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Java Programmers FAQ

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Peter van der Linden

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Aug 14, 1997, 3:00:00 AM8/14/97
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Archive-name: java/programmer
Posting-Frequency: twice-weekly
Last-modified: 1997/08/07
URL: http://www.best.co14
Copyright: (c) 1997 Peter van der Linden
Maintainer: Peter van der Linden (pv...@best.com)


A list of Frequently Asked Questions (with answers) for Java programmers.
Most of the Java FAQ's here are intended for experienced programmers.
To distinguish this from other Java FAQs, this is the "Programmer's FAQ"
and will be posted mostly in comp.lang.java.programmer, comp.lang.java.help,
comp.lang.java.gui, and comp.answers.

Latest copy of this FAQ is available at: http://www.best.com/~pvdl

This FAQ reflects the latest and most current version of Java: JDK 1.1.3.

0. A WORD FROM OUR SPONSOR
1. LOOKING FOR INFORMATION
2. COMPILERS, ETC.
3. LANGUAGE ISSUES
4. AWT
4.1 APPLETS
4.2 BROWSERS
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

-------------------------------------------------------------------------
0. A WORD FROM OUR SPONSOR

This Java FAQ is compiled and maintained as a service to the Java community.
Please consider these books when you are looking for a programming text:

0.1 "Not Just Java" by Peter van der Linden, $35
publ 1997 Sun Press/Prentice-Hall, ISBN 0-13-864638-4

This should be your first book on Java -- and it doesn't teach you Java
programming. It explains why Java technology is different and important,
and why Java is more than a language.

There are hundreds of books that teach you Java programming. This is
the *only* book that instead describes in plain English the industry trends
and implications of Java and related technologies. Clear explanations with
diagrams covering Java, CORBA, thin clients, network computers, RMI,
multi-tier systems, Java threads, ActiveX, OLE, JavaBeans, JDBC, etc.
A book for engineers, executives, and all the job titles in between.

You can see more info on this book at http://www.best.com/~pvdl

0.2 "Just Java Second Ed." by Peter van der Linden, $35
publ 1996 Sun Press/Prentice-Hall, ISBN 0-13-272303-4

An introduction to Java Programming, for people who can already program
in another language. Filled with common sense examples, and anecdotes
from the computer industry. Learning Java is enjoyable, stimulating fun
with this book. Comes with a CD including the compiler and sample code.

The only book on the market that teaches you Java, has a recording of the
anti-Barney, and shows you how to fold a water-carrying paper plane.
[see http://www.best.com/~pvdl/fold.html for the Java origami applet]

0.3 "Expert C Programming" by Peter van der Linden, $35,
publ 1994 Sun Press/Prentice-Hall, ISBN 0-13-177429-8.

This book should be everyone's *second* book on C. Unix Review gave it
"5 stars". "Expert C Programming" teaches you the tips and techniques
used by the very best C programmers in the world.

These books can be ordered online from http://www.cbooks.com
Or by phone from Quantum Books 617 494-5042 (in USA) during business hours.

-------------------------------------------------------------------------
1. LOOKING FOR INFORMATION

1.1 What is the best way to refer someone to the FAQ when they ask a question
I know is answered there?

A. The Java Programmers FAQ <http://www.best.com/~pvdl> gives the answer to
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, providing an incentive to go there next time.)

1.2 I'm looking for a Java style guide on naming conventions.

A. Check out the section "Naming Conventions" in the language specification
http://java.sun.com/docs/books/jls/html/6.doc.html#11186


1.3 Where can I find a full list of Java books and book reviews?

A. http://lightyear.ncsa.uiuc.edu/~srp/java/javabooks.html

1.4 How do I search Deja News for past postings on Java topics, e.g. the FAQ?

A. Go to http://www.dejanews.com/home_sf.shtml
Under "Newsgroups" enter "comp.lang.java.programmer" (or whatever)
Under "Subject" enter "Frotteur" (or other topic you find pressing)
Click "Create Filter"
It will go to a new document, and you should click the link labeled
"nnn Documents" ("nnn" is some number).

This makes finding information very easy.

Also look at http://sunsite.unc.edu/java/cgi-bin/query
and look at http://asknpac.npac.syr.edu/ for Java newsgroup search

http://www.javaworld.com/search.html can search the Javaworld newspaper

1.5 How do I check on known bugs in JDK 1.1?

A. Look at http://java.sun.com/products/jdk/1.1/knownbugs/index.html

Note that only bugs that Sun deems "important" are listed there.
The criteria for "important" are not published.

After you have checked whether the bug is already listed, you can
submit a bug report through:
http://www.javasoft.com:80/cgi-bin/bugreport.cgi

You can even send in an RFE or ease-of-use issue there!

If you go to the JDC (that's the Java Developer's Connection) you can
do a keyword search against the internal Javasoft bugtraq database,
looking by keyword or bug number. JDC has free registration.
http://developer.javasoft.com should do it.

Also, there's longer list of known bugs at:
http://www2.vo.lu/homepages/gmid/java.htm

1.6 Is there a Java port to Windows 3.1?

A. IBM's ADK1.02 beta 3 is available at the following locations:
http://ncc.hursley.ibm.com/javainfo/latest/answers/faq0.html
http://www.alphaworks.ibm.com/formula

Netscape Navigator for Win3.1 has Java support [anyone know the version?]
Microsoft is preparing a Win3.1 port of the JDK.

You should 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.

A list of JDK ports is available from
http://java.sun.com/products/jdk/jdk-ports.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.
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
http://http.cs.berkeley.edu/~engberg/guavac/
Kaffe (a JVM) can be found at
http://www.kaffe.org

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:
http://www.sun.com/sunservice/suned/java_certification.html

It costs $150 to sit the Java Programmer exam.
Note that passing Java certification is not trivial, and does
require 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.

-------------------------------------------------------------------------
2. COMPILERS, ETC.

2.1 Why did I get an OutOfMemory error when porting working code
from jdk 1.02 -> 1.1

A. The preset memory limit has changed. It went down to 16Mb so as
not to penalize low memory machines.
You can adjust it with
java -mx32m Frotz
to get a 32Mb extent.

Also see the Runtime methods freeMemory() and totalMemory().

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
JavaLex and JavaCup: http://www.cs.princeton.edu/~appel/modern/java/

LL(k) parser
JavaCC: http://www.suntest.com/Jack/

2.4 Where can I find a byte code obfuscator?

A. HashJava: http://www.blackdown.org/~kbs/hashjava.html
http://www.primenet.com/~ej/
http://www.math.gatech.edu/~mladue/HoseMocha.java
Some people have reported problems using these with JDK 1.1.

This obfuscator has been updated to be fully compatible with
JDK 1.0 and 1.1:

http://www.monmouth.com/~neil/Obfuscate.html

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

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);
Unix command: zip -r0 classes.zip <directories>
2. Make sure the main class has no directory beside it inside the
archive, but your packages do;
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>

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, but it takes away one of the
most significant benefits of Java: portability of executables.
Consider carefully why you want to do this, 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. 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.

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 a real pain. 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. One reliable way is javac *.java
Another way is javac -depend tip.java
where "tip.java" is a class that everyone else depends on.
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 the
compiler 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.

Without the "-depend" option, javac doesn't
look beyond the immediately adjacent dependencies to find
classes lower down the hierarchy where the source has changed.
However, there have been reports that "-depend" doesn't work
on very complicated source bases. It also doesn't issue compile
commands in parallel to make use of multi-processor systems.

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.

-------------------------------------------------------------------------
3. LANGUAGE ISSUES
3.1 Why doesn't my "hello world" program work?

A. Two very common causes of failure are:
* the 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.
In JDK 1.1.2, it is generally an ERROR if the user sets the CLASSPATH!
(except that use of RMI apparently still requires it).

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 memory management 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 Beta's). Then it
was removed in JDK 1.0.1. It was complicated to explain, it was an
ugly hack syntax-wise, and it didn't fit consistently with the other
access modifiers. More people disliked it than liked it, and 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's constructor at the start, a call to the
superclass's 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's 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://www.javasoft.com/products/jdk/1.1/compatibility.html#incompatibilities
http://www.javasoft.com/doc/language_specification/javalang.doc.html#1466
http://www.javasoft.com/doc/language_specification/javalang.doc.html#5899

If you want the rounded floating point output that most languages have,
use the new java.text package of Java 1.1 to limit the number of digits
that are output. If you need more precision than about 16 digits, use
the BigInteger and BigDecimal classes of Java 1.1.


3.9 Why do I get the compiler error message
urltest.java:8: Variable test may not have been initialized.

URL test;
try { test = new URL("http://osprey.avs.dec.com");
} catch (MalformedURLException e) {}
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 either to declare test with an explicit initial value of null, or
(better) to print out the e.getMessage() string of the exception.

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. Variable declarations inside are never given the
"public", "static", or "final" keywords. They are not
public or static because they are local to a method.
They are not final by convention.
Move your constant 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
Thing.java:96: Can't access protected method clone in class
java.lang.Object. OtherThing is not a subclass of the current class.

A. The method clone() is protected. It is protected because cloning is an
operation which the object might want to restrict access to.
If Object.clone() was declared as public, the object could never decide
to make it more restrictive later. By declaring it protected, the object
has the choice of making it *less* restrictive.

This means that a method can clone its own objects, but a method cannot
clone objects of another class, unless you do something like:
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 I transform a char into the corresponding int value,
that represents the code value of the char?

A. char c = 'A';
int i = c;

Going the other way is just 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's constructor is provided
for you. If the superclass doesn't have a no-arg constructor, see 3.9

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
Int Iarr[] = new Int[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 is even a value to represent "void", like this:

Class myCl1 = Character.class;
Class myCl2 = Void.class;
Class myCl3 = Object.class;

You might use it like this:
Class cl = thing.getClass();
if (cl.equals(myCl1))
System.out.println("It's a Character class");


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. 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 JITC's 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 java.lang.Math were not final, you could instantiate a local copy
with a shorter name, but the final prevents it.

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) );

-------------------------------------------------------------------------
4. AWT

4.1 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
xlsfonts > ~/fonts.txt

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" &


4.2 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 to be introduced
as part of the Java Foundation Classes with JDK 1.2.

4.3 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);


4.4 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
"Ariel" as a font and expecting it to work on something other than Windows.

On 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()


4.5 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.

4.6 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 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.

4.7 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.

4.8 What is the difference between
Component's setForeground(Color c)
and
Graphics's setColor(Color c) ?

A. First of all, these methods both do the same thing: set the foreground
color to the value of the parameter. The difference lies in where you
use them.

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.

4.9 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.

4.10 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.

4.11 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( ...

4.12 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:
1L << Toolkit.getDefaultToolkit().getColorModel().getPixelSize()

That does a shift left to get the power of two as a long.

4.13 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().

4.14. 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.

4.15 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);

4.16 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 around.
For example, Component.setBounds calls Component.reshape, instead of
reshape calling setBounds.


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.
A Servlet is an application, but (like an applet) requires a context
in which to run, namely web-server software. Servlets are used like
CGI, but allows 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.

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:
http://www.acme.com/java/software/Acme.Serve.Serve.html


4.1.2 My applet works on my machine, but fails when I put it on our web server.
Why?

A. It could be one of several reasons, and unfortunately the messages
that you get in this situation aren't much help. In general, you can
assume that either your applet's class files are corrupted somehow, or
the web server can't find one or more of them when the browser needs
them.

Be careful of the following things:
- Make sure you transfer the class files in binary mode, rather than
text or ASCII mode.
- Make sure you transfer *all* of the class files which are a part
of your applet. Sometimes people are surprised by how many there
are. There will be a class file for every class and interface you
define, even if you define more than one in a single source file.
If you use the Java 1.1 "inner classes" feature, there will be class
files for each inner class as well.
- Make sure you maintain the appropriate case distinctions in your
filenames. If a class is called StUdLy, it must be found in a
file called StUdLy.class.
- Make sure you maintain the directory structure that matches your
package structure. If you declare a class in package
COM.foo.util, the class file needs to be in directory COM/foo/util
under the applet's codebase directory. Again, case distinctions
are important for package/directory names, just as they are for
class/file names.
- Make sure that the web server process will have read access to the
class files, and search access to the directories that the files
are in. For example, if the web server runs on a Unix machine, use
the command "chmod o+r filename" for the files, and
"chmod o+x dirname" for the directories.

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:
http://www.geocities.com/SiliconValley/Vista/1337/ (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?

A. there is no way to give an applet a transparent background that
lets the web browser background show through. You can simulate it
by giving the applet a background that matches the underlying browser
background. It doesn't produce satisfactory results with a patterned
background because of problems aligning the edges.

Lightweight components (new in JDK 1.1) have a transparent background.

4.1.5 How do you do file I/O from an applet?

A. See answer to question is 5.2 and 6.8

4.1.6 How do you get a MenuBar/Menu in an applet?

A. In your applet's init() method, create a Frame instance
and then attach the Menus, Menubar etc to that frame. You cannot
attach the Menu or a Menubar to an applet directly.

Or get the parent Frame like this:
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.

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).

4.1.10 I want to know about {applets,applications} but the lousy book I bought
just talks about {applications,applets}. What can I do?

A. The truth is that 95% of the material is the same, whichever your book
chooses to focus on. Some people write their apps to work completely in
a Panel, then depending on whether they're running stand-alone or
in a browser the Panel is either added to a Frame or an Applet.

In this scenario the following code will tell you which environment you're
running in:

public boolean isRunningInBrowser() {
Component p = getParent();

while(p != null && !(p instanceof Frame)) {
p = p.getParent();
}
return (p == null);
}


4.2 BROWSERS

4.2.1 When will my favorite browser support Java 1.1?

A: Netscape Communicator 4.0 supports most new features of Java 1.1,
but not the two most important ones: the new Event model, and Beans.
Communicator is scheduled to fully support Java 1.1 in Fall 1997, and
it will use "smart update" to change your copy of the browser (with
your "ok") without requiring a full new download.
URL: http://developer.netscape.com/support/faqs/champions/java.html#21

Sun's HotJava browser fully supports the JDK 1.1 features.

Internet Explorer 4.0, anticipated to be released in Fall 1997,
will fully support Java 1.1, ... apart from the pieces of the language
that Microsoft is saying they will not support: RMI, JFC, JNI, etc.

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: The DevEdge site on Netscape's home page has a
javascript-java example on getting cookies. 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.


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.


-------------------------------------------------------------------------
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 - print command
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. No, Java 1.1 does not have a core library widget that automatically
formats HTML. At least one person has written one though.
Search at http://www.gamelan.com or http://www.yahoo.com for details.

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.

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.
Use properties instead. 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 (MS-DOS)
This sets the "foo" property to the value of the environment
variable foo. 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 where JavaSoft is).

A. To make things easier for debugging Sun has decided that
the toString() method should always use one format,
if you want a different format you should use the new
internationalization routines.

As a Date is stored internally in GMT the obvious choice
for a standard format is... PST time? JavaSoft is in the PST zone
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.

Don't forget this code too, to prevent GMT being interpreted as
GMT+1:00 hour.
// + 1 is to work around bug in GregorianCalendar
// XXX - need FIX
// should probably be formatter.format( this );
// or formatter.format(new Date());

return formatter.format( new Date(getTime() + 1) );

5.9 How are dates represented in Java?

A. java.util.Date stores dates as integers representing the number of
seconds since 00:00:00 UTC Jan 1, 1970 (the birth of Unix, a date
known as the "Epoch").

Dates earlier than the Epoch are represented as negative numbers,
counting away from 1/1/1970. It is a poorly-conceived representation,
spanning just the years 1901-2038, and more classes were added
in JDK 1.1 to try to fix it up. They didn't really improve matters.
Note that this differs from the Java specification which says that
Dates should be stored as a long, and contain milliseconds since the
Epoch. That actually gives a reasonable range (although making dates
before the Epoch be negative is still suspect, as it doesn't correspond
to the real world representation).
Dates are the lemon of Java, as Roedy Green truly notes.

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.

5.10 That may be, but how do I extract day, month, year from a Date?

A. In JDK 1.1, Date represents an instant in time.
Calendar translates between an instant in time, and individual fields
like year, month, day, etc
So get your Date -- the current Date can be had by
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).
The default time zone for SimpleDateFormats is PST (i.e. California).


5.11 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.
Also check the JDBC FAQ listed at the end of this document.

5.12 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.

5.13 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();

5.14 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.


5.15 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.

Where things start to get tricky 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. You just take the abs() value and
then mod it into the right range:

int dice_throw = 1 + Math.abs(i) % 6;

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.
[... to be continued... ]

-------------------------------------------------------------------------
6. I/O

6.1 How do I read a String/int/boolean/etc from the keyboard?

The easiest way is to pick up the source for the 100% pure Java
class EasyIn from http://www.best.com/~pvdl (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.

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?

NOTE: This answer needs to be proved by a practical example.
We don't have a satisfactory example of this so far. Provide one, and
get your name in the FAQ for all time!
A. 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.
Some people say that writing a formfeed at the end of the file is needed.

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 on
Windows 95, NT, OS/2. See
http://www.sc-systems.com
http://www.cd.com/portio

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.

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 three 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 delete" and have all processes (Java
and non-Java) follow the same protocol: if the file was deleted by you,
you have the lock, and you create the file again to give up the lock.
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.

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).

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 10.4


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 suggestions are for server-side I/O.
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. Read a file by opening a connection using the URL
class and then using a DataInputStream to read the data.
This allows reading but not writing. It requires an http demon
running on the server, which will usually be the case.
2. Or 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.
3. 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.
You can read a file from the server by referencing it via URL, like this:
try
{
URL url = new URL("http://somewhere.com/test.txt");
URLConnection uc = url.openConnection();
DataInputStream dis = new DataInputStream(uc.getInputStream());
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.

4. 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.

5. 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?

A. Java does everything in network byte order (big-endian order), as do
many computers including Motorola, and SPARC. The Intel x86 uses
little endian order in which the 4 bytes of an int are stored least
significant first. Rearranging the bytes on the way in will get you
the results you need. This is only necessary when the file was written
by a non-Java program on a little endian machine such as a PC.

The following code will byte-swap little-endian integers into network
standard order

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;
}

6.10 How do I make I/O faster? My file copy program is slow.

A. This is the purpose of BufferedInputStream.

6.11 How do I do formatted I/O of floating point numbers?

A. Look at http://www.apl.jhu.edu/~hall/java/CoreJava-Format.html
for the html with the javadoc info. The actual file is called
http://www.apl.jhu.edu/~hall/java/CoreJava-Format.java
However, you must rename the file to Format.java for it to compile.

Although many utilities claim to handle all varieties of C's printf,
as far as has been found, this is the only one to correctly handle
the equivalent of %e in printf.

6.12 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();

6.13 How do I print the hex value of an int?

A. int i = 0xf1;
System.out.println("i is hex " + Integer.toHexString(i) );
-------------------------------------------------------------------------
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.

In July 1997, Sun announced that it was aligning RMI to work more
closely with CORBA, and perhaps even use IIOP as its on the wire
protocol.

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 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 avoid the problem by giving your system another way to
resolve DNS names. Edit the hosts file for your system (found in
%windir%\hosts on Win95 and %windir%\system32\drivers\etc\hosts on NT)
so that localhost and the full domain name are both mentioned. So
if my system is called goober.best.com change the hosts file from
127.0.0.1 localhost
to
127.0.0.1 goober.best.com localhost

[If any networking guru's have improvements to this process, send'em in!]

There is a longer, different procedure at http://www.best.com/~pvdl
Please send me mail (pv...@best.com) if either or both of these work
or don't work for you, so I can update the FAQ.

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");
should always work.

[Note: this info is still to be confirmed. Net guru's?]


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:
http://www.best.com/~rmlynch/saveit.html

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.

7.8 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.

7.8 My socket code looks good, but is broken!

A. When using sockets you typically open both inward and outward
streams. If you close one of them, the other seems to 'break' instantly.
Check whether this is happening for you, by adding the matched pair.

[comments from net gurus welcome]

7.9 How do I map between IP address and hostname?
In Java 1.1 (earlier releases were buggy) use:

String host = InetAddress.getByName("211.10.2.119").getHostName();

7.10 How do I embed an anchor in a URL? Just putting it as part of the
string in the constructor doesn't work.

A. Like this:
URL url = new URL("http://www.my_domain.com/my_page.html");
URL anchor = new URL(url, "#section2");
this.getAppletContext().showDocument(anchor);


7.11 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.

There are several very good sources available from SUN which
cover many simple and advanced RMI problems. They are:
* The documentation, of course:
http://www.javasoft.com/products/jdk/1.1/docs/guide/rmi/index.html
* Dedicated FAQs on RMI and Object Serialization
http://chatsubo.javasoft.com/current/faq.html
* Mailing list RMI-...@JAVASOFT.COM with archive at
http://chatsubo.javasoft.com/email/rmi-users/
Visit the archive!


-------------------------------------------------------------------------
8. MULTI-MEDIA
8.1 Why won't my audio file play?

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.

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

8.2 Does Java support Animated GIFs?

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. You can use cliprect() with a negative x coordinate 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.

8.2.1 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
1. ImageUpdate is

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
http://www.javasoft.com/products/java-media/jmf/mediaplayer/

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 http://developer.intel.com/ial/jmedia

SGI has released an implementation of JMF for IRIX:
See http://www.sgi.com/Products/motion/

8.5 How can I play *.au files from an application?

A. You've got 2 options:

a. Use the AudioClip class in sun.audio
http://www.javaworld.com/javaworld/javatips/jw-javatip24.html
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.

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?

A. Use
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.

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:
http://www.best.com/~pvdl
[pointers to other sites welcome]

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.


-------------------------------------------------------------------------
9. SECURITY

9.1 What is a "trusted applet"?

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?

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 are legitimate commercial products of that name).
If anyone has more concrete information about a virus that can attack a
Java applet (again, this is thought to be impossible), please could 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 to work around
this message.

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
http://www.systemics.com/software

Another crypto library for Java is at
http://www.acme.com/java/software/Package-Acme.Crypto.html
Blowfish, CRC16, CRC32, DES, DES3, IDEA, RC4, ROT13 (can they really
call that "crypto"?), and more.

One commercial Java encryption source (from Ireland) is
http://www.baltimore.ie/jcrypto.htm

A complete crypto API for Java (with HTML documentation) at:
http://www.geocities.com/SiliconValley/Heights/8298

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, a beta of Sun's Java Cryptography Extension (JCE) became available
for JDK 1.1.1 in May 1997. Download it from
http://java.sun.com/jdk/1.1/jce/
There's a Q&A archive at
http://jeeves.javasoft.com/hypermail/java-security-archive/index.html

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.

-------------------------------------------------------------------------
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 classes for Double, Float, Long, etc.
int i = Integer.valueOf(my_str).intValue();
also works but involves the creation of an extra object.

10.2 How do I convert an int to a string?

A. String s = String.valueOf(i);
or
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.

10.3 How can you send a function pointer as an argument?

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.

10.4 How do I execute a command from Java?
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());
}
}
}


10.5 OK, how do I read the input from a command?

A. As before, 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 = data.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 ) {
...
}


10.6 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


10.7 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


10.8 How can I pretty-print Java source?

A. Try http://www.CS.ORST.EDU/~speton/percolator/ (currently in Beta)
Or http://www.parallax.co.uk/~rolf/download/jpp.pl
Some Unix utilities work adequately:
indent (fails with "//" comments though)
cb (very few style choices though)

alias printjava 'vgrind -lC++ -t -w \!* | lp'
works pretty well too.

10.9 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.

-------------------------------------------------------------------------
11. FOR C and C++ AFFICIONADOS

11.1 How do I translate C/C++ into Java or vice-versa?

A. In general it is not simple to translate C/C++ into Java, as
Java lacks the arbitrary pointer arithmetic of those languages. If
your C code does not use pointer arithmetic, automatic translation
gets a lot simpler. Try these URLs:
http://www.ist.co.uk (search for X-Designer 4.6: Java edition).
http://members.aol.com/laffra/c2j.html
http://www.ilog.com/
http://www.cs.wustl.edu/~schmidt/java_notes.html

Going the other way there are currently three freely-available
tools to translate Java into C. It seems that these have been
done for hacking value, rather than practical purposes.
- j2c from Japan,
http://www.webcity.co.jp/info/andoh/java/j2c.html
- Toba from the Sumatra research project,
http://www.cs.arizona.edu/sumatra/toba
- JCC from Nik Shaylor.
http://www.digiserve.com/nshaylor/jcc.html
None of them support the AWT yet, and both j2c and JCC have
additional restrictions.

There's a product to convert Visual Basic to Java. Details at
http://www.blackdirt.com and http://www.tvobjects.com

This program dumps info about the class file:
http://www.professionals.com/~cmcmanis/java/dump/index.html
Chuck McManis was one of Sun's original Java whackers.

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.

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.

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.

public class AssertionException extends RuntimeException {
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 (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:
1234.567

There are lots of different characters you can feed to the
DecimalFormat constructor, not just "0". See source for details.


11.8 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/
[Some Java vendors are bundling it with their next release]


-------------------------------------------------------------------------
12. FURTHER RESOURCES

12.1 Useful URLS

Latest copy of this FAQ: http://www.best.com/~pvdl/

Other Java resources:
The Java "Hall" of Fame: http://www.apl.jhu.edu/~hall/java/
good glossary: http://oberon.ark.com/~roedy
conversions: http://oberon.ark.com/~roedy/convert.htm
good JDBC FAQ: http://www.yoyoweb.com/Javanese/JDBC/FAQ.html
general Java: http://java.miningco.com/
tutorial:
http://www.javasoft.com:80/nav/read/Tutorial/index.html
http://www.phrantic.com/scoop/onjava.html

Java Book lists:
http://lightyear.ncsa.uiuc.edu/~srp/java/javabooks.html
http://www.netcharts.com/majug/reviews.html
http://www.avena.net/~dong/book.htm
http://wwwiz.com/books

Javasoft site: http://java.sun.com

General sharing and exchange of Java info:
http://www.gamelan.com
http://www.JavaShareware.com

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

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.
Or 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. 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!

A. Somebody has done just that. Look at http://www.jsh.net/

-------------------------------------------------------------------------

Acknowledgements:
Original FAQ copyright February 1997 Peter van der Linden

Contributions 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,
Jake Cormier, Ian Macgregor, Mike Faulkner, Rich Koch, Rick Simkin,
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, Rob Mayoff

[<your name here>: send in a suggested FAQ *with the answer*]

----

I am maintaining a FAQ list to address specifically programming issues
(not a general tutorial on Java). Please feel free to mail me entries for it.
Question with answer gets you a credit in the FAQ.
I can be emailed at: pv...@best.com
Sun Certified Java Programmer.

-- end --


--
Peter van der Linden, Java Programmer's FAQ
Sun Certified Java Programmer. ---> http://www.best.com/~pvdl

Inventor of the Lambada-Calculus: logic predicates using Spanish dance steps.

Steve Robbins

unread,
Aug 21, 1997, 3:00:00 AM8/21/97
to

I'm trying to understand 'javac -depend'. Unless otherwise noted, all
the examples below were done using javac from JDK 1.0.2 as well as JDK
1.1.1, on linux.

Suppose I have three classes:

class a { b ib; }
class b { c ic; }
class c { }

Clearly class a depends on class b, and class b depends on class c.
Suppose each is in their own file: a.java, b.java, and c.java.

If I do "javac a.java", the compiler will create all three
corresponding .class files. OK.

Now, if I change c.java, compiling a.java will not rebuild the (out of
date) c.class file, no matter whether I use the -depend flag, or not.

So I don't understand the "tip.java" answer given below. What have I
missed?


> 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. One reliable way is javac *.java
> Another way is javac -depend tip.java
> where "tip.java" is a class that everyone else depends on.
> The -depend options searches recursively for depending classes
> and recompiles it.

When you say '"tip.java" is a class that everyone else depends on', I
think you really meant '... that depends on everything else'. In
other words, the role played by a.java in my example.

If you really meant what you typed, then my examples would have been:
touch a.java, and run "javac c.java". Clearly that will never
recompile anything other than c.java.


> This option doesn't help when you have
> dynamically loaded classes whose names cannot be determined by the
> compiler 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.
>
> Without the "-depend" option, javac doesn't
> look beyond the immediately adjacent dependencies to find
> classes lower down the hierarchy where the source has changed.
> However, there have been reports that "-depend" doesn't work
> on very complicated source bases. It also doesn't issue compile
> commands in parallel to make use of multi-processor systems.

--
--
Steve Robbins <st...@nyongwa.montreal.qc.ca>

I was looking back to see
if you were looking back at me
to see me looking back at you.

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