for other people to write programs which can read .ZIP files.
NOTE THAT THE PKZIP, PKUNZIP, PKSFX PROGRAMS AND THEIR ASSOCIATED SOURCE
CODE AND SUPPORT PROGRAMS ARE THE EXCLUSIVE PROPERTY OF PKWARE INC. AND
ARE NOT PUBLIC DOMAIN SOFTWARE PROGRAMS.
The "other people" PKZIP's FAQ refers to is the InfoZIP project, a group
of public-minded programmers spread over the world producing
free software that works on most ANSI C compilers and platforms. See
InfoZIP's free (C language) source code was the basis for the JDK1.1 Zip
package and the GNU GZIP program.
Jar files are in ZIP format, but are not as complete as a full filesystem
archive format since file permissions are not saved.
Some versions of WinZip are known to
be inadequate for processing the full PKZIP format. Use InfoZIP instead.
6.17 How can I use characters other than ASCII
in Java?
A.Search for the article titled "Adding Fonts
to the Java Runtime" at
http://java.sun.com.
The article explains how to add fonts to Sun's JDK, using the font.properties
file. [If anyone has summarised the information, please send
it in].
6.18 I did a read from a Buffered stream, and
I got fewer bytes than I specified.
A.
This is the way that BufferedInputStream works up to and including the
current release. The behavior is so unintuitive that it really
represents a bug. Javasoft
has "resolved" the bug by writing the comments in the
program so that the behavior is not disallowed.
When you instantiate a buffered input stream, you can specify the size of
buffer it should use. Let's call this the internal buffer.
When you call read() you can say how many bytes to read. Let's call
this the request. If the request is smaller than the internal buffer and
not a multiple of the internal buffer, then the last read returns only the
odd bytes left in the internal buffer! The more reasonable
and intuitive behavior would
be for the internal buffer to be refilled, so that the whole request
can be granted.
For example, if you create a BufferedInputStream
with an internal buffer of 1000 bytes,
and try to read 512 byte chunks, your first read will return 512 bytes,
but your second read will only return (1000-512), or 488, bytes.
(Assuming that the file has at least that many bytes in it).
The following code illustrates the problem.
// troubleshooting by Tov Are Jacobsen
import java.io.*;
class filebug {
public static void main(String args[]) throws
FileNotFoundException,IOException {
BufferedInputStream bis =
new BufferedInputStream( new FileInputStream("test.txt"),
1000 );
byte[] buf = new byte[2000];
int numread;
System.out.println( "Available: "+bis.available() );
while (true) {
numread = bis.read(buf,0,512);
if (numread<0) break;
System.out.println( "got "+numread+", avail:"+ bis.available());
}
}
}
Of course, a valid reason for getting less than you asked for is that you
asked for more data than is actually available in the Stream, e.g. you
requested 512 bytes from a file that only contains 40 bytes. In
general, there are no guarantees about how much data is returned for
a given buffered input stream read request. To avoid this problem, push
a DataInputStream on top of your buffered stream. Then
you can call readFully() which will do what you want.
A similar "got less than I asked for" occurs when reading a socket.
Network protocols frequently packetize data and send it across in bursts.
Nothing is lost of
course, and you are always told how many bytes you actually got. You will
get the remaining bytes on a subsequent read. This happens regardless of
the language used.
Be sure to check the "amount of data returned"
when using the read(byte[], int, int) method of BufferedInputStream,
or when reading from a socket.
Another problem with java.io.InputStream.read(byte[], int, int) is
that it catches and ignores IOExceptions. Instead, these
exceptions should be passed on to the caller.
Ace programmer Jef Poskanzer, jef@acme.com, has a version to
do this at http://www.acme.com/jef/.
6.19
How do I get a directory listing
of the root directory C:\ on a PC?
A. The obvious approach of calling
File.list("C:\"); does not work. There are two reasons why this
fails. First, slash is an escape character in Java, so if you want a
literal slash, you have to repeat it. Second, you need to give the
name of the directory, i.e. dot. Putting this together, either of
the following calls will work
File.list("C:\\.");
or
File.list("C:/.");
Note: a file separator of "/" works just as well as "\" in
most MS Windows programs and library calls. It is an artifact of DOS's
origin's as a ripped-off port of CP/M.
They changed the direction of the CP/M file separator,
so it looked different to a casual onlooker, but they
also made the old one continue to work in the shell and elsewhere.
That code has stayed there ever since.
6.20
How do I redirect the System.err stream to a file?
A. You cannot assign a new FileOutputStream
to System.err, as it is final. Instead use the System.setErr() library
call, like this:
System.setErr(new FileOutputStream("myerrors.txt"));
This was introduced with JDK 1.1.
There is also a corresponding setIn() for redirecting standard in, and
a setOut() for standard out.
7. NETWORKING
& DISTRIBUTED OBJECTS
7.1 Should
I use CORBA in preference to RMI? Or DCOM? Or what?
A. If your distributed programs are all
in Java, then RMI provides a simpler mechanism that allows the transfer
of code, pass-by-value of real Java objects, and automatic garbage collection
of remote objects.
If you need to connect to C++ (or other language) systems or you need
CORBA-specific services, then CORBA is your choice.
has a good intro to CORBA in the Java world.
In July 1997, Sun announced that it was aligning RMI to work more closely
with CORBA. Sun is simply adding an IIOP transport layer to RMI to support
interoperability with CORBA. Java programs can then
access CORBA-based objects through IIOP, the OMG's CORBA-based protocol.
This is very good news for those building heterogenous Enterprise systems,
although
it will take some additions to IIOP to support the pieces that RMI uses.
Microsoft spokespeople have tried to promote DCOM by
spreading misinformation that RMI is
changing or being dropped. That is totally wrong.
The RMI API continues unchanged in its current form.
Using DCOM would restrict your code to only ever run on Microsoft platforms
using Intel hardware, and negates the "write once, run anywhere" Java philosophy.
Non-portable, single vendor code should be avoided.
7.2 Why
does <my java debugger/IDE/other> hang for a couple of minutes if my
Windows PC is not dialed up to the Internet?
A. Java has networking support built in.
When the Java program starts the Winsock dll automatically gets loaded.
The first thing this does is to try to resolve the fully qualified domain
name for your machine under the name "localhost". If your system doesn't
have this name mapped, it will try to query a nameserver on the internet,
which is typically (on a PC) your dialup ISP. So it either prompts you
to connect to the ISP, or waits till the attempt times out.
You can sometimes minimize the Win95 problem by giving
your system another way to resolve
DNS names. Edit the hosts file for your system
so that localhost
and the full domain name are both mentioned.
On Windows 95 systems the hosts file is: %windir%\HOSTS
(for example, C:\WINDOWS\HOSTS)
On Windows NT systems the hosts file is:
%windir%\System32\DRIVERS\ETC\HOSTS
(for example, C:\WINNT\System32\DRIVERS\ETC\HOSTS)
So if my system is called
goober.best.com change the hosts file from
to
127.0.0.1 goober.best.com localhost
or (showing more of the file) to
# Hosts file
127.0.0.1 localhost
129.146.77.177 goober
Another alternative is to dial up with a PPP connection to your ISP
whenever you want to run networking programs.
Fundamentally the experience of some people has been that networking
is not completely satisfactory on Windows95, and is subject to sporadic
unexplained failures. If this occurs to you, there is little choice
but to reboot and start again. Microsoft has several network-related
patches at its site http://www.microsoft.com
7.3 If I call the InetAddress.getByName() method
with an IP-address-string argument, like "192.168.0.1", I
get an UnknownHostException on some platforms, but not others. Code like
Socket sock = new Socket("155.152.5.1", 23);
triggers the exception. Why?
A. This is a platform difference that arises
out of different semantics in the underlying network libraries, and is
[said to be, but subject to confirmation] fixed in JDK 1.1. On Solaris
and Windows NT, the IP address string only works for IP addresses that
have an associated hostname. On Linux and Windows 95, the IP address string
works in all cases.
http://www.cdt.luth.se/~peppar/java/InetAddress/
has a workaround.
When InetAddress is instantiated with an IP address, a reverse DNS lookup
is done. If the IP address is not associated with a valid hostname, the
instantiation will fail. This is part of anti DNS-spoofing, and in JDK
1.1 works because the reverse lookup will not occur until the hostname
is asked for. So in JDK 1.1,
InetAddress in = InetAddress.getByName("155.152.5.1");
[Note: this info is still to be confirmed. Net gurus?]
7.4 I am using JDK 1.1.1 on Windows95, and
when I start jdb I get "Uncaught exception: java.lang.UnsatisfiedLinkError no
winawt in shared library path"
The same program works OK using jdk1.1
A. It *sounds* like your java\bin directory
is not on your PATH and so the system can't find the winawt DLL.
But actually the problem is the version of Microsoft's Visual C++ that
was used to build the product. VC++ 4.2 incorrectly generates code that
depends on MSCVRT.DLL or in the case of java_g, MSVCRTD.DLL. These DLLs
are not present in (some versions of) Win95. To make things even more interesting,
some versions of Win95 (yes, there are at least four different ones...)
ship with a broken MSVCRT.DLL (and MSVCRTD.DLL?) that seems to work, only
it doesn't, and after a while it dies.
Sun linked the winawt_g.dll with VC++ 4.2, which wrongly brought in
MSVCRTD.DLL, the debug version of the VC++ runtime. You have to get that
library from somewhere (like, say, VC++)
in order to get jdb to run.
You'll hit this problem any time you try to debug 1.1.1 code with jdb
on a win95 system that doesn't have VC++ (or the MSVCRTD.DLL library from
some other source) installed. At least this is a problem you can solve
without waiting for the next release.
Some people say that the missing library has been seen at http://cag-www.lcs.mit.edu/curl/Binaries/PC/
Others say you need to buy VC++ to get it.
7.5 I
want to pass a class file to willing recipients who are using my applet.
Any ideas how?
A. You could use a trick: put your .class
file(s) in a .zip archive and use showDocument() on the URL. A person accessing
this will get a dialog box put up asking them about saving the file to
their local hard disk. You can see this in action and try it out yourself
at:
7.6 How
do you succeed with new URL(someURL) from behind a proxy server?
A. Tell the run time system what you are
trying to do, like this:
java -DproxySet=true -DproxyHost=proxy_host -DproxyPort=proxy_port
\MyJavaProgram
note proxyPort is optional and it defaults to 80.
7.7 What
is "swizzle", as in "Swizzle this object?"
A. It means serialize. To swizzle an object
is to recursively serialize or flatten composed objects.
7.8 I
have been using the Serializing capabilities in 1.1 to save some objects
to disk. I added a new field to one of my objects that get serialized and
now deserializing my old data no longer works. I get this exception:
java.io.InvalidClassException: MacroData; Local class not compatible
A. You need to add a declaration such as
static final long serialVersionUID = 4021215565287364875L;
in the modified class. The actual value of this long is supplied by the
"serialver" utilitity suppied with the JDK. Any versions of a class other
than the first version require this static to be defined
in the class. This is how versioning is achieved.
7.9 My
socket code looks good, but is broken!
7.10 How
do I map between IP address and hostname?
A. In Java 1.1 (earlier releases were buggy)
use:
String host = InetAddress.getByName("211.10.2.119").getHostName();
7.11 How
do I embed an anchor in a URL? Just putting it as part of the string in
the constructor doesn't work.
URL url = new URL("http://www.my_domain.com/my_page.html");
URL anchor = new URL(url, "#section2");
this.getAppletContext().showDocument(anchor);
7.12 RMI
seems to have stopped working for me in JDK 1.1. Why is this?
A. The rules for where the client looks
for a stub class seem to have changed making it necessary to reset your
class path on the client after starting the rmi registry. In particular,
it looks like rmic was not updated to the new "don't need $CLASSPATH" convention
as the compiler was.
There are several very good sources available from Sun which cover many
simple and advanced RMI problems. They are:
-
The documentation, of course:
-
Dedicated FAQs on RMI and Object Serialization
-
Mailing list RMI-USERS@JAVASOFT.COM with archive at
Visit the archive!
7.13 After
a number of RMI client to server connections (55 on my system), subsequent
RMI clients trying to connect fail. Why?
A. You are hitting the default limit of
64 open file descriptors. Try increasing the limit in your OS.
In addition there is currently a practical RMI connection limit imposed
by the scalability of the VM and the performance of object serialization.
In JDK 1.2 this is addressed. The actual number of active clients you will
be able to support will depend on the workload mix you have (i.e. the number
of clients, how often they talk to the server, and how much work must be
done per call).
7.14 How do I POST to a CGI script from an
applet?
A. Let's start by noting that this is
more troublesome than it might seem at first, and that GET is preferred.
For an untrusted applet, the CGI script
can only be on the server that served the applet. Then use code like this:
try { sock = new Socket(host, 80);
dock = new DataOutputStream(sock.getOutputStream());
dock.writeBytes("POST "+cgiloc+" HTTP/1.0\n");
dock.writeBytes("Content-type: text/html\n");
dock.writeBytes("Content-length: " +my_string.length() + "\n\n");
dock.writeBytes(my_string+"\n");
dock.close();
sock.close();
uresp = new URL(getDocumentBase(),"respond.html");
getAppletContext().showDocument(uresp); }
The my_string contains the data you want to POST to the CGI script. The
string should be encoded in the special way CGI expects. The class method
java.net.URLEncoder.encode(my_string) will do it.
The CGI has to write its output to
respond.html so that it can be displayed by the browser. Even this
won't really work, because respond.html could be overwritten by a
subsequent request to the same CGI before the results of the first
POST are read back.
To get an acceptable solution takes quite a lot of
effort. There's a pretty good explanation at
http://www.javaworld.com/javaworld/javatips/jw-javatip41.html
In general you should prefer GET
to POST for CGI access from Java. As it says on the Javaworld page,
the answers to the question
are really: you can't, don't POST (use GET), use a bean, or cheat.
Finally, if you request a url via
the URLConnection/HttpURLConnection, the server
sets the content type, and your applet can use URLConnection.getContentType()
to get the type. Alternatively, use setRequestProperty to set it, like
this:
url = new URL(cgiUrl);
urlc = url.openConnection();
urlc.setRequestProperty("Content-type", "application/x-www-form-urlencoded");
8. MULTI-MEDIA
8.1 Why
won't my audio file play?
A. Java 1.1 and earlier releases use one
audio format exclusively. The audio file must be in .au format, recorded
at 8 KHz, mono, in mu-law encoding. If your audio clip is in a different
format (e.g., .wav) or a different frequency it must be converted to the
exact specifications above before Java can play it. Support for .wav and
other formats is part of the Java Media Framework coming in JDK 1.2.
Search at www.yahoo.com for GoldWave for Win 95, sox for Unix and similar
conversion utilities for other systems. One conversion utility in Java
is at http://saturn.math.uaa.alaska.edu/~hursha
The source of a Java class to play linear PCM .WAV files is at: http://www.shef.ac.uk/~cs1mjp/Java/WhiteBoard/WavePlayer.html
It can be used in any Java application or applet.
8.2 Does
Java support Animated GIFs?
A. Java 1.0.2 and earlier releases use
GIF and JPEG formats, and do not use the GIF89 animated GIF format. (An
animated GIF is one that contains successive frames of an image, so when
they are displayed in quick sequence the image appears to contain movement).
When you display an animated GIF in Java 1.0.2, you will just get the first
frame. There doesn't appear to be any easy way to get other
frames from the image.
The advantage of an animated GIF file is that there is only one file
to download, and it is simple to do simple animations. The advantage of
programmatic control over individual frames is that you control the rate
and order of displaying them.
Here's a surprise: JDK 1.1 supports the animated display of animated
GIFs. For simple animations animated GIFs are a lot easier and lighter-weight
than coding an animation explicitly.
8.2.0
How do I create animated GIFs?
A.
Use
GIFanimator from ULead (said to be the best)
http://www.ulead.com, or
GIF Construction Set from Alchemy Mindworks
8.2.1 How
do I prevent animated GIFs from flashing while displaying?
A. The problem is most likely that in your
paint method you have
g.drawImage(img, ix, iy, this);
You should change this to
g.drawImage(img, ix, iy, getBackground(), this);
This will change all the transparent regions of the image to the background
color before painting to the screen. If you paint transparent images directly
to the screen they flicker.
If that does not solve it then check that:
public boolean imageUpdate(Image img, int flags, int x,
int y, int width, int height) {
if ((flags & (FRAMEBITS|ALLBITS))!= 0) {
repaint();
}
return (flags & (ALLBITS|ABORT)) == 0;
}
2. update is
public void update(Graphics g) {
paint(g);
}
If you have a background Image behind the partly transparent animated GIF
you will have to double buffer. You can crop the backgound image so you
won't have to double buffer the full app and waste too much memory.
8.3 Does
Java support transparent GIFs?
A. GIF89a images with a transparent background
show up as transparent without further filtering. This has been supported
from 1.0 on. Java correctly displays both animated GIFs and transparent
GIFs.
Even better, you can fill the transparent pixels with a color (so they
appear non-transparent in Java). Just pass the fill color explicitly:
drawImage(img, x, y, w, h, fillcolor, this);
Further, you can filter the pixels of an Image to turn any bits you wish
transparent. However, the most you can do is reveal what is underneath
the image. You cannot reveal what is underneath the applet (i.e. on the
browser itself). By default applets have a plain grey background.
8.4 How
do I play video in Java?
A. Use the Java Media Framework Player
API
The spec can be found at
Intel has released a SDK for the Java Media Framework Player API. The SDK
is for Windows 95 and Windows NT For more information, see
SGI has released an implementation of JMF for IRIX:
8.5 How
can I play *.au files from an application?
A. You've got 2 options:
a. Use the AudioClip or AudioPlayer class in sun.audio
If you take this option, you can only play AU files. Worse, your code is
no longer 100% pure Java, as it relies on a vendor library.
import sun.audio.*;
URL url; ...
AudioStream audiostream = new AudioStream(url.openStream());
AudioPlayer.player.start(audiostream);
...
AudioPlayer.player.stop(audiostream);
b. Use the new Java Media Framework API, allowing a wide range of
video and audio formats to be played back. See previous question for implementations
of this.
8.6 How
do I read in an image file, in an application?
Image img = Toolkit.getDefaultToolkit().getImage(fname);
8.7 When I initialize a component, I
call MyComponent.getImage() to get its image. createImage() returns null!
I know the image works elsewhere. What's wrong?
A. A peer component needs to exist for
your component before you can get its image. This is done by the method
addNotify() (surely one of the most poorly named methods in all Java --
it doesn't mean "add a Notify". It means "Notify that the Component has
been added to a Container". This tells the system, "whoops, I'd better
create the peer for this Component right away). addNotify will be called
for you when you add your component to a container.
If you override addNotify(), don't forget to call super.addNotify()
in your overriding version.
8.8 How can I force
a reload a fresh version of an image into my applet? My image file is changed
periodically, and I want the applet to go and retrieve it, not cache it.
A. You need to turn off caching for the
URL.
URL url = null;
URLConnection con;
try {
url = new URL(getDocumentBase(),"image.jpg");
con = url.openConnection();
con.setUseCaches(false);
} catch (MalformedURLException e1) {System.err.println(e1.getMessage());}
catch (IOException e2) {System.er.println(e2.getMessage());}
Note: some programmers have reported that it caches anyway, even if that
do this. That is a browser bug.
One programmer reported that even after turning off caching and calling
image.flush() before getImage(..), he was still seeing the same picture
even though it had been changed on the server.
He worked out a solution: access the image via a cgi script that returned
a URL. This redirects the browser, and he put in an Expires: header as
well to force the reload. Painful and laborious, but it got the result.
8.9 How can I save
an Image file to disk in jpg or gif format?
A. A number of people have written utilities
to do that. One of them is available at the same place as this FAQ:
8.10 What causes this problem?
$ appletviewer m.html
Premature end of JPEG file
sun.awt.image.ImageFormatException: JPEG datastream contains no image
at sun.awt.image.JPEGImageDecoder.produceImage(JPEGImageDecoder.java:133)
at sun.awt.image.InputStreamImageSource.doFetch(InputStreamImageSource.java:215)
at sun.awt.image.ImageFetcher.run(ImageFetcher.java:98)
A. There's a known bug in early releases of
the JDK which can cause the above failure when reading a JPEG across a
slow connection. The failure only occurs if the JPEG contains a large application
data block (APPn marker) --- the problem is that the JPEG decoder is trying
to skip over the APPn and failing if not all of the APPn has been received
yet. The quoted error message is only one of several possible complaints,
but they all stem from the same root.
Photoshop is the most common source of JPEGs containing oversize APPn
blocks. In particular, if you allow Photoshop 4 to save a thumbnail (preview)
in a JPEG, the thumbnail plumps up Photoshop's private APPn marker to several
K, which is usually enough to cause this problem.
There are several possible workarounds:
1. Get a newer JDK --- this problem is said to be fixed in 1.1. (If
you are putting images up on the Web, this isn't much of a solution, because
you can't assume visitors to your site have an up-to-date Java installation.)
2. When making JPEGs for Web use from Photoshop, make sure you have
turned off the "save thumbnails" preference. (This is a good idea quite
aside from bug workarounds, because the thumbnail is just a waste of download
time as far as a Web browser is
concerned.) You might still have a problem if you've got verbose comments
or lots of paths being saved into the file, but 99% of the time, getting
rid of the thumbnail will make Photoshop's APPn small enough to not trigger
the Java bug.
3. Use a tool such as 'jpegtran' (from the Independent JPEG Group) to
strip out the Photoshop APPn entirely without any loss of image quality.
Recommended answer for the compulsive byte-trimmer.
4. (Last resort) Load and resave the image in a different image editor
that won't insert any APPn or other overhead data. This implies a JPEG
generational loss, so I don't recommend it if you are picky about image
quality.
Any large overhead marker will cause the same problem; 4K of comment
text, say, in a COM marker. So Photoshop is not the only source of tickling
this bug.
8.11 How
can I convert between GIF and JPEG formats?
A. In a word: don't.
There's hardly any overlap between the set of images that JPEG works
well on and the set that GIF works well on. Sometimes, with enough care,
you can get an acceptable conversion ... but most of the time gif<->jpeg
conversion will just turn your image to mush. It's better to pick the right
format in the first place.
If you're determined to convert formats anyway, try
the GBM (Generalized Bitmap Module).
The package is GNU licensed, in C and is very good.
Find it at
http://www.interalpha.net/customer/nyangau/
GBM does a good job converting to JPG, and
'lossiness' is adjustable to 0%.
It also converts to/from about 20 other formats, does
cropping, sizing, color mapping, gamma correction,
halftoning, everything you could want.
GBM source doesn't support jpeg directly, but utilizes
jpg source from IJG called jpeg-6a and found at
ftp://sun2.urz.uni-heidelberg.de/pub/simtel/graphics/jpegsr6a.zip
For more info see the JPEG FAQ at http://www.faqs.org/faqs/jpeg-faq/
8.12 If
you have an InputStream (rather than a file) that contains an Image, how
can you display it?
A: Using this method, and some adroit shuffling.
Toolkit.getImage(URL url)
Create a thread that pretends to be an http server. Make it listen to some
port (8888 for example) for incoming requests. When the thread gets a request,
it should simply whisk up the appropriate http headers and follow it by
the InputStream. Thus the component that has the input stream and wants
to do the getImage(url) can now invoke:
Toolkit.getImage("localhost:8888/")
The thread will act as a stream-to-url adapter, and send back the data
It saves you from having to read 200K of JPEG data before you can begin
drawing anything.
8.13 How can
I record sounds in Java?
A. The Java Media Framework coming in JDK
1.2 will support this. In the meantime, there is a package for Win95/NT
available at http://www.scrawl.com/store/
It supports 8, 16-bit, stereo, mono, 11025, 22050, 44100 Hz record/play,
load/save .WAV files.
8.14 Does Java have any built-in support for
displaying HTML?
A. The JDK doesn't currently have any support
for rendering HTML.
The Swing software that will appear in JDK 1.2 has an
elementary (graphics, tables, text) HTML bean that is good enough for
simple rendering (help files, email, etc). Other alternatives include:
9. SECURITY
9.1 What
is a "trusted applet"?
A. JDK 1.1 introduced the notion of a "trusted
applet" which is one that has been cryptographically-signed to guarantee
its origin and make it tamper resistant. Trusted applets can be granted
more system access privileges than untrusted applets.
You preconfigure your browser with a list of whose X.509 certificate
you trust, and then applets arrive with X.509's attesting to their keys.
It's easier than it sounds.
9.2 What is the story with Java
and viruses? What is the blackwidow virus?
A. Java was designed with security in mind.
The security features make it very difficult, probably impossible, to attach
a virus (self- copying code) to a Java applet. As far as is known, there
has never been a Java virus.
There has been mention of a "Java virus" called "BlackWidow" in the
media (it was mentioned in Unigram in late 1996, and obliquely on the RISKS
newsletter in February 1997). A request to the editor of Unigram for more
information brought the answer that there was no more information, it was
just a report of a rumor. As far as is known, this story exists *only*
as rumors reported on by the press. There is no actual Java virus or blackwidow
virus (there was a legitimate commercial product of that name, since renamed).
If anyone has more concrete information about a virus that can attack a
Java applet (again, this is thought to be impossible), please would they
contact the FAQ maintainer with details.
9.3 Why do I get the warning string at the
bottom of popup windows "Unsigned
Java Applet Window" in my applets?
A. This is a security feature, to make
certain that users can always tell that a window asking for their password
and credit card details (or whatever) is from an applet. There should be
no way for an untrusted applet to work around this message. See also the
answer to 4.1.3.
9.4 Where
can I find crypto libraries for Java?
A. Cryptographic libraries are not part
of the Java release because US Government policy classifies strong cryptography
under the same rules as munitions. Its export is regulated under the International
Traffic in Arms Regulations. Many people regard this as a Kafka-esque (and
futile) attempt to stem the use of cryptography inside the US.
A comprehensive and free crypto library (called Cryptix) is at
Another crypto library for Java is at
Blowfish, CRC16, CRC32, DES, DES3, IDEA, RC4, ROT13 (can they really call
that "crypto"?), and more.
One commercial Java encryption source (from Ireland) is
A complete crypto API for Java (with HTML documentation) at:
The library provides comprehensive and complete range of crypto library
and functions covering DES, 3DES, IDEA, Blowfish ...and RSA, DH, DSA and
PGP access to Java programmers. The crypto functions are based on the C
cryptlib, by Peter Gutmann. It would be illegal to export this under current
US government rules, but the author of the code is outside the US, and
not subject to US export regulations. Download it today before it becomes
illegal.
Also, early access to Sun's Java Cryptography Extension (JCE) is available
for JDK 1.1 at:
(This may not be exported outside the USA and Canada).
There's a Q&A archive at
An actual port of PGP v2.6.3i to Java is at
http://tassun.math.nsc.ru
9.5 How
do I find out what these terms mean?
A. Read Bruce Schneier's excellent book
"Applied Cryptography 2nd Ed." for more info on what these terms mean.
Read David Kahn's excellent (if exhaustive) book "The Codebreakers" for
more info on the history and background of encryption.
9.6 Where
is Javasofts Security FAQ?
10. Java IDIOMS
10.1
How do I convert a String to an int?
A. There are several ways. The most straightforward
is:
int i = Integer.parseInt(<String>);
or
i = Integer.parseInt(<String>,<int radix>);
Note: there are similar methods for Byte, Short, and Long.
int i = Integer.valueOf(my_str).intValue();
also works but involves the creation of an extra object.
Note: use this for floating point values, as there are no parseDouble or
parseFloat methods.
float f = Float.valueOf(my_str).floatValue();
10.2 How
do I convert an int to a string?
A. String s = String.valueOf(i);
String s = Integer.toString(i);
or
String s = Integer.toString(i, radix);
or
String s = "" + i; // briefer but may result in extra object allocation.
Note: there are similar classes for Double, Float, Long, etc.
10.3How do I print
the hex value of an int?
A.You can print the hex equivalent of an int with:
int i = 0xf1;
System.out.println("i is hex " + Integer.toHexString(i) );
10.4 How can you
send a function pointer as an argument?
A. Simple answer: use a "callback". Make
the parameter an interface and pass an argument instance that implements
that interface.
public interface CallShow { public void Show( ); }
public class ShowOff implements CallShow {
public void Show( ) { .... }
public class ShowMe implements CallShow {
public void Show( ) { .... }
public class UseShow { CallShow callthis;
UseShow( CallShow withthis ) { callthis = withthis; }
void ReadyToShow( ) { callthis.Show( ); }
// in some other class that uses all this stuff:
UseShow use_1 = new UseShow( new ShowOff() );
UseShow use_2 = new UseShow( new ShowMe() );
and then the ReadyToShow() method on use_1 or use_2 will call the appropriate
method, as if you had stored a pointer to the method.
10.5 How
do I execute a command from Java?
A. Use Runtime.getRuntime().exec( myCommandString
) Where myCommandString is something like "/full/pathname/command"
10.6 How
do I do I/O redirection in Java using exec() ?
A. This solution works on Unix platforms
using either JDK 1.0.2, or JDK 1.1. The trick is to use an array of Strings
for the command line:
String[] command = {"/bin/sh", "-c", "/bin/ls > out.dat"};
If you don't do this, and simply use a single string, the shell will see
the -c and /bin/ls and ignore everything else after that. It only expects
a single argument after the -c.
import java.io.*;
import java.util.*;
class IoRedirect {
public static void main(String Argv[]) {
try {
String[] command = {"/bin/sh", "-c", "/bin/ls > out.dat"};
Process p = Runtime.getRuntime().exec(command);
p.waitFor();
System.out.println("return code: "+ p.exitValue());
}
catch (IOException e) {
System.err.println("IO error: " + e);
}
catch (InterruptedException e1) {
System.err.println("Exception: " + e1.getMessage());
}
}
}
10.7 So why can't I exec common DOS commands
this way (as in 10.6)?
10.8 OK, how
do I read the input from a command?
A. As above (10.5, 10.6), adjusted like
this:
BufferedReader pOut=
new BufferedReader(
new InputStreamReader(p.getInputStream()));
try {
String s = pOut.readLine();
while (s != null) {
System.out.println(s);
s = pOut.readLine();
}
} catch (IOException e) { }
Another possibility is to read chunks of whatever length as they come in:
...
p = r.exec(cmd);
InputStream is = p.getInputStream();
int len;
byte buf[] = new byte[1000];
try {
while( (len = is.read(buf)) != -1 ) {
String str = new String(buf,0,0,len);
System.out.println( "Process out: " + str );
}
}
catch( java.io.EOFException eof ) {
...
}
catch( java.io.IOException ioe ) {
...
}
10.9 How do I compile
code which has a cyclic dependency, i.e class pkg1.X contains a reference
to class pkg2.Y ?
A. You throw both classes at the compiler
at the same time.
javac pkg1/X.java pkg2/Y.java
10.10 How can I store
the errors from the javac compiler in a DOS file? javac foo.java > errorfile
seems not to work
A. javac write errors to stderr, so on
NT use:
javac myfile.java 2> errors.dat
On Win95, this doesn't work (as command.com is very poor software), so
you have to use the javac error redirection mechanism:
javac -J-Djavac.pipe.output=true myfile.java > errors.txt
10.11 How
can I pretty-print Java source?
10.12 What
is the point of creating the temporary reference to this.layoutMgr? This
code is from the 1.0 AWT, and the programmer was probably pretty skilled.
public synchronized void layout() {
LayoutManager layoutMgr = this.layoutMgr;
if (layoutMgr != null) {
layoutMgr.layoutContainer(this);
}
}
A. The code makes a local copy of a global
variable for one or both of two reasons.
The first reason is that accessing local variables can be faster than
accessing (non final) member variables It's good for loops or where there
are many references in the source.
The second reason is so that even if other threads update the global,
this.layoutMgr = someOtherLayoutMgr;
this method will still have a pointer to the original layoutMgr.
If the local variable were omitted, and another thread used the setLayout()
method to change layoutMgr to null between when the layout method checked
for null and when it invoked layoutMgr's layoutContainer method, a NullPointerException
would result.
Note that the synchronized keyword on the layout method doesn't help
any, since setLayout (which could make such a dire change) isn't synchronized.
Synchronized methods only lock out other synchronized methods on this object.
(The unhelpful synchronized keyword on the layout method is gone in JDK
1.1.)
There are two alternative solutions. One would be to make setLayout
synchronized and make layoutMgr private, so that it can't be set other
ways. This provides a stronger form of thread serialization, in that you
would never be able to see an old layout manager being used after it had
been replaced. However, it is slower. Another option that provides no increase
in thread serialization over the original would be to catch the NullPointerException.
Threads programming is hard! This idiom was probably put in place by
someone who got really bitten by this in the past.
10.13 What
is the difference between "a & b" and "a && b" ?
A. "a & b" takes two boolean operands,
or two integer operands. It always evaluates both operands. For booleans,
it ANDs both operands together producing a boolean result. For integer
types, it bitwise ANDs both operands together, producing a result that
is the promoted type of the operands (i.e. long, or int). "|" is the corresponding
bitwise OR operation. "^" is the corresponding bitwise XOR operation.
"a && b" is a "conditional AND" which only takes boolean operands.
It always avoids evaluating its second operand if possible. If a is evaluated
to false, the AND result must be "false" and the b operand is not evaluated.
This is sometimes called "short-circuited" evaluation. "||" is the corresponding
short-circuited OR operation.
Possible mnemonic: The longer operators "&&" or "||" try to
shorten themselves by not evaluating the second operator if they can.
10.14 If
I create a thread, and then null out the reference to it, what happens
to the thread? Does it get interrupted or what?
A. The code looks like this:
Thread t = new Thread( my_runnable_obj );
t.start();
...
t = null; // what happens to the thread?
The answer is that *you* may no longer have a reference to the thread,
but the JVM still does. Once a thread is started, and as long as it keeps
running, it is a root object. Root objects are the starting points for
"things in use" that the garbage collector uses.
10.15 How do
I calculate the number of days between two dates?
A. There is no API for this (there should
be), but you can calculate it by hand, like this:
Calendar earlierDate = new GregorianCalendar();
Calendar laterDate = new GregorianCalendar();
earlierDate.set(1997, 1, 1, 0, 0, 0); // Jan 01, 1997
laterDate.set(1998, 1, 1, 0, 0, 0); // Jan 01, 1998
// the first getTime() returns a Date, the second takes
// that Date object and returns millesecs since 1/1/70.
// The API has misleading and horrible naming here, sorry.
long duration = laterDate.getTime().getTime()
- earlierDate.getTime().getTime();
long nDays = duration / (24 * 60 * 60 * 1000);
System.out.println("difference in days: " + nDays);
10.16 How can
a Java program determine the level of JDK support given by the underlying
VM? I.e. is it running in a JDK 1.0.2 or 1.1 VM?
A. Look at the java.version system property
with:
String ver = System.getProperty("java.version");
There isn't a lot of standardization on the string contents however.
Another possibility is to try { ... } a library method that didn't exist in
JDK 1.0.2, such as SomeComponent.setSize(), and catch the exception.
If you got an exception, it's a 1.0 implementation. In 1.0 setSize()
was known as resize().
11. FOR
C and C++ AFFICIONADOS
11.1 How
do I translate C/C++ into Java or vice-versa?
11.2 How
are finalizers different from C++ destructors?
A. Java objects are not explicitly deleted
and do not have destructors. Instead they are implicitly garbage collected
when the JVM realises your program can no longer access them. Typically
this technology is _not_ based on reference counting and _will_ cope with
circular references.
Every object has a routine called finalize() which will be called before
the object is collected. This is Java's nearest equivalent to C++'s destructor.
However, it is not a good idea to rely on finalization for the timely freeing
of resources.
This is because garbage collection and hence finalization may be arbitrarily
delayed, and may never happen at all if the program terminates before it
runs out of memory. You should instead provide your objects with methods
similar to Graphics.dispose() to free resources, and call the
dispose() method explicitly when you have finished using them - typically
within the "finally" clause of a "try/catch" block. You may then call your
dispose() method from within your finalize() method as a last-ditch attempt
to free the resource if someone forgets.
Alas, all this means the C++ idiom of "object construction is resource
aquisition" does not translate well to Java. However, note that 90% of
destructors in C++ are there to free memory, and the GC means you don't
need to do that in Java. As well as fixing an important source of bugs,
the GC is essential to Java's security model; without it you could forge
object references by preserving the reference after the object has been
deleted.
If your program appears to be crashing due to running out of some system
resource (like File, Window or Graphics handles), it probably because the
system is running out of handles before it has run out of memory. Check
that you have called the dispose() method (or equivalent) on every object
that uses system resources. You can help the GC a little bit more by explicitly
NULLing out references that you've finished with.
11.3 What's
the Java equivalent of sizeof()?
A: There isn't one. sizeof() in C and C++
is used in three main places:
1) To check on the size of a primitive type. In Java, the sizes of
primitive types are fixed in the language specification (a short is _always_
16 bits; an int is _always_ 32 bits, etc), so this is no longer necessary.
2) In memory allocation (i.e. malloc (32 * (sizeof(int));) In Java you
always allocate a specific type of object, rather than a block of raw memory
that you will fill as you like. The system always knows the size of the
kind of objects you are allocating. So sizeof is not needed.
3) in pointer arithmetic (i.e. p += sizeof (int)) Pointer arithmetic
of this type is not allowed in Java, so this isn't necessary, either.
For all these reasons, there is no need for a Java sizeof() operator.
If you absolutely must know the size of an object,
you can create a ByteArrayOutputStream, and have the object serialize itself
to the stream using the object serialization that is available with JDK1.1.
After it is serialized, get the byte array that the ByteArrayOutputStream
built and look at its .length property (the bytearray.length).
Now you have the object in a form you can read/write to any I/O stream, and
you also know its size. Only use this knowledge for good, never for evil
purposes.
11.4 Does
Java have the equivalent of "const" arguments in C and C++?
A. Java 1.1 adds the ability to use the
"final" keyword to make arguments constant. When used to qualify a reference
type, however, this keyword indicates that the reference is constant, not
that the object or array referred to is constant. For example, the following
Java code:
void foo(final MyClass c, final int a[]) {
c.field = 7; // allowed
a[0] = 7; // allowed
c = new MyClass(); // final means this is NOT allowed
a = new int[13]; // final means this is NOT allowed
}
is roughly equivalent to the following C/C++ code:
void foo(MyClass * const c, int * const a) {
c->field = 7; // allowed
a[0] = 7; // allowed
c = new MyClass(); // const means this is NOT allowed
a = new int[13]; // const means this is NOT allowed
}
Java does not have any equivalent to the following C/C++ function declarations:
void foo(const MyClass *c); // a pointer to a const class
void foo(const int *a); // a pointer to a const int
void foo(const int a[]); // a pointer to an array of const ints
11.5 Are
there any hacks around this?
A. Certainly! There are always hacks around
stuff. One way of enforcing constant values is to have two interfaces,
a constant one and a non-constant one, e.g.
public interface ConstFoo {
int getValue();
}
public interface Foo extends ConstFoo {
int getValue();
void setValue(int i);
}
Then when you want to receive a parameter that cannot be modified you have:
void noChange(ConstFoo foo);
For a parameter that can be modified
void change(Foo foo);
11.6 How
can I write C/C++ style assertions in Java?
A. The two classes shown below provide
an assertion facility in Java. Set Assert.enabled to true to enable the
assertions, and to false to disable assertions in production code. The
AssertionException is not meant to be caught--instead, let it print a trace.
Since the exception is not meant to be caught, we just extend Error instead
of RuntimeException. As with RuntimeException, a method does not need to
declare that it throws Error. In addition programmers are less likely to
write "catch(Error) ..." than "catch(RuntimeException)".
With a good optimizing compiler there will be no run time overhead for
many uses of these assertions when Assert.enabled is set to false. However,
if the condition in the assertion may have side effects, the condition
code cannot be optimized away. For example, in the assertion
Assert.assert(size() <= maxSize, "Maximum size exceeded");
the call to size() cannot be optimized away unless the compiler can see
that the call has no side effects. C and C++ use the preprocessor to guarantee
that assertions will never cause overhead in production code. Without a
preprocessor, it seems the best we can do in Java is to write
Assert.assert(Assert.enabled && size() <= maxSize, "Too big");
In this case, when Assert.enabled is false, the method call can always
be optimized away, even if it has side effects. However, an opposing view holds
that even this might not work in the face of java's late binding.
If the Assert class is in a package, and if the computer
the applet/application is running on has a different version
with Assert.enabled being true, then it's possible that
assertions *should* be enabled in that case. Therefore, the
compiler can't assume that enabled is *false* at runtime just because
it *is* false in the compilation environment.
This may be thought perverse, but it could happen.
If a language
lawyer wants to weigh in on this theory, please go ahead.
public class AssertionException extends Error {
public AssertionException(String s) {
super(s);
}
}
public final class Assert {
public static final boolean enabled = true;
public static final void assert(boolean b, String s) {
if (enabled && !b)
throw new AssertionException(s);
}
}
11.7 How
do I do stuff like scanf and sscanf in C/C++? And how do I do stuff like
sprintf, e.g.
float x = 12345.6789;
printf("%6.3f/n", x);
A. You can break a string like "5 loaves 2
fishes" into its parts by using java.util.StringTokenizer. This is the
Java equivalent of sscanf().
StreamTokenizer does a similar thing on a file or any stream (i.e, what
scanf() and fscanf() do in C).
To do formatted character output, create a format string, and then use
that to format your binary value, e.g.
import java.text.*;
float fi = 1234.56789F;
DecimalFormat df2 = new DecimalFormat( "0.000" );
System.out.println( df2.format(fi) );
gives:
There are lots of different characters you can feed to the DecimalFormat
constructor, not just "0". See $JAVAHOME/src/java/DecimalFormat.java
source for details.
11.8
What is the Java equivalent of C++ friend"?
A: The keyword "friend" in C++ is a hack to
allow a piece of code to access the private member declarations of another class.
In Java, you would do this by labelling, not the friend, but the private members.
Instead of making them private, make them protected.
Classes in the same package have automatic "friendship" in that they
can access each other's protected fields.
11.9 Does
anything like the C++ Standard Template Library exist for Java ?
A: Yes, only it's better and simpler to
use in Java. It's called the Java Generic Library. This library (JGL) is
freely downloadable from http://www.objectspace.com/
It includes about a dozen nice data structures (including sets and bags)
and algorithms like unions, searching, and sorting.
[Some Java vendors are bundling it with their next release]
12. FURTHER
RESOURCES
12.1 Useful URLS
12.2 Newsgroups
These are the Java newsgroups since the reorganization that I
arranged in April 1997
comp.lang.java.help
simple programming and setup questions
comp.lang.java.announce
(moderated) announcements
comp.lang.java.advocacy
for arguments: no it isn't, yes it is
comp.lang.java.programmer
programming in Java
comp.lang.java.security
security issues
comp.lang.java.machine
JVM and native interfaces
comp.lang.java.databases
JDBC,ODBC, java access to DBs.
comp.lang.java.softwaretools
IDES, editors, compilers, tools, etc
comp.lang.java.gui
AWT, IFC, JFC, AFC, Vibe, etc etc
comp.lang.java.beans
Software components in Java
comp.lang.java.corba
(newsgroup added Dec 3 1997) interaction between Java and CORBA.
Please make an effort to post only to the single most appropriate group.
As with the other language groups on Usenet (comp.lang.c, comp.lang.c++,
etc) questions about products from specific vendors that only work on one
specific platform are best posted to other newsgroups. For example, questions
about ActiveX belong in comp.os.ms-windows.programmer.ole, not the Java
groups. For questions about J/Direct try the group microsoft.public.java.visualj++
or microsoft.public.java.sdk. These are available from Microsoft's news
server msnews.microsoft.com .
12.3 Are
there any Java libraries for sorting, etc?
A. A sort class has not yet been made part
of the standard library. It should be. If you have the demos that come
with the JDK, take a look at the class SortDemo. Alternatively, use one
of the several classic sorts available from Roedy Green. They are supplied
free with heavily commented Java source code.
See "QuickSort",
"HeapSort" and
"RadixSort" in
the Java glossary at
http://oberon.ark.com/~roedy/index.html
.
Also, try the Java Generic Library. This library (JGL) is freely downloadable
from http://www.objectspace.com/
Also Visual Engineering has JChart at: http://www.ve.com.
No licensing fees.
12.4 Why doesn't
somebody write a shell in Java? Then they could use it on all platforms!
12.5
Are there any URLs for other libraries?
12.6 Are
there any URLs for regular expression handlers in Java?
13.
MOST COMMON JAVA NOVICE PROBLEMS
13.1
I can't seem to get my classpath environment variable set correctly.
A. Classpath was a hack from the early releases of Java.
It is not needed in JDK 1.1 until you start creating your own packages.
Don't set your CLASSPATH at all when starting out.
Once you set it, be sure to include "." (to represent the current directory)
In either case, you don't have to include the location
of the Java runtime .zip or .jar file; Java
knows about that automatically in a correct installation.
13.2
How do I do keyboard (interactive) I/O in Java?
A. Interactive I/O in Java is very poorly supported.
Programmers must piece together several library classes in non-obvious
ways to get the required functionality. See the answer to question 6.1.
13.3
How do I do file I/O in an applet?
A. In general, an applet can read files on the server, but
not write them, and has no access to the client. This is for reasons
of security. It would be very unsafe to let any old applet
that you downloaded from an unknown origin on the Internet read/write
your files.
It would be as unwise as allowing this kind of access to an ActiveX
control (which is one reason ActiveX is dead on the Internet).
There are several different ways to relax the rules.
See the answer to question 6.8.
13.4
How do I do I/O to the serial port on my computer?
A. Use JDK 1.2. Java 1.0 and 1.1 do not have a
serial port API. There
are various hacks around this. JDK 1.2 introduces access to the
serial and parallel ports as an extension (optional extra)
library. See also the answer to question 6.3.
13.5
How do I do formatted I/O like printf and scanf in C/C++?
A. The java.text package introduced with Java 1.1 supports
formatted I/O. See also
the answer to questions 6.11, 6.12.
13.6
I have spent more debugging time finding case (upper vs lower)
typos than everything else put together and squared!
A. Do not forget that your remark must be phrased in
the form of a question to win on FAQ Jeopardy. In any event, it is
worth reminding those new to Java that letter case really matters
in Java, and that the names of public classes should exactly match
(including case) the names of the files they live in.
See also the answer to question 4.1.2
13.7
Why do I get a compiler error with this statement: "double y = sin(90);"?
A.
You need to write it this way:
double cvtDegToRad = Math.PI/180;
double x = 90*cvtDegToRad;
double y = Math.sin(x);
This is because the Math package works in radians, not degrees.
360 degrees = 2 pi radians. You need to use the "Math" classname, e.g. Math.sin
instead of plain sin, because you have to say what class or object these
methods belong to. These are static methods of the Math class, so you
give the name of the class in invoking them.
13.8
Why do I get this compiler error: "Can't make static reference to method..."?
A. Your code probably looks something like this:
class myclass {
public static void main(String args[]) {
myMethod();
}
public void myMethod() { //some code
}
}
The issue is this: a static method means it belongs to the class,
not each individual object. If you leave the static keyword off
(the usual case) as is done here with the method "myMethod()", it means
that method can only be invoked on an object. But your call from
main() has not told myMethod() which object it is to be invoked on.
Inside a non-static method, you don't have to provide this information,
as it assumes you mean the same object on which it was invoked.
When calling from a static method, you must provide the information.
A common fix is to instantiate a member of the class, on which to
invoke myMethod(), like this:
public static void main(String args[]) {
myclass m = new myclass();
m.myMethod();
}
This problem is especially common when you are writing code that
you want to run as an applet and as an application. Naturally, you
call init() and start() from main. What you really need to do is:
public static void main(String[] args) {
Applet ma = new myApplet();
ma.init();
ma.start();
}
13.9
How do I close a Java window by using the icon in the
upper right hand corner of a window?
A.
If you're extending Frame, have your subclass of Frame extend
WindowAdapter. Then override WindowAdapter's windowClosing():
public class MyFrame extends Frame extends WindowAdapter {
public void windowClosing(WindowEvent e)
{
// Do any other cleanup here
hide();
dispose();
// possibly also System.exit(0);
}
// do your other Frame stuff
}
Somewhere in your initialization code, put:
addWindowListener(this);
See also the answer to Q.4.0.30, and 4.0.19.
13.10
To be completed.
See also "The 10 Most Common Java Programming Mistakes":
http://webreview.com/wr/pub/97/12/19/feature/sidebar.html
Acknowledgements
FAQ copyright 1997, 1998 by Peter van der Linden.
Contributions and help from:
Matt Kennel, Patric Jonsson, Brad Van Tighem, Tony Hursh , Glenn L Vanderburg,
Peter Jones, John McDowall, Jim Driscoll, Uday, Dave Harris, Bill Wilkinson,
Tom Valesky, Dan Drake, Giles Thomas, Mitch Baltuch, Guy Ruth Hammond,
Gordon Keith, Jason Brome, Shani Kerr, Steve Chapel, Timothy Wolters, Robert
Lynch, Jake Cormier, Sean C Sullivan, Joseph A. Millar, Jim Frost, Jim
Balter, Jeff Bauer, John Kochmar, Carl Burke, William Stubbs, Mark Smith,
Volker Turau, Real Gagnon, Russell Gold, Max Hailperin, Bill Tschumy, Marco
Nijdam, Marc Pawlowsky, Laurence Vanhelsuwe,Ian Macgregor, Mike Faulkner,
Rich Koch, Will Clark, Govind Seshadri, Rich Simkin, Ian Stiles, Kieren,
Darren Christie, Tom Lane, Michael Jungmann, Rob Mayoff, George Ruban,
Tom McCann, David Hopwood, Thomas Phan, Kai Stuke, Rolf Howarth,
Derek Snider, David Boydston, Andy Godwin, John F. Dumas, Doug Bell, David
J. Biesack, Tiger Quimpo, Martin Hugh Rogers, Brian Krahmer, Ian Burrell,
Nikki Locke, Bin Li, Jackson Thompson, Steve Odendahl, Greg Smith,
Jeffrey C. Ollie, Mark Halvin, Jeremy Cook, Lak Ming Lam, Peter S. Morris,
Mark Halvin, Juergen Keil, Alex Stewart, Mike Abney, Rodney Stephenson
Mark Gritter, Satish Talim, Tamminen Eero, Alexander Gridnev, Eric Hodges,
Jamey Graham, Will Lockhart, Scott Plante, Tom Sanfilippo, Jan Newmarch,
Sean Breslin, Stuart D. Gathman, rhino@wwdc.com, C Matthew Curtin,
Tor Iver Wilhelmsen, A.N.Pryke, Phil Race, David Holmes, David Rodal,
Dominique Plante, Trent Jarvi, Ingrid Biery, Gopal Unni Krishnan, Grant Lewis,
Tov Are Jacobsen, Gary McGath, Marty Hall, Will Forster, Colin Mummery,
Darin McBride, Mayank Shah, Jens Alfke, Glen Stampoultzis, Philip Brown,
Peter Steiner, Kurt Spaugh, Rasmus Ekman
I am maintaining a FAQ list to address specifically programming issues
(not a general tutorial on Java). Please feel free to mail
suggestions to me at pvdl@best.com.
Question with answer gets you a credit in the FAQ.
Peter van der Linden, Sun Certified Java Programmer.
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