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发信人: afei (飞色精灵), 信区: Java
标 题: Java FAQ
发信站: BBS 水木清华站 (Mon Aug 4 09:08:58 1997)
From: elharo@sunsite.unc.edu
Newsgroups: comp.lang.java.programmer,comp.lang.java.softwaretools,comp.lang.java.gui,comp.lang.java.help,alt.www.hotjava
Organization: Cafe Au Lait
Followup-To: poster
Subject: comp.lang.java FAQ
Archive-name: computer-lang/java-faq/part1
Posting-Frequency: weekly
Last-modified: 1997/06/15
Version: 1.4.4
URL: http://sunsite.unc.edu/javafaq/javafaq.html
The comp.lang.java FAQ List
This is a Java FAQ list for the comp.lang.java newsgroups. If you're reading the text version via
Usenet, I do apologize that parts of it may be less intelligible than possible in HTML. Please refer
to the HTML version at http://sunsite.unc.edu/javafaq/javafaq.html.
The only official mirrors of this site are at the other sunsites. If the URL where you found this file
is not a sunsite, then this is an illegal mirror copy and is likely to be out of date. Please refer to the
official site instead.
This document is Copyright 1995-1997 by Elliotte Rusty Harold. For information about
reproduction, redistribution and other such rights please see the end of this file.
What's New
I fixed a URL for Intermetrics.
I added question 4.10: How do I load and display formatted HTML in window or TextArea?
I no longer list inability to communicate with hosts other than the applet host as a problem with
Netscape.
11.4: When I load the page Netscape gives me a java.lang.ClassFormatError.
This error may be thrown if Netscape can't find the desired .class file.
For the most part, this FAQ list covers Java 1.0.2. I have begun the process of converting this list
over to Java 1.1. I am aware of the changes in Java 1.1 and will incorporate them as time permits.
Please don't send me email pointing out errors or omissions that are only relevant to 1.1. I am fully
cognizant of these.
Table of Contents
What's New
Table of Contents
1: What is Java?
1.1: Where did Java come from?
1.2: Doesn't the World Wide Web have something to do with this?
1.3: What platforms does Java run on?
1.4: Do I need to know C++ to learn Java?
1.5: Where does Javascript fit in?
1.6: What's the difference between an application and an applet?
2: What other Java Resources are available?
2.1Web Sites
2.2: Mailing Lists
2.3: Newsgroups
2.5: Books
3: Java for C++ programmers
3.1: Data Types
3.2: Control Statements
3.3: Command Line Arguments
3.4: Comments
3.5: Classes
3.6: How is Java unlike C++?
4: How Do I...
4.1: Make a linked list without pointers?
4.2: How do I scanf, readln, etc. in Java?
4.3: How do I convert strings to numbers?
4.4: How Do I Call C Code from Java?
4.5: How Do I Call Java from C?
4.6: How do you do multiple inheritance?
4.7: Can I write objects to and read objects from a file or other stream?
4.8: How do I call the native API from Java?
4.9: How do I pass a function pointer to a method?
4.10: How do I load and display formatted HTML in window or TextArea?
5: Programming Applets
5.1: Can applets communicate with each other?
5.2: Can applets launch programs on the server?
5.3: Can applets launch programs on the client?
5.4: Can I record audio from an applet?
6: Language Issues
6.1: What does it mean that a class or member is "final"?
6.2: What does it mean that a method or field is "static"?
6.3: What does it mean that a method or class is abstract?
6.4: What's an interface?
6.5: Why doesn't Java include insert your favorite feature here?
6.6: Is Java CORBA compliant?
6.7: Can I cast an int to an Integer? a float to a Float?
6.8: How do I version a class?
6.9: Why isn't there operator overloading?
6.10: Does Java have pointers?
6.11: Does Java pass method arguments by value or by reference?
6.12: Are there parameterized types (templates)?
6.13: How does garbage collection work?
6.14: PERFORM and BECOME
7: java.awt
7.1: What Does AWT Stand For?
7.2: What are peer "classes"?
7.3: Can you explain how events are passed?
7.4: Clipping
7.5: How do I change the thickness of the line?
7.6: What fonts does Java support?
8: java.io
8.1:How do I read data from a file?
8.2: How do I write data to a file?
8.3: How do I append data to a file?
9: java.net
9.1: How do I convert a numeric IP address like 199.1.32.90 into a hostname like
star.blackstar.com?
9.2: How can a Java program talk to a CGI program?
9.3: How can an applet send email?
9.4: How can I use ICMP in Java?
10: java.util
10.1: How random is Random()?
10.2: How do I generate a random integer between a and b?
11: Common Errors and Problems
11.1: How Can I Avoid Flicker in an Applet?
11.2: Can you explain CLASSPATH?
11.3: Why won't my audio clip play?
11.4: When I load the page Netscape gives me a java.lang.ClassFormatError.
11.5: Netscape gives me "Applet Not Initialized Error"
11.6: Other Netscape Problems
11.7: I keep getting access privilege violations, " Java tried to read file foo.class in
directory bar."
11.8: Javac keeps insisting that it can't find the right constructor function in my class,
but I'm sure it's in a file I imported. Why is it even looking in my class instead of the
imported file?
11.9: NullPointerExceptions with arrays of objects
11.10: Can't make static reference to method type myMethod in class myClass
11.11: Why do I have trouble when using == to compare Strings?
12: Security
12.1: System Level Security
12.2: User Level Security
13: Tools
13.1: IDE
13.2: Debugger
13.3: Editors
13.4: YACC grammar
13.5: Profilers
14: The Java Virtual Machine and Byte
14.1: Can I program directly in byte code?
14.2: Optimizing Java
15: Legal Issues
16: FAQ's about the FAQ
16.1: Who is this Elliotte guy, and just what qualified him to write a Java FAQ
anyway?
16.2: Why don't you include X in your FAQ?
16.3: This is really cool. Can I republish or translate it?
1: What is Java?
Java, formerly known as oak, is an object-oriented programming language developed by Sun. It
shares many superficial similarities with C, C++, and Objective C (for instance for loops have the
same syntax in all four languages); but it is not based on any of those languages, nor have efforts
been made to make it compatible with them.
Java is sometimes referred to as C++ ++ --. The language was originally created because C++
proved inadequate for certain tasks. Since the designers were not burdened with compatibility with
existing languages, they were able to learn from the experience and mistakes of previous
object-oriented languages. They added a few things C++ doesn't have like garbage collection and
multithreading; and they threw away C++ features that had proven to be better in theory than in
practice like multiple inheritance and operator overloading. (There's still argument over whether
they made the right choices. I tend to think they were correct to throw out operator overloading
and probably correct to throw out multiple inheritance. For now let's just say that neither choice is
likely to be reviewed soon.)
Even more importantly Java was designed from the ground up to allow for secure execution of
code across a network, even when the source of that code was untrusted and possibly malicious.
This required the elimination of more features of C and C++. Most notably there are no pointers in
Java. Java programs cannot (at least in theory) access arbitrary addresses in memory.
Furthermore Java was designed not only to be cross-platform in source form like C, but also in
compiled binary form. Since this is frankly impossible across processor architectures, Java is
compiled to an intermediate byte-code which is interpreted on the fly by the Java interpreter. Thus
to port Java programs to a new platform all that is needed is a port of the interpreter and a few
native code libraries.
Finally Java was designed to make it a lot easier to write bugfree code. Shipping C code has, on
average, one bug per 55 lines of code. About half of these bugs are related to memory allocation
and deallocation. Thus Java has a number of features to make bugs less common:
Strong Typing
No unsafe constructs
The language is small so its easy to become fluent.
The language is easy to read and write. Obfuscated Java isn't nearly as common as
obfuscated C.
There are no undefined or architecture dependent constructs.
Java is object oriented so reuse is easy.
Java has concurrency.
1.1: Where did Java come from?
In the late 1970's Bill Joy thought about doing a language that would merge the best features of
MESA and C. However other projects (like cofounding Sun) intervened. In the late 1980's he got
Sun's engineers started on a complete revision of the UNIX operating system that involved
merging SunOS4.x with AT&T's SYSVR4.
In 1989 Joy sold his Sun stock, invested heavily in Microsoft and moved out of mainstream Sun to
Aspen, Colorado. By the early 90's Bill was getting tired of huge programs. He decided that he
wanted to be able to write a 10,000 line program that made a difference. In late 1990 Bill wrote a
paper called Further which outlined his pitch to Sun engineers that they should produce an object
environment based on C++. Today Joy freely admits that C++ was too complicated and wasn't up
to the job.
Around this time James Gosling (of emacs fame) had been working for several months on an
SGML editor called "Imagination" using C++. The Oak language (now Java) grew out of Gosling's
frustration with C++ on his "Imagination" project.
Patrick Naughton, then of Sun, now vice-president of technology at StarWave, started the Green
Project on December 5th, 1990. Naughton defined the project as an effort to "do fewer things
better". That December he recruited Gosling and Mike Sheridan to help start the project. Joy
showed them his Further paper, and work began on graphics and user interface issues for several
months in C.
In April of 1991 the Green Project (Naughton, Gosling and Sheridan) settled on smart consumer
electronics as the delivery platform, and Gosling started working in earnest on Oak. Gosling wrote
the original compiler in C; and Naughton, Gosling and Sheridan wrote the runtime-interpreter,
also in C. (The Green project never wrote a single line of C++.) Oak was running its first programs
in August of 1991. Joy got his first demos of the system that winter, when Gosling and Naugton
went skiing at Joy's place in Aspen.
By the fall of 1992 "*7", a cross between a PDA and a remote control, was ready This was demoed
to Scott McNealy, Sun's president, in October. He was blown away. Following that the Green
Project was set up as First Person Inc., a wholly owned Sun subsidiary.
In early 1993 the Green team heard about a Time-Warner request for proposal for a settop box
operating system. First Person quickly shifted focus from smart consumer electronics (which was
proving to be more hype than reality) to the set-top box OS market, and placed a bid with
Time-Warner.
Fortuitously, Sun lost the bid. The Time-Warner project went nowhere, the same place it probably
would have gone if Sun had won the bid. First Person continued work on settop boxes until early
1994, when it concluded that like smart consumer electronics settop boxes were more hype than
reality.
Without a market to be seen First Person was rolled back into Sun in 1994. However around this
time it was realized that the requirements for smart consumer electronics and settop box software
(small, platform independent secure reliable code) were the same requirements for the nascent
web.
For a third time the project was redirected, this time at the web. A prototype browser called
WebRunner was written by Patrick Naughton in one weekend of inspired hacking. After additional
work by Naughton and Jonathan Payne this browser became HotJava. The rest, as they say, is
history.
Information in this section is primarily based on the first hand accounts of Bill Joy and Patrick
Naughton (which don't always agree). No doubt other people have still different memories of what
occurred. If you've got any more first hand information about what went on in the Green project
I'd like to hear from you.
1.2: Doesn't the World Wide Web have something to do with this?
Not necessarily. Java is a programming language. When work began on what has become Java, the
World Wide Web was just getting started at CERN; and Mosaic wasn't even a glint in Marc
Andreesen's eye. The original use of the Java language (settop boxes) required security and the
ability to execute code from untrusted hosts. It turns out these are virtually the same requirements
for allowing people to download and run programs from the Web. No other language has the
built-in security of Java. The key here is the security features. The object-oriented nature of Java
is secondary, and mainly reflects the preferences and prejudices of the developers who set out to
write a secure language. The C-like syntax of the language is even less crucial.
At the lowest level the advantage of Java to the web is that it provides a secure, cross-platform
way for code to be executed. At a somewhat higher level Java adds several features to existing web
sites:
Arbitrary Graphics
Java lets the server draw pictures in a window on the client. In theory this allows a web page
to do anything a regular program can do by drawing in a window.
Arbitrary Data Types
In practice rather than using graphics primitives to create your desired web page you'd use a
graphics program to draw the page and then write a program that could read and display the
file formats of that program. Java lets you write content handlers that display any particular
data format. This way you can download your data and your data display program rather
than downloading a bitmapped snapshot of the display. People are already using this to add
sound and animation to web pages. Rather than having to download a file and spawn an
external viewer, the viewer is included with the data; and the data is displayed right on the
page.
Less Load On The Server
CPU intensive cgi-bin scripts place a large load on a server, particularly at busy sites. With
Java you can offload the calculations to the client's PC. I've written an applet that calculates
all possible ram configurations for a given Mac model. However on models with many
memory slots the sheer number of permutations can tie up even a fast machine for several
hours. Publishing this as a cgi-bin would bring my server to its knees, but by publishing it as
a Java applet I can distribute the load across all the machines that want to run it.
More User Interaction
Finally Java allows for more interaction with the user. Java not only allows you to paint
arbitrary data on the screen. It also allows you to collect input from the user in the form of
mouse clicks, keystrokes and the like. This lets you put almost any application on your web
page that doesn't require disk access.
1.3: What platforms does Java run on?
Sun has made a Java development kit available for Sparc and X86 Solaris, Windows NT for Intel,
Windows 95, MacOS 7.5 on PowerMacs and 68030 (25 MHz and faster) and 68040 Macs.
An IRIX port has mostly been completed by Simon Leinen. SGI's own port is available at
http://www.sgi.com/Fun/Free_webtools.html
A Linux port is in fairly good shape. See http://java.blackdown.org/java-linux.
html.
IBM has ported Java to Windows 3.1, OS/2, and AIX. It is working on ports for MVS and OS/400.
See http://ncc.hursley.ibm.com/javainfo/hurindex.html.
The OSF has ported Java to Unixware, the Bull Estrella or other PowerPC running AIX4.1, X86
running DASCOM OSF/1 , the Digital Alpha running Digital UNIX 3.2 , the HP700 series
running HPUX 10.x, the NCR Globalyst (Pentium) running UNIX SysV, and Sony NEWS (MIPS)
running Sony NEWS 6.1.1. See http://www.osf.org/mall/web/JDK/.
Other ports are underway for Nextstep, SunOS 4.1, the Amiga and possibly other platforms.
In the past new versions of Java have most often been made available first for Solaris. If you have
to have the latest version as soon as it's released, or if you're developing "Bet your company"
applications with Java, you should probably be running Solaris on a SparcStation. Otherwise, if
you just want to learn the language, you can get away with an Intel based Windows 95 or NT
machine with a lot of RAM.
Netscape 2.0 and later plays Java applets on Windows NT, Windows 95, Solaris, SunOS 4.1, Linux
and most other Unix platforms. Netscape 3.0 for the Mac also supports Java. Netscape 2.0 can even
compile Java programs. (Netscape 3.0 cannot).
First download the classes.zip file from the Solaris distribution of the JDK. (You have to
download the entire JDK. You can't get just the classes.zip file.) Next set your CLASSPATH
environment variable to include the classes.zip file. For example, on Unix, if you put the file in
/usr/local/lib/classes.zip, then you would set it like this:
sh: % CLASSPATH=$CLASSPATH:/usr/local/lib/classes.zip
csh: % setenv CLASSPATH "$CLASSPATH:/usr/local/lib/classes.zip"
You'll probably want to put this line in your .login or .cshrc file. Of course you'll need to adjust
this to match where you've installed these files and to add any other classes you use. Finally you
compile programs from the command line like this:
% netscape -java sun.tools.javac.Main HelloWorld.java
You can always alias netscape -java sun.tools.javac.Main to just javac to make this
more transparent.
You can also run command-line programs that don't use the AWT in the same way, like this:
% netscape -java HelloWorld.class
You can't use netscape -java to run programs that use the AWT, but you can of course play
applets in Netscape..
1.4: Do I need to know C++ to learn Java?
No. Java is in fact a much easier language to learn than C++.
A little further out in left field, it isn't even necessary to know Java to write Java programs.
Intermetrics is beta testing an ADA-95 to Java byte code compiler. Other such cross-compilers
are probably possible including ANSI Fortran-77. However the one most people probably want, a
C/C++ to Java byte code compiler, is probably not possible due to Java's lack of pointers.
1.5: Where does Javascript fit in?
To quote from p. 31 of the first edition of my book, The Java Developer's Resource,
Java and JavaScript are about as closely related as the Trump Taj Mahal in Atlantic
City is to the Taj Mahal in India. In other words Java and JavaScript both have the
word Java in their names. JavaScript is a programming language from Netscape which
is incorporated in their browsers. It is superficially similar to Java in the same way C
is similar to Java but differs in all important respects.
1.6: What's the difference between an application and an applet?
This question can be answered on many levels. Technically an application is a Java class that has a
main() method. An applet is a Java class which extends java.applet.Applet. A class which
extends java.applet.Applet and also has a main() method is both an application and an
applet.
More generally and less technically an application is a stand-alone program, normally launched
from the command line, and which has more or less unrestricted access to the host system. An
applet is a program which is run in the context of an applet viewer or web browser, and which has
strictly limited access to the host system. For instance an applet can normally not read or write
files on the host system whereas an application normally can.
The actions of both applets and applications, however, can be controlled by SecurityManager
objects. If you can change the SecurityManager that's used you can change what an applet or an
application is and is not allowed to do. Thus these are not hard and fast differences, though this is
normally how they separate out in practice.
2 What other Java Resources are available?
2.1Web Sites
The key site for Java information is http://www.javasoft.com/.This is Sun's official site for Java,
and contains the latest published version of all official Java information. The most important page
on this site is the Documentation page at http://www.javasoft.com/doc/.Most of the other pages
are linked off of this page or its children.
For many more web sites see the Cafe Au Lait links page at
http://sunsite.unc.edu/javafaq/links.html.
2.2: Mailing Lists
There are over sixty different Java related mailing lists. A more-or-less complete list is at the
Cafe Au Lait mailing list page at http://sunsite.unc.edu/javafaq/mailinglists.h
tml.
2.3: Newsgroups
alt.www.hotjava
A newsgroup for the discussion of the HotJava browser.
comp.lang.java.announce
A newsgroup for announcements that may be of interest to Java developers. See
http://www.cs.latrobe.edu.au/~le ... /guidelines.htmlfor the ridiculously
complicated submission guidelines and submission address.
comp.lang.java.programmer
A newsgroup for the discussion of the Java language.
comp.lang.java.machine
Technical issues about Java not related directly to programming, including virtual machines,
byte code, classfile format, performance and optimization, possible extensions, porting,
native methods and interfacing Java with other languages.
comp.lang.java.security
Discussion concerning any of Java's security features such as byte code verification,
SecurityManagers, class loaders, public-key encryption and authentication of classes, holes
in the security model, and similar topics.
comp.lang.java.help
This group is for end-users of Java systems, not programmers. It deals with installation
problems, CLASSPATH problems, the availability of ports to specific platforms, and the
like. Subject lines should include the platform that the problem occurs on, and the browser
version, for example MacOS 7.5 Netscape 3.0.
comp.lang.java.beans
The JavaBeans component software API
comp.lang.java.databases
Java and databases, including middleware like java.sql, JDBC, ODBC, and Java based
databases like jDB.
comp.lang.java.gui
The AWT and other GUI enrvironments: windows, dialogs, menus, components, buttons,
radio buttons, printing, cut and paste, etc.
comp.lang.java.softwaretools
Integrated Development Environments (IDEs), web browsers, compilers, applet viewers,
Rapid Application Development (RAD) tools, class browsers, visual interface builders, and
other tools.
2.5: Books
As of March, 1997 I recommend four books in particular for those learning the language. The first
is my own book, The Java Developer's Resource, ISBN: 0-13-570789-7, $26.95 from Prentice
Hall. This is an introduction to Java for programmers, regardless of previous experience with C or
OOP. It should be available in most bookstores that stock computer books as well as from
amazon.com, http://www.amazon.com/exec/obidos/ISBN=0135707897/
The second book I recommend is Java in a Nutshell: A Desktop Quick Reference for Java
Programmers by David Flanagan from O'Reilly & Associates, Inc., ISBN: 1-56592-183-6,
$19.95. This is an excellent introduction to Java for C and C++ programmers and an excellent
reference for everyone.
For people with no prior experience in programming I recommend Kris Jamsa's Java Now!, ISBN:
1-884133-30-4, $16.95, Jamsa Press.
Finally The Java Handbook by Patrick Naughton, $27.95, Osborne/Mcgraw-Hill, is a moderately
comprehensive introduction to Java.
None of these books cover Java 1.1. In fact as of this writing, no book truly covers Java 1.1 though
several claim to.
For details about these and many more books see the Cafe Au Lait books page at
http://sunsite.unc.edu/javafaq/books.html
3: Java for C++ programmers
In the large Java looks like Smalltalk. In the small it looks like C. The syntax of Java is
deliberately similar to C. If you know C you already know large chunks of Java. Here, in brief, is
Java syntax for C programmers:
3.1: Data Types
Java's primitive data types are very similar to those of C. Strings, booleans, and true arrays have
been added. However the implementation of the data types has been substantially cleaned up in
several ways.
1.Where C and C++ leave a number of issues to be machine and compiler dependent (for
instance the size of an int) Java specifies everything.
2.Java prevents casting between arbitrary variables. Only casts between numeric variables and
between sub and superclasses of the same object are allowed.
3.All numeric variables in Java are signed.
Here are the detailed data types:
boolean
1-bit. May take on the values true and false only.
true and false are defined constants of the language and are not the same as True and
False, TRUE and FALSE, zero and nonzero, 1 and 0 or any other numeric value. Booleans
may not be cast into any other type of variable nor may any other variable be cast into a
boolean.
byte
1 signed byte (two's complement). Covers values from -128 to 127.
short
2 bytes signed (two's complement), -32,768 to 32,767
int
4 bytes, signed (two's complement). -2,147,483,648 to 2,147,483,647. Like all numeric types
ints may be cast into other numeric types (byte, short, long, float, double). When lossy casts
are done (e.g. int to byte) the conversion is done modulo the length of the smaller type.
long
8 bytes signed (two's complement). Ranges from -9,223,372,036,854,775,808 to
+9,223,372,036,854,775,807.
float
4 bytes, IEEE 754. Covers a range from 1.40129846432481707e-45 to
3.40282346638528860e+38 (positive or negative).
Like all numeric types floats may be cast into other numeric types (byte, short, long,
int, double). When lossy casts to integer types are done (e.g. float to short) the
fractional part is truncated and the conversion is done modulo the length of the smaller type.
double
8 bytes IEEE 754. Covers a range from 4.94065645841246544e-324d to
1.79769313486231570e+308d (positive or negative).
char
2 unsigned bytes, Unicode.
Chars are not the same as bytes, ints, shorts or Strings.
Strings
Strings are a special object defined in java.lang.String. They are not null terminated and are
not the same as an array of chars.
array
Arrays are objects. Multidimensional arrays are created via arrays of arrays.
sizeof isn't necessary in Java because all sizes are precisely defined. i.e. an int is always 4 bytes.
This may not seem to be adequate when dealing with objects that aren't base data types. However
even if you did know the size of a particular object, you couldn't do anything with it anyway. You
cannot convert an arbitrary object into bytes and back again.
3.2: Control Statements
Java contains if, else, for, while, do while and switch statements. The syntax is identical
to C's. However all condition tests must return boolean values. Since non-boolean assignment
statements and arithmetic statements do not return a boolean value, some of the more obfuscated
condition tests in C are prohibited.
3.3: Command Line Arguments
Command line arguments are like C's except that argv has become a string array commonly
called args and args[0] is the first command line argument, not the name of the program. The
other arguments are all shifted one to the left from where they'd be in C or C++.
3.4: Comments
Java supports both the /* This is a comment */ comment from C and the
// This is a C++ comment
comment from C++.
However comments that begin with a /** are special. These comments should only be used before
a method or class declaration. They indicate that the comment should be included in automatically
generated documentation for that declaration.
3.5: Classes
Java does not support multiple inheritance.
Superclasses of a class are indicated with the extends keyword rather than with a :.
Methods must be defined inside the class to which they belong. They may not be declared inside
the class and defined outside the class as is common in C++.
3.6: How is Java unlike C++?
Two classes of language features have been removed from C++ to make it Java. These are those
language features which make C++ unsafe and those which make it hard to read.
Features removed that make Java easier to read and understand than C++ include #define,
typedef, operator overloading, enum, unions and structs.
The main feature removed to make Java safer and more robust than C++ is pointer arithmetic.
Other features removed include global variables, standalone functions (everything is a method),
friend functions (Everything in a package is a friend of everything else in the package.) and
non-virtual functions.
A number of features have been added to Java to make it safer including true arrays with bounds
checking, garbage collection, concurrency, interfaces (from Objective C) and packages. There is no
need to explicitly allocate or free memory in Java.
4: How Do I...
4.1: Make a linked list without pointers?
Short answer: Use the Vector class in java.util. It can do anything a linked list can do and a
little more and saves you a lot of coding which, after all, is the point of OOP and the class library.
However it is array based so insertions or deletions from the middle of a Vector are not as
efficient as with a true linked list.
Long answer: Object variables in Java are all references. A reference acts like a pointer in most
other languages. (Though they're handles, not pointers, in most Java implementations. The notable
exception is Microsoft's.) The main difference is that you can't do pointer arithmetic on
references. Therefore wherever you'd use a pointer to an object in C++, in Java you should just use
the object itself.
On the other hand the primitive data types (int, float, double, char, byte, short,
long and boolean) are not references. If you want to get a reference to one of these you need to
wrap it in a class first. Java provides ready-made type-wrapper classes in the java.lang package for
Boolean, Character, Integer, Double, Float, and Long. Bytes and shorts can be stored in
the Integer class as well. Java 1.1 will add Byte, Short, and Void classes too.
4.2: How do I scanf, readln, etc. in Java?
Java has no exact equivalent to C's scanf(), fscanf() and sscanf() functions, Pascal's
read() and readln() function, or Fortran's READ* function. In particular there's no one method
that lets you get input from the user as a numeric value.
However, roughly equivalent functionality is scattered across several classes. You first read an
input line into a String using DataInputStream.readline() or BufferedReader (in Java
1.1)
Next use the StringTokenizer class in java.util to split the String into tokens. By default
StringTokenizer splits on white space (spaces, tabs, carriage returns and newlines), but this is
user definable.
For example,
import java.util.StringTokenizer;
class STTest {
public static void main(String args[]) {
String s = "9 23 45.4 56.7";
StringTokenizer st = new StringTokenizer(s);
while (st.hasMoreTokens()) {
System.out.println(st.nextToken());
}
}
}
prints the following output:
9
23
45.4
56.7
Finally you convert these tokens into numbers using the type wrapper classes as described in the
next question.
4.3: How do I convert strings to numbers?
You can convert strings into numbers using the Integer, Float, Double and Long type wrapper
classes as indicated by the following code snippet:
class ConvertTest {
public static void main (String args[]) {
String str;
str = "25";
int i = Integer.valueOf(str).intValue();
System.out.println(i);
long l = Long.valueOf(str).longValue();
System.out.println(l);
str = "25.6";
float f = Float.valueOf(str).floatValue();
System.out.println(f);
double d = Double.valueOf(str).doubleValue();
System.out.println(d);
}
}
There are no equivalent Short and Byte classes in Java 1.0. There are in Java 1.1. For shorts and
bytes use the Integer class but use the byteValue() or shortValue() methods instead.
4.4: How Do I Call C Code from Java?
First of all for security reasons you can only call external code from an application. You cannot
call external code from an applet. (Otherwise all the security would go out the window.)
Furthermore all code you write in this way will be non-portable. If these aren't problems for you,
check out javah in the Java developer's kit.
4.5: How Do I Call Java from C?
Since Java is not as of yet a true compiled language, the only way to call Java code from C in Java
1.0 is by using the system() call (or your OS's equivalent) to execute the java interpreter with
appropriate command line arguments.
In Java 1.1 the Java Native Method Interface in combination with the Invocation API allows native
applications to load and access the Java virtual machine.
4.6: How do you do multiple inheritance?
Java does not support multiple inheritance. Instead declare interfaces for each additional class you
want to inherit from, and implement those interfaces in your subclass. For more details, see
question 6.1 below.
4.7: Can I write objects to and read objects from a file or other stream?
In theory yes, but once again all the coding is up to you. There is no general method for doing this
in Java 1.0. The problem is made harder by Java's security features that don't let you forge
arbitrary byte streams into objects.
In Java 1.1 a serialization interface has been added to the language. However only objects that
explicitly implement the java.io.Serializable interface can be serialized.
4.8: How do I call the native API from Java?
You can do this only in an application, not in an applet. You will need to write some intermediary
code in C or another traditional language and call your C code from Java.
4.9: How do I pass a function pointer to a method?
Methods are not first class objects in Java. Only objects and primitive data types can be passed to
methods.
Instead you should declare an interface which declares the function you want to pass; for example
public interface Comparable {
public abstract int compare(Object o1, Object o2);
}
Then declare that your method takes an object of this type. For example,
public void sort(Object[] o, Comparable c) {
boolean done = false;
while (!done) {
done = true;
for (int i = 0; i < o.length - 1; i++) {
if (c.compare(o, o[i+1]) < 0) {
swap(o, o[i+1]);
done = false;
}
}
}
Each class you want to compare then needs its own class which implements Comparable. For
example,
public class StringCompare implements Comparable {
public int compare(Object o1, Object o2) {
String s1 = (String) o1;
String s2 = (String) o2;
return s1.compareTo(s2);
}
}
It helps that Object is a superclass for all object types.
4.10: How do I load and display formatted HTML in window or TextArea?
If you need to do this, you should probably be looking at JavaScript instead of Java. It's possible in
Java, but only with GREAT difficulty. There is no simple answer at this point in time. The
problem is that java does not contain a built-in HTML rendering engine, and I'm not aware of any
third-party classes that perform this function. You either need to generate the file on the server,
then load the URL of that file into the browser; or you need to write a complete HTML parser and
display engine.
5: Programming Applets
5.1: Can applets communicate with each other?
At this point in time applets may communicate with other applets running in the same virtual
machine. If the applets are of the same class, they can communicate via shared static variables. If
the applets are of different classes, then each will need a reference to the same class with static
variables. In any case the basic idea is to pass the information back and forth through a static
variable.
An applet can also get references to all other applets on the same page using the getApplets()
method of java.applet.AppletContext. Once you've got a reference to an applet, you can
communicate with it by using its public members.
It is conceivable to have applets in different virtual machines that talk to a server somewhere on
the Internet and store any data that needs to be serialized there. Then, when another applet needs
this data, it could connect to this same server. Implementing this is non-trivial.
5.2: Can applets launch programs on the server?
Yes, using CGI. Any other implementation would be server dependent. Of course this requires a
lot of coding and is non-trivial. A simple way to do this is not built into Java because that would
require a special server. One of Java's strengths is that it is http server independent.
A number of organizations have developed special http servers that allow applets or other clients
to run Java programs on the server in a secure environment. Most notably Sun's Jeeves implements
a servlet interface for this purpose. The W3C's JigSaw implements a similar idea called resource
objects.
5.3: Can applets launch programs on the client?
Absolutely not. This would be a security hole big enough to walk three herds of elephants, two
marching bands and at least one quarter of the people AT&T laid off through.
5.4: Can I record audio from an applet?
This is what is known as "a third-party opportunity". In other words not this year and probably not
next year either. This would make a neat Java plug-in if anyone cares to write one in C.
6: Language Issues
6.1: What does it mean that a class or member is "final"?
A final class can no longer be subclassed. Mostly this is done for security reasons with basic
classes like String and Integer. It also allows the compiler to make some optimizations, and
makes thread safety a little easier to achieve.
Methods may be declared final as well. This means they may not be overridden in a subclass.
Fileds can be declared final, too. However, this has a completely different meaning. A final field
cannot be changed after it's initialized, and it must include an initializer statement where it's
declared. For example,
public final double c = 2.998;
It's also possible to make a static field final to get the effect of C++'s const statement or
some uses of C's #define, e.g.
public static final double c = 2.998;
6.2: What does it mean that a method or field is "static"?
Static variables and methods are instantiated only once per class. In other words they are class
variables, not instance variables. If you change the value of a static variable in a particular
object, the value of that variable changes for all instances of that class.
Static methods can be referenced with the name of the class rather than the name of a particular
object of the class (though that works too). That's how library methods like
System.out.println() work.
6.3: What does it mean that a method or class is abstract?
An abstract class cannot be instantiated. Only its subclasses can be instantiated. You indicate that a
class is abstract with the abstract keyword like this:
public abstract class Container extends Component {
Abstract classes may contain abstract methods. A method declared abstract is not actually
implemented in the current class. It exists only to be overridden in subclasses. It has no body. For
example,
public abstract float price();
Abstract methods may only be included in abstract classes. However, an abstract class is not
required to have any abstract methods, though most of them do.
Each subclass of an abstract class must override the abstract methods of its superclasses or itself be
declared abstract.
For more details, see section 8.1.2.1 of the Java Language Specification.
6.4: What's an interface?
An interface is an idea taken from Objective C. It describes the public methods that a class
implements and their calling conventions without saying anything about how those methods are
implemented. It is the responsibility of each class that implements an interface to provide code to
handle the cases where the methods of the interface are called.
For example suppose you're writing an inventory database. The inventory may include many
different items of many different types and classes. However each item in the warehouse needs to
be able to tell you its price. Normally you would implement this by having each class extend a
common superclass. However that's not always convenient. Instead you can declare an interface
called Price with a price() method like this:
public interface Price {
public float price();
}
Any class which implements the Price interface must contain a method with the signature
public float price(). The code of the price() method is included separately in each
separate class which implements Price, not in the Price interface itself.
Different classes in your warehouse can each implement the Price interface like this:
public class Monopoly extends BoardGame implements Price {
// other methods
public float price() {
return 14.95;
}
}
When other code is passed an object, it can test whether the object implements Price with the
instanceof operator. For example,
if (o instanceof Price) System.out.println("Subtotal is " + o.price());
In fact, interfaces can be used to tag objects. The java.rmi.Remote interface declares no
methods. Its sole purpose is to indicate that an object is a remote object. In general, sub-interfaces
of java.rmi.Remote will declare remote methods, however. For example,
public interface Hello extends java.rmi.Remote {
public String sayHello();
}
public class HelloImpl extends UnicastRemoteServer implements Hello {
public String sayHello() {
return "Hello";
}
}
For more information about the java.rmi package, see
http://chatsubo.javasoft.com/current/rmi/index.htmlor Chapter 14 of my book, Network
Programming in Java, from O'Reilly & Associates (available early 1997).
6.5: Why doesn't Java include insert your favorite feature here?
The Java language has been extensively debated and argued about within Sun. Almost every
language construct of existing languages has already been considered for inclusion in Java. While
there may still be room for addition, it is very unlikely that your pet feature will be added to the
language spec if it isn't already there. In a couple of years parameterized types (i.e. templates) may
be added to the language. Otherwise the spec is pretty much frozen except for minor changes and
bug fixes.
Extensions are planned for the class library though. In particular Sun is working on extensions for
3D, multimedia, telephony, and improved graphics.
6.6: Is Java CORBA compliant?
Not yet. However work is underway for a Java ORB and IDL.
http://splash.javasoft.com/JavaIDL/pages/index.html.
6.7: Can I cast an int to an Integer? a float to a Float?
No, you cannot promote a base data type like int or float to an object such as an Integer or a
Float. However the proper way to do this isn't very hard. Instead do
int x = 5;
myInteger = new Integer(x);
6.8: How do I version a class?
There is no support for versioning classes in Java 1.0. However in Java 1.1 the serialver tool
provides a serialVersionUID for one or more classes you can add to your class as a field. This is
used in object serialization.
6.9: Why isn't there operator overloading?
Because C++ has proven by example that operator overloading makes code almost impossible to
maintain. In fact there very nearly wasn't even method overloading in Java, but it was thought that
this was too useful for some very basic methods like print(). Note that some of the classes like
DataOutputStream have unoverloaded methods like writeInt() and writeByte().
6.10: Does Java have pointers?
No, no, a thousand times no. Java does not have pointers, no way, no how, the daily email I get
from people who think differently not withstanding.
Java does have references. A reference is an abstract identifier for an object. It is not a pointer. A
reference tags a particular object with a name in the Java virtual machine so that the programmer
may refer to it. How exactly the virtual machine implements references at the level of machine
code is VM-dependent and completely hidden from the programmer in any case. Most VMs
including Sun's use handles, not pointers. A handle is a pointer to a pointer. At the level of
machine code in the CPU a reference is an address in memory where the address of the object is
stored. This way the objects can be moved around in memory and only the master pointer needs to
be updated rather than all references to the object. This is completely hidden from the Java
programmer, though. Only the implementer of the virtual machine needs to worry about it. Indeed,
this is not the only way references can be implemented. Microsoft's VM actually does use pointers
rather than handles. Other schemes are possible.
6.11: Does Java pass method arguments by value or by reference?
Java passes all arguments by value, not by reference. However this is one of the few places where
the distinction between an object and a reference to an object becomes important. Object and array
variables in Java are really references to the object or array. This can make it look like an object is
passed by reference if you only modify the fields of the object or array, but do not change the
reference itself. For example, consider this program:
import java.awt.Point;
class changePoint {
public static void main(String args[]) {
Point p1 = new Point(0, 0);
changePoint(p1);
System.out.println(p1);
}
static void changePoint(Point p) {
p.x = 38;
p.y = 97;
}
}
It prints:
java.awt.Point[x=38,y=97]
Therefore the point has been changed. However the reference, which is what was really passed, has
not been changed. To see that consider the following program.
import java.awt.Point;
class dontChangePoint {
public static void main(String args[]) {
Point p1 = new Point(0, 0);
dontChangePoint(p1);
System.out.println(p1);
}
static void dontChangePoint(Point p) {
p = new Point(38, 97);
}
}
It prints:
java.awt.Point[x=0,y=0]
What happened in this example was that a copy of the reference p1 was passed to the
dontChangePoint() method. A new Point object was then assigned to that copy. However this
did not change the old reference in the main method. In the previous example the reference p in the
changePoint() method and p1 in the main() method both referred to the same object. In this
example p and p1 refer to different objects after the new Point is assigned to p.
6.12: Are there parameterized types (templates)?
Not in Java 1.0 or 1.1. However this is being seriously considered for future versions.
6.13: How does garbage collection work?
Current implementations of Java use a mark and sweep garbage collector. Reference counting is
not used. Thus circular linked lists do not lead to memory leaks. It is theoretically possible that
future versions of Java will use some other garbage collection algorithm.
6.14: PERFORM and BECOME
Sorry Smalltalkers. There's no equivalent for PERFORM or BECOME in Java 1.0. These would
probably open security holes. There are workarounds for some things you might want to do using
interfaces.
In Java 1.1 the Core Reflection API and the java.lang.reflect package provides most of the
functionality you need. However this API mostly unavailable to applets due to security issues.
7: java.awt
7.1: What Does AWT Stand For?
So far I've heard it claimed that AWT stands for:
Abstract Window Toolkit
Advanced Window Toolkit
Another Window Toolkit
Applet Window Toolkit
Awkward Window Toolkit
Annoying Window Toolkit
I do not know which if any of these are correct, but Abstract Window Toolkit seems to be the most
popular.
7.2: What are peer "classes"?
Peer classes exist mainly for the convenience of the people who wrote the Java environment. They
help in translating between the AWT user interface and the native (Windows, OpenWindows, Mac
etc.) interfaces. Unless you're porting Java to a new platform you shouldn't have to use them.
7.3: Can you explain how events are passed?
The following applies to Java 1.0 only. The event mechanism has changed completely in Java 1.1,
though the following is retained for backwards compatibility.
Components are subclasses of java.awt.Component. Examples of components include buttons,
scrollbars, textfields, frames, windows, dialogs, panels, canvases, and checkboxes. One subclass of
java.awt.Component is java.awt.Container. A container is a component which can hold
other components. Examples of containers include Frames, Windows, Dialogs, Panels and Applets.
An applet is a subclass of Panel. Panel is a subclass of Container. Container is a subclass of
Component. Therefore an applet is both a container and a component.
When the user clicks the mouse, types on the keyboard, drags and drops, or does any of a few other
things, the operating system produces an event. This event is passed to Java, and the Java runtime
tries to figure out which component the event was intended for.
The Java runtime then passes that event to the handleEvent(Event e) method of some
Component. The Component's handleEvent() method contains a big if-else statement to look
at the type of event and respond appropriately. What the handleEvent() method does depends
on the type of component. Generally some events are ignored and other events are passed to
methods that know how to respond to those events. For instance a MOUSE_DOWN event is
passed to mouseDown(). The table below shows the events the default handleEvent() can deal
with. When one of these events occurs, handleEvent() passes it and various other information
to the specified method.
Event Method Called
--------------------------------------------------------
Event.MOUSE_ENTER: mouseEnter(evt, evt.x, evt.y)
Event.MOUSE_EXIT: mouseExit(evt, evt.x, evt.y)
Event.MOUSE_MOVE: mouseMove(evt, evt.x, evt.y)
Event.MOUSE_DOWN: mouseDown(evt, evt.x, evt.y)
Event.MOUSE_DRAG: mouseDrag(evt, evt.x, evt.y)
Event.MOUSE_UP: mouseUp(evt, evt.x, evt.y)
Event.KEY_PRESS: keyDown(evt, evt.key)
Event.KEY_ACTION: keyDown(evt, evt.key)
Event.KEY_RELEASE: keyUp(evt, evt.key)
Event.KEY_ACTION_RELEASE: keyUp(evt, evt.key)
Event.ACTION_EVENT: action(evt, evt.arg)
Event.GOT_FOCUS: gotFocus(evt, evt.arg)
Event.LOST_FOCUS: lostFocus(evt, evt.arg)
By default each of these methods does nothing. Furthermore, by default all other events do
nothing. If you want to do something when one of the above events occurs in your component,
subclass the component and override the appropriate method. For example, to create a Canvas that
prints the message "Don't Tread on Me!" every time the user clicks on it, use the following
subclass of Canvas:
public class noTread extends Canvas {
public boolean mouseDown(Event e, int x, int y) {
System.out.println("Don't Tread on Me!");
return true;
}
}
Button, Choice, TextField, and Checkbox are special because of how they're implemented as
native widgets. They do not see MOUSE_DOWN, MOUSE_UP, KEY_PRESS, and other such events.
Rather they receive one event and one event only: Event.ACTION_EVENT. When a Button is
pressed, the user hits return in a TextField, the user double clicks on a List item, clicks a
Checkbox, or selects from a Choice, an ACTION_EVENT is generated. If you haven't overridden
handleEvent(), this is passed to the action() method of the component's container. These
four components are the only things which generate ACTION_EVENTs. No other component will
generate an ACTION_EVENT. (Menu items, which are not components but are implemented as
native peers, also generate action events.)
Each event handler method, including handleEvent(), returns true if the event was completely
handled and false if it was not. If false is returned, then the event is passed to the component's
container to see if the container wants to process the event.
If you want to respond to a different set of events, you must override handleEvent(). You can
either completely replace it or just add the extra events you want to handle and then return
super.handleEvent() for other types of events. For example to print all the events as they
happen in your applet you might use this handleEvent() method:
public boolean handleEvent(Event e, int x, int y) {
System.out.println(e);
return super.handleEvent(e, x, y);
}
mouseDown(), mouseUp(), keyDown(), and all the other event methods would still be called by
the superclass. If you don't handle an event in the handleEvent() method, return
super.handleEvent(e) to give your superclass an opportunity to process the event. In this
case, we've handled the event but not completely so we still let the superclass process it by
returning super.handleEvent(e, int x, int y).
The return value of handleEvent() is important. If handleEvent() returns true, that means
the event has been fully digested and no further processing is needed. On the other hand if
handleEvent() returns false, then the event is passed to the component's container to be
handled.
Note that this all works for any component at all. You'll most commonly override these methods
in a subclass of Applet or perhaps Frame. However the same methods and techniques apply to all
subclasses of Component.
The API documentation confuses the issue because of the ambigious use of the word parent. In
object-oriented circles the word parent usually refers to the superclass. However in the context of
the AWT, the word parent refers to the container that contains a component. Thus in the statement:
return true if the event has been handled and no further action is necessary; false if
the event is to be given to the component's parent
the word parent means the component's container, not its superclass.
You must distinguish between the container/component hierarchy and the inheritance hierarchy.
By default, the way an event travels has nothing to do with the inheritance hieararchy. If you
override an event handling method, then events passed to that method will not be passed to a
component's superclass unless you specifically ask them to be. However, in most cases your
handleEvent() method will return super.handleEvent() to give the superclass a crack at
handling the event rather than simply returning false.
To sum up, when you write a subclass of Component, you can either respond to specific events by
overriding the methods that correspond to those events, such as mouseDown(); or you can respond
to all events by overriding handleEvent(). Each of your event handling methods must return a
boolean. Return true when an event has been completely processed, and return false when you
want the component's container to try to handle the event. When overriding handleEvent(), you
also have an option to return super.handleEvent() to ask the superclass to handle the event
and decide whether or not to pass it along to the component's container.
7.4: Clipping
java.awt.Graphics.clipRect(int, int, int, int) and related methods are hopelessly flawed, at least as
of 1.0. Ignore them completely.
Instead if you need to do clipping, create separate offscreen Images for each clipping region. Each
Image should be the size of the clipping region you desire. Draw into those offscreen images, and
then copy them onto the appropriate section of the of your applet window using
java.awt.Graphics.drawImage(). Some coordinate conversion will almost certainly be necessary.
If the background image isn't a simple color then you'll first need to copy the appropriate part of
that image to your offscreen clipping Image. You can do this by drawing your background Image
into your offscreen region with Graphics.drawImage() and a suitable shifting of coordinates.
This all works for rectangular regions only since all Images are rectangular. More complicated
geometries can be faked if all but one section contains only simple colors.
7.5: How do I change the thickness of the line?
Java 1.1 and earlier only support 1 pixel wide lines. There's no easy way around this. You can,
however, draw multiple, parallel lines offset from each other by one pixel:
public void paint(Graphics g) {
int x1=5;
int x2=278;
int y1=8;
int y2=93;
// Draw a ten pixel thick line
for (int i = -5; i < 5; i++) {
g.drawLine(x1+i,y1+i,x2+i,y2+i);
}
}
This isn't perfect. The ends of the line are excessively tapered. You really need to take the slope of
the line into account when incrementing x and y, but this should give you the idea. If you're doing
a lot of this, you can write a class or method to do it for you.
There are other hacks you can use. For example, a thick line is essentially a filled rectangle.
Therefore you can calculate the endpoints of the rectangle and use fillPolygon() to draw it.
The real solution is going to have to wait for a more complete graphics API for Java, possibly in
Java 1.2.
7.6: What fonts does Java support?
Java implementations are guaranteed to have the fonts Helvetica, Courier, TimesRoman, and
Symbol or some reasonable facsimile thereof. Whether fonts installed on the client are available to
Java is implementation depenedent. You can get a String array of the names of the available
fonts by calling the getFontList() method from java.awt.Toolkit. For example,
String[] fonts = Toolkit.getDefaultToolkit().getFontList()
8: java.io
8.1:How do I read data from a file?
There are a number of ways to read data from a file. If you're reading a file as raw binary data, you
open a file using a FileInputStream(String) constructor and use one of the various read()
methods to read the data into an array of bytes. For example the following program reads raw data
from a file specified on the command line. It then writes the same data to the standard output.
import java.io.*;
class ReadRawData {
public static void main (String args[]) {
boolean done = false;
byte b[] = new byte[1024];
int num_bytes = 0;
FileInputStream fin = null;
try {
fin = new FileInputStream(args[0]);
}
catch(ArrayIndexOutOfBoundsException e) {
System.out.println("You have to give me the name of a file to open.");
System.exit(0);
}
catch (FileNotFoundException e) {
System.out.println("Could not open input file " + args[0]);
System.exit(0);
}
catch(IOException e) {
System.out.println("Error while opening input file" + args[0]);
System.exit(0);
}
catch (Exception e) {
System.out.println("Unexpected exception: " + e);
System.exit(0);
}
try {
num_bytes = fin.read(b);
}
catch(IOException e) {
System.out.println("Finished Reading: " + e);
done = true;
}
catch (Exception e) {
System.out.println("Unexpected exception: " + e);
System.exit(0);
}
while(!done) {
System.out.write(b, 0, num_bytes);
try {
num_bytes = fin.read(b);
}
catch(IOException e) {
System.out.println("Finished Reading: " + e);
done = true;
}
catch (Exception e) {
System.out.println("Unexpected exception: " + e);
System.exit(0);
}
if (num_bytes == -1) done = true;
} // end while
} // end main
} // end ReadRawData
On the other hand if you're reading a text file in Java 1.0 you'll probably want to use a
DataInputStream which gives you a readLine() method that returns successive lines of the
file as Java Strings. You can then process each String as you see fit.
// Implement the Unix cat utility in java
import java.io.*;
class cat {
public static void main (String args[]) {
String thisLine;
//Loop across the arguments
for (int i=0; i < args.length; i++) {
//Open the file for reading
try {
FileInputStream fin = new FileInputStream(args);
// now turn the FileInputStream into a TextFileStream
try {
DataInputStream myInput = new DataInputStream(fin);
try {
while ((thisLine = myInput.readLine()) != null) { // while loop begins here
System.out.println(thisLine);
} // while loop ends here
}
catch (Exception e) {
System.out.println("Error: " + e);
}
} // end try
catch (Exception e) {
System.out.println("Error: " + e);
}
} // end try
catch (Exception e) {
System.out.println("failed to open file " + args);
System.out.println("Error: " + e);
}
} // for ends here
} // main ends here
}
This code emulates the Unix "cat" command. Given a series of filenames on the command line it
concatenates the files onto the standard output.
In Java 1.1 DataInputStream.readLine() is deprecated. You should use a BufferedReader
instead.
8.2: How do I write data to a file?
You should only assume you'll be able to write to a file from an application. Although it may be
possible to write data into a file from an applet if the browser viewing the applet is HotJava, this
ability will generally be disabled. From within Netscape there is no way for an applet to write to a
file on the local hard drive.
Within an application, however, file access is straight-forward. There are several ways but here is
a simple example using formatted output streams:
import java.io.*;
class PrintToAFile {
public static void main (String args[]) {
//First open the file you want to write into
try {
FileOutputStream fout = new FileOutputStream("test.out");
// now convert the FileOutputStream into a PrintStream
PrintStream myOutput = new PrintStream(fout);
// Now you're able to use println statements just as if you were using System.out.println
// to write to the terminal
myOutput.println("Hello There!");
myOutput.println(1 + " + " + 1 + " = " + (1+1));
}
catch (IOException e) {
System.out.println("Error opening file: " + e);
System.exit(1);
}
} // main ends here
}
There are a number of other things to note about writing data to a file. This program creates or
opens a file called "test.out" in the same directory as the running program. However you could
pass it a full pathname to a file in a different directory instead.
You should also learn about the DataOutputStream class and the write() method when you
get a chance. DataOutputStreams and DataInputStreams are used for moving data between
Java programs in a portable way. The various incarnations of the write() method are used for
writing and reading arbitrary byte streams. What I've demonstrated here is more suitable for
human consumption.
8.3: How do I append data to a file?
The java.io.RandomAccessFile class lets you read and write bytes from arbitrary locations
in a file. It also implements DataInput and DataOutput so you have all the methods of
DataInputStream and DataOutputStream available to you. To create a new random access
file pass the name of the file and the mode to the constructor. The mode is either "r" (read-only) or
"rw" (read and write). The length() method returns a long that tells you how many bytes there are
in a file and the seek(long p) method lets you position the file pointer at a particular point in
the file. Thus to start writing at the end of a RandomAccessFile raf, you first
raf.seek(raf.length()). The following example demonstrates by appending the string
"Kilroy was here!" to every file specified on the command line.
import java.io.*;
class AppendToAFile {
public static void main (String args[]) {
for (int i = 0; i < args.length; i++) {
//First open the file you want to append to
try {
RandomAccessFile raf = new RandomAccessFile(args, "rw");
// Position yourself at the end of the file
raf.seek(raf.length());
// Write the String into the file. Note that you must
// explicitly handle line breaks.
raf.writeBytes("\nKilroy was here!\n");
}
catch (IOException e) {
System.out.println("Error opening file: " + e);
}
}
}
}
9: java.net
Network programming in Java is covered in much more detail in my book, Java Network
Programming from O'Reilly & Associates. Most of what is new and exciting about Java centers
around the potential for new kinds of dynamic, networked applications; and Java Network
Programming shows you how to write them. Topics covered include sockets, URLs, InetAddresses,
UDP, TCP/IP, multicasting, applets, servlets, RMI, and more.
Manning Publications has also recently published a book titled Java Network Programming. It's
not a bad book, and is surprisingly orthogonal to mine. About 2/3 of that book is streams and
encryption which I only touch on. My book covers servlets, applets, RMI, multicast sockets, and
Java 1.1 which that book doesn't discuss. The matching titles appear to be just unlucky choices.
Both publishers went with the most obvious title they could think of. However the cover of the
Manning book has a big fish, and looks suspiciously like an O'Reilly book. Don't be fooled. The
real O'Reilly book has a gyroscope on the cover.
O'Reilly has the table of contents and the index on their web site. You can buy Java Network
Programming at any bookstore that stocks computer books, or you can order it from amazon.com
or cbooks.
9.1: How do I convert a numeric IP address like 199.1.32.90 into a hostname like
star.blackstar.com?
Unfortunately due to an unintended side effect (i.e. a bug) in Java's caching of IP addresses and
hostnames, Java 1.0 can't convert numeric IP addresses into hostnames. However this is
straightforward in Java 1.1:
String hostname = InetAddress.getByName("199.1.32.90").getHostName()
9.2: How can a Java program talk to a CGI program?
Web browsers display forms, read user input, encode that input into a standard format called a
"query string", and send that data to CGI programs that live on the web server. When you write an
applet that talks to a CGI program, you have to do all this yourself.
The first thing to know is that there are two ways a CGI program can accept data from a web
browser, GET and POST. CGIs that use GET take their arguments from the URL. Programs that
use POST read their arguments from standard input.
The second thing to know is that when you submit data to a form through a web browser, the web
browser encodes the data for you. In an applet, however, you need to encode the data yourself. The
data is encoded like this: Each form entry is a name-value pair. Names and values are separated
from each other by equals signs (=). Pairs are separated from each other by ampersands (&). For
example, consider this form:
<Form method=GET action="http://sunsite.unc.edu/javafaq/cgi-bin/getform.pl>
Email: <Input NAME="email" size=40>
Name: <Input NAME="realname" size=40>
<Input TYPE="submit" VALUE="Subscribe">
</Form>
You see that this uses the GET method to communicate with a cgi-bin program at
http://sunsite.unc.edu/javafaq/cgi-bin/getform.pl.It sends two fields to the CGI program, email
and realname. Let's say you want to send the string "elharo@sunsite.unc.edu" for the email
address, and the string "Elliotte Harold" for the real name. Then the query string would look like
this:
String qs = "email=elharo%40sunsite.unc.edu&realname=Elliotte%20Harold";
The spaces in "Elliotte Harold" and the @ in "elharo@sunsite.unc.edu" have been converted into
percent escapes. All non-alphanumeric characters in the values must be replaced with a %
followed by their ASCII value. Thus a space becomes %20 and the @ becomes %40.
To send this data to the server, append a question mark (?) and the query string to the URL of the
CGI program, and request that URL from the server. Thus the URL you want is:
http://sunsite.unc.edu/javafaq/c ... ro%40sunsite.unc.ed
u;realname=Elliotte%20Harold";
In Java terms this requires constructing a URL object from this string, and opening that URL's
InputStream to read the response. The following code fragment demonstrates:
try {
String thisLine;
String qs = ""email=elharo%40sunsite.unc.edu&realname=Elliotte%20Harold";
URL u = new URL("http://sunsite.unc.edu/javafaq/cgi-bin/getform.pl? + qs);
DataInputStream theHTML = new DataInputStream(u.openStream());
while ((thisLine = theHTML.readLine()) != null) {
System.out.println(thisLine);
}
}
catch (Exception e) {
System.err.println(e);
}
Communicating with CGI programs that use POST is somewhat more complex, and it doesn't
work very well in Java 1.0.2. It may be improved in Java 1.1. When POSTing to a CGI, you encode
the query string exactly as you do for GET requests. However instead of merely requesting a
URL's InputStream, you open a URLConnection to the CGI program.
Do not append the query string to the URL as you did with GET. Instead set the
URLConnection's doOutput and doInput fields to true and set AllowUserInteraction to
false. Chain the URLConnection's OutputStream to a DataOutputStream and use the
DataOutputStream's writeBytes() method to send the query string to the server.
If you want to read the response, then chain the URLConnection's InputStream to a
DataInputStream, and use the DataInputStream's readLine() method to read the response
in a while loop. The following code fragment demonstrates:
String query = "email=elharo%40sunsite.unc.edu;realname=Elliotte%20Harold";
try {
// open the connection and prepare it to POST
URL u = new URL("http://sunsite.unc.edu/javafaq/cgi-bin/postform.pl);
URLConnection uc = u.openConnection();
uc.setDoOutput(true);
uc.setDoInput(true);
uc.setAllowUserInteraction(false);
DataOutputStream dos = new DataOutputStream(uc.getOutputStream());
// Send the data
dos.writeBytes(query);
dos.close();
// Read the response
DataInputStream dis = new DataInputStream(uc.getInputStream());
String nextline;
while((nextline = dis.readLine()) != null) {
System.out.println(nextline);
}
dis.close();
}
catch (Exception e) {
System.err.println(e);
}
As you see, posting forms is considerably more complex than using the GET method. However on
some platforms, GET has an annoying habit of failing once the query string grows past 200
characters. The exact point where GET fails varies depending on the operating system and the web
server.
9.3: How can an applet send email?
The simplest way to answer this question is to tell you to write a CGI program which sends email,
and then refer you to the previous question to learn how to communicate with the CGI program.
There really isn't any other reliable, cross-platform way to send email. The problem is that email
programs are platform dependent.
The Simple Mail Transport Protocol is an Internet standard that you may be able to use some of
the time. The basic idea is to open a socket to the SMTP port, 25, on the web server, and send your
email through the server. However not all web servers, especially those based on Macs or
Windows NT, run SMTP servers, and an applet can't open sockets to other hosts. Even if you do
have SMTP software running on your web server, the applet may be behind a firewall that
disallows outgoing connections to port 25 on hosts outside the firewall.
9.4: How can I use ICMP in Java?
Java does not support ICMP, the Internet Control Message Protocol, at this time; nor does it allow
you to send raw IP packets. You must use TCP or UDP. Therefore protocols that rely on ICMP
like ping and traceroute cannot yet be implemented in Java.
10: java.util
10.1: How random is Random()?
It's good enough for games. I wouldn't use it for cryptography.
Unlike most random functions in other libraries the Math.random() method seeds itself with the
current time in milliseconds. Thus you do not need to seed it explicitly at the start of your
program. If you require a non-random Random() for test purposes or you need more randomness
than the current time in milliseconds can provide, then you can use java.util.Random() which
has a constructor that lets you specify a seed.
10.2: How do I generate a random integer between a and b?
Generate a random double between 0.0 and 1.0, multiply by the number of int values you want to
choose from, add the smallest int value you want, round it down to the nearest integer using
Math.floor(), and cast the result to an int. For example, the following class simulates a
six-sided die by producing random, uniformly distributed ints between 1 and 6.
public class Die {
Random generator = new Random();
public static int roll() {
// get a rcandom number between 0 and 1
double r = generator.nextDouble();
// multiply by 6 so it's now between 0 and 6
r *= 6.0;
// add 1
r += 1.0;
//truncate it to an int
r = Math.floor(r);
// handle one special if unlikely case
if (r == 7.0) r = 6.0;
// convert to an int and return
return (int) r;
}
}
11: Common Errors and Problems
11.1: How Can I Avoid Flicker in an Applet?
The key to fixing flicker is realizing that the screen isn't actually painted in the paint() method.
The pixels get put on the screen in the update() method which most applets don't override.
However by overriding the update() method you can do all your painting in an offscreen Image
and then just copy the final Image onto the screen with no visible flicker.
The cookbook approach is simple. Add the following three private fields to your applet and the
public update() method. Flicker will magically disappear.
private Image offScreenImage;
private Dimension offScreenSize;
private Graphics offScreenGraphics;
public final synchronized void update (Graphics g) {
Dimension d = size();
if((offScreenImage == null) || (d.width != offScreenSize.width) || (d.height != offScreenSize.height)) {
offScreenImage = createImage(d.width, d.height);
offScreenSize = d;
offScreenGraphics = offScreenImage.getGraphics();
}
offScreenGraphics.clearRect(0, 0, d.width, d.height);
paint(offScreenGraphics);
g.drawImage(offScreenImage, 0, 0, null);
}
11.2: Can you explain CLASSPATH?
A path specifies the name and location of a file on the file system. It starts with the name of the
disk or the root of the filesystem and works its way down through various directories until reaches
the file. File, directory, and path naming conventions are platform specific. For example a Unix
path looks like /home/users/elharo/html/javafaq.html. A DOS/Windows path looks
like C:\html\javafaq.htm. A Macintosh path looks like My Hard Drive:html:Java FAQ
List v1.1. All three of these examples point to a file. Paths can also point to a directory. For
example, /home/users/elharo/html, C:\html, or My Hard Drive:html:.
The character that separates one directory from the next in a path is called the separator character.
It is a slash (/) on Unix, a backslash (\) in Windows and a colon (:) on the Mac. You can get its
value on a particular platform by looking at the static variable
java.io.File.separatorCharacter.
If you actually check this on the Mac, you'll note something funny.
java.io.File.separatorCharacter appears to be a slash (/) like on Unix, not a colon like a
Mac programmer would expect. Why Java had to be different from every other Mac program in
the universe I don't know. This is problematic because Mac file names can include slashes.
The CLASSPATH is an environment variable that contains a list of directories where Java looks
for classes referenced in a program. If the CLASSPATH isn't set properly no program written in
Java will be able to run, and the compiler won't be able to compile. Each entry in this list is
separated from the other entries by the java.io.File.pathSeparatorChar. This is
semicolon (;) on Windows and a colon (:) on Unix and the Mac. For example
Unix: ~/classes:/usr/local/netscape/classes
Windows: C:\java\classes;C:\netscape\classes
Mac: My Hard Drive/JDK/classes:My Hard Drive/My Project:My Hard Drive/classes
On most platforms, the JDK's java interpreter appends some directories to the CLASSPATH you
set manually. These are set relative to where the java interpreter itself is. For example, if the java
program is installed in /usr/local/java/bin, then it will append /usr/local/java/classes and
/usr/local/java/lib/classes.zip to the CLASSPATH. Another way of thinking about it: if the
directory where the java interpreter is installed is $JAVA, then $JAVA/../classes and
$JAVA/../lib/classes.zip are automatically in your CLASSPATH.
Java applets and applications aren't self-contained. They need access to other classes to do their
work. For instance when you call System.out.println() Java needs to know where to look to
find the file that includes the System class.
The directories in the CLASSPATH are where Java starts searching for classes. To find a class
Java first changes the periods in the full package-qualified name of the class (e.g.
java.util.Date and not just Date) into directory separators (/ on Unix, \ on Windows, : on the
Mac). Thus if it wants the java.awt.GridBagLayout class, it looks for the file
java/awt/GridBagLayout.class in each of the root directories listed in the CLASSPATH
variable from left to right until it finds the file. With the Unix CLASSPATH listed above, Java
first looks for ~/classes/java/awt/GridBagLayout.class Then for,
/usr/local/netscape/classes/java/awt/GridBagLayout.class.
The specification of the CLASSPATH is somewhat platform dependent. For instance ~ means the
home directory on Unix but has no meaning on the Mac.
Under Unix you set CLASSPATH variables like this:
csh: % setenv CLASSPATH my_class_path
sh: % CLASSPATH=my_class_path
You'll probably want to add one of these lines to your .login or .cshrc file so it will be
automatically set every time.
Under Windows you set the CLASSPATH environment variable with a DOS command like
C:\> SET CLASSPATH=C:\JDK\JAVA\CLASSES;c:\java\lib\classes.zip
You can also add this to your autoexec.bat file. You should of course point it at whichever
directories actually contain your classes.
The CLASSPATH variable is also important when you run Java applets, not just when you
compile them. It tells the web browser or applet viewer where it should look to find the referenced
.class files. If the CLASSPATH is set improperly, you'll probably see messages like "Applet could
not start."
Since large packages can contain many, many .class files Sun has built the capability to read zip
archives into Java. Therefore an entire directory structure of class files can be zipped to save space.
If you want to see what's inside the zip file, unzip it. Java doesn't care whether or not a directory
has been zipped. You just need to make sure that the .zip file is named the same as the directory it
replaces plus the .zip extension and that it is in the same location.
In Netscape you should make sure that the first directory in the CLASSPATH is the directory that
contains Netscape's class files (The defaults are /usr/local/netscape/java/classes on Unix and
C:\NETSCAPE\NAVIGATOR\Program\java\classes in Windows.)
Finally note that if you install additional packages such as Jeeves or any third party package, you
need to add the directory where the package is installed to your CLASSPATH. For example let's
say you buy a package of statistics classes from SPSS, and you put those classes in
/opt/classes/stats. Then you you need to add /opt/classes/stats to the end of your CLASSPATH.
You can temporarily add a directory to the CLASSPATH by giving the -classpath option to the
java interpreter or the javac compiler. For example,
javac -classpath $CLASSPATH:/opt/classes/stats
To use just the classes in /opt/classes/stats and not the classes normally found in your
CLASSPATH, omit $CLASSPATH like this:
javac -classpath /opt/classes/stats
Finally if the CLASSPATH environment variable has not been set, and you do not specify one on
the command line, then Java sets the CLASSPATH to the default:
Unix: .:$JAVA/classes:$JAVA/lib/classes.zip
Windows: .:$JAVA\classes:$JAVA\lib\classes.zip
Mac: ./$JAVA:classes/$JAVA:lib:classes.zip
Here . is the current directory and $JAVA is the main Java directory where the different tools like
javac were installed.
11.3: Why won't my audio clip play?
It's probably in the wrong format. Java 1.0 only understands 8 bit, mulaw-encoded, 8000 Hz,
one-channel files. Many sound conversion programs will convert various formats to .au files but
not necessarily to 8 bit, mulaw-encoded, 8000 Hz, one-channel files.
11.4: When I load the page Netscape gives me a java.lang.ClassFormatError.
Something is mangling the .class file. Most likely the .class files were uploaded to the server as
text or MacBinary rather than as raw binary data. Make sure you put your ftp program in binary
mode before sending the files to the server.
The other possibility is that the web server is sending the file to clients as text rather than binary
data. Make sure the web server is configured to send files that end in ".class" with a MIME type of
application/octet-stream. Many web servers send files as type text/plain which often works but
causes problems on a few servers. In particular, WebStar needs to change the action to "binary"
and the MIME type to "application/octet-stream".
It's also possible on some platforms that Netscape just can't find the .class file; that is, it isn't in
the directory where Netscape is looking for it. Technically, this isn't really a ClassFormatError,
but this is how Netscape reports it on some platforms and versions.
11.5: Netscape gives me "Applet Not Initialized Error"
This is almost always means Netscape can't find one of the classes it needs to run the applet. Check
to make sure that the classes your program uses are in the CODEBASE, the CLASSPATH, or
somewhere else Netscape can find them. It's not uncommon to get this error when you first test a
new package or class you've written with Netscape. If you've only tested it with the applet viewer
or an IDE, then the applet viewer or the IDE may have included the current directory in the
CLASSPATH where Netscape does not. Therefore the applet viewer can find the right class, but
Netscape can't. Explicitly add the path containing your class or package to the CLASSPATH as
specified in the previous question.
11.6: Other Netscape Problems
As of version 3.0, Netscape has many problems handling Java applets.In no particular order they
are:
Netscape can only run applets that are compiled with the Java 1.0 beta compilers through the
Java 1.0.2 compiler. It cannot handle .class files compiled for the various alpha versions of
Java or Java 1.1. As a general rule, make sure you're using Sun's javac 1.0.2 to compile all
applets you distribute. (Microsoft and Natural Intelligence's compilers have non-trivial bugs
of their own.)
Not all versions of Netscape support Java. Notably the Windows 3.1 version does not,
though a beta version that does has recently become available.
Netscape will not import any user-defined packages from the local CLASSPATH.
Netscape does not flush classes when you reload a page. If you need to reload a changed
applet, for instance if you made a change to the applet source code and regenerated the .class
file, there are a number of things you can try. On some platforms holding down the shift key
and pressing the reload button will get Netscape to reload the .class file. You can also try
manually clearing both the memory and disk caches, in order to reload an applet. Turning
Java off and then on again in the Options/Security Preferences... menu may also
do the trick. If none of these work, you'll simply have to quit and relaunch Netscape.
Netscape calls your init() method many times more than the appletviewer will. In
particular Netscape calls init() anytime your reload, resize or otherwise restart your
applet. I am not sure whether or not this is a bug, but it does indicate that you should try to
keep your init() methods as small and quick as possible. Similarly it may call
destroy() when the applet viewer would only call stop().
Netscape won't run any applet that calls Object.clone().
Netscape can't print applets.
11.7: I keep getting access privilege violations, " Java tried to read file foo.class in
directory bar."
This happens when you try to load a class from a local hard disk instead of a server, the class is not
a subclass of Applet, and the directory where the class resides is not in the HotJava read path. The
solution is to move the class file into a directory in the HotJava read path. On Unix one such
directory is ~/public_html/classes (where ~ is your home directory). Alternatively you can change
the HOT_JAVA_READPATH environment variable to point to the directory containing your
classes directory.
11.8: Javac keeps insisting that it can't find the right constructor function in my
class, but I'm sure it's in a file I imported. Why is it even looking in my class
instead of the imported file?
You cannot construct instances of a class on the fly using syntax like
System.out.println(Double(0.56).toString());
You must use the new operator like:
Double myDouble = new Double(0.56);
System.out.println(myDouble.toString());
or
System.out.println((new Double(0.56)).toString());
In other words, constructors are only called after a new operator.
11.9: NullPointerExceptions with arrays of objects
When you allocate an array of objects, each component of the array is initialized to null. The
individual components of the array must still be initialized with a constructor or an assignment
statement. For example, consider this statement:
Integer[] scores = new Integer[10];
int m = scores[5].intValue(); // throws NullPointerException
This creates an array called scores containing ten references to Integer objects. Then it tries to
get the value of the fifth component. However, each of those references is initially set to null.
Thus when you try to call a method on one of the components of the array or pass the component to
a method that expects a non-null argument, a NullPointerException is thrown.
To fix this, you need to initialize the components of the array, either with constructors or with
assignment statements For example:
Integer[] scores = new Integer[10];
for (int i = 0; i < scores.length; i++) scores = new Integer(i);
int m = scores[5].intValue();
You do not need to initialize all the components of the array though it's a good idea to do so. You
can initialize just those you'll use, or you can make sure you catch and handle
NullPointerExceptions in the appropriate places.
This is different from how Java handles uninitialized non-array reference variables. By way of
contrast when you write,
Integer score;
int m = score.intValue();
the compiler catches the null reference in score and complains. You have to fix the problem
before you can compile the program. However in general the compiler has no way to know
whether an array component has or has not been initialized. Therefore the check for the
non-nullness of an array component is deferred till runtime when the NullPointerException
may be thrown.
11.10: Can't make static reference to method type myMethod in class myClass
This common error occurs when you attempt to call an instance method from a static method, most
commonly main(). For example,
class StaticTest {
public static void main(String[] args) {
int i = getX();
}
public int getX() {
return 3;
}
}
The static method, main(), belongs to the class. However, getX() belongs to an object in the
class. The compiler doesn't know on which object it's invoking the getX() method.
There are a couple of ways around this problem. You could declare that getX() is also static;
that is:
public static int getX() {
Alternately, you can instantiate an object in the StaticTest class in the main() method and invoke
that object's getX() method, like this:
public static void main(String[] args) {
StaticTest st = new StaticTest();
int i = st.getX();
}
11.11: Why do I have trouble when using == to compare Strings?
When used on objects, == tests whether the two objects are the same object, not whether they have
the same value. Consider this code fragment:
String s1 = new String("Hello World");
String s2 = new String("Hello World");
if (s1 == s2) {
System.out.println("The strings are the same.");
}
else {
System.out.println("The strings are different.");
}
This prints "The strings are different."
To compare to objects for equality, rather than identity, you should use the equals() method,
like this:
String s1 = new String("Hello World");
String s2 = new String("Hello World");
if (s1.equals(s2)) {
System.out.println("The strings are the same.");
}
else {
System.out.println("The strings are different.");
}
This prints "The strings are the same." The default equals() method all objects inherit from
java.lang.Object just tests for object identity with ==. However many classes, including
java.lang.String, override equals() to test the state of the object.
However, the issue is a little confused when string literals are considered. Consider this code
fragment:
String s1 = "Hello World";
String s2 = "Hello World";
if (s1 == s2) {
System.out.println("The strings are the same");
}
else {
System.out.println("The strings are different");
}
This prints "The strings are the same". The compiler recognizes that the two string literals have the
same value and it performs a simple optimization of only creating one String object. Thus s1
and s2 both refer to the same object and are therefore equal. The Java Language Specification
requires this behavior. However, not all compilers get this right so in practice this behavior here is
implementation dependent.
12: Security
12.1: System Level Security
On a system level Java applets live in a cage. They can do pretty much anything they want inside
their cage, but they cannot get outside the cage and do damage unless the user unlocks the door to
the cage. Even then they wear shackles that prevent them from doing things like writing to
arbitrary addresses in memory.
In Netscape the user doesn't even get the keys to the cage so you can't let the applets out into your
system even if you wanted to. With the Applet Viewer it is possible to give your applets more
access to the network. However they still can't do everything a C program can do.
Specifically a Java applet cannot write anything to a hard drive. It cannot write directly to
memory, and it cannot introduce a virus into your system. A Java applet can use a lot of your CPU
time though not 100% of it. It cannot crash your system (though a Java enabled browser can).
12.2: User Level Security
A Java applet can trick the user into doing something stupid such as revealing their root password
though there are efforts to avoid this. A Java applet can also send this information back to the
applet's server.
Finally an applet can display pictures or text or play sounds which the user may find annoying or
offensive, but this hardly qualifies as a security flaw and is not unique to Java enabled browsers.
13: Tools
13.1: IDE
Symantec publishes Cafe, an applet development environment for Windows 95, NT and the Mac.
It includes a source code editor and a class browser, and a just-in-time compiler. It is still beta
quality at best. I do not recommend it.
Natural Intelligence's Roaster is a Macintosh hosted applet development environment. It has been
designed from the public specs with no help from Sun. It is also still beta quality. I do not
recommend it.
Metrowerks includes Java support in Code Warrior Gold. Metrowerks has licensed Sun's source
code. The Java support is alpha quality, and is not by itself worth the price of Code Warrior, even
the $99 Java only edition. However if you also need a C compiler it doesn't cost you anymore to
get Java support.
Sun publishes a payware IDE called Java Workshop. It is fairly cheap, ($99) but again not yet
worth the download time. Java Workshop is similar to Sun's existing Workshop products for
Fortran and C++. However it is written entirely in Java. You'll need at least a Pentium Pro to
squeeze adequate performance out of this system.
Microsoft's Visual J++ is one of the faster VM/JIT combinations available. However the compiler
has some bugs that prevent it from compiling certain legal Java code. Once again, despite the 1.0
designation this is beta software at best, and is not worth spending money on.
The bottom line is that you should use Sun's JDK (or your local platform's equivalent) and the text
editor of your choice. None of the available IDEs are worth what they cost. All will cause you
more problems than they solve. Unless you enjoy paying to beta test products, there's no reason to
purchase any of these products at this time.
13.2: Debugger
A preliminary jdb is available with Java Developer's Kit from Sun. Various payware IDEs include
debuggers of beta quality at best.
13.3: Editors
I use BBEdit 4.0 on the Mac. The free, light version works well too although it doesn't include
syntax coloring. See
http://www.barebones.com/bbedit.html
However, in general any normal text or programmer's editor should be fine.
The Windows Notepad program, however, is not fine. It appends the extension ".txt" to all files it
saves. Java source code files must end in ".java". If you must use Notepad, enclose the filename in
double quotes when you save it; that is don't just type HelloWorld.java but rather
"HelloWorld.java". This will force Notepad not to append a ".txt" extension to the file.
13.4: YACC grammar
An LALR grammar for Java is included in The Java Language Specification.
13.5: Profilers
Using the -prof option to the java interpreter produces basic profile information in a file called
java.prof. That is,
% java -prof HelloWorld.class
14: The Java Virtual Machine and Byte Codes
The most recent Java Class File / Java Virtual Machine specs are available in various formats from
http://www.javasoft.com/java.sun.com/newdocs.html
14.1: Can I program directly in byte code?
Yes, you can. Jonathan Meyer's Jasmin Java assembler is a small assembler for Java byte code, and
would be of great assistance to you in such an endeavor. The Java interpreter will check your byte
codes to make sure they don't violate security. If you write security violating byte codes the
interpreter won't run them.
14.2: Optimizing Java
1.Make your classes final.
2.Use the -O flag to the compiler.
3.If you're reading data off a disk read it in as large a chunk as possible. Do not read it a byte
at a time. If you need to process the data a byte at a time read it into a temporary buffer first.
4.Creating objects is expensive. Try to reuse objects rather than creating new ones, especially
for temporary objects that exist only to support a calculation.
5.Avoid synchronization where possible.
15: Legal Issues
Some portions of Java and HotJava are neither in the public domain nor copylefted. However no
license is required to write and distribute most Java applications and applets. Nor is any license
from Sun is required to distribute the Java or HotJava in binary form.
The source code for Java 1.0 is freely available for non-commercial use though a license must be
signed. Source code for the java packages is included with the various versions of the JDK,
including 1.1. However, if you want to license the Java source code for commercial use or get
source code for Java 1.1, you will need to negotiate a source code license with Sun. Complete
details are at http://www.javasoft.com/java.sun.com/source.html.
Furthermore whether your product is commercial or non-commercial you may not use the
trademark "Java" to describe your product unless you pass an as yet unreleased verification and test
suite.
16: FAQ's about the FAQ
16.1: Who is this Elliotte guy, and just what qualified him to write a Java FAQ
anyway?
He was willing to do it. Seriously, months on the Java mailing lists made it obvious that a FAQ
was needed and since none seemed to be forthcoming from those more qualified than I, I boldly
and with a complete disregard for my absolute lack of qualifications stepped forward and
volunteered. I do have some hope that my previous experience with FAQ lists, the World Wide
Web and a variety of programming languages may somehow cover up the fact that I have no idea
what I'm doing. You've read the FAQ list by now. Did I succeed?
Why am I taking on such a Herculean task? It's often said that you never really understand a
subject until you teach it. I'm going to endeavor to learn Java and understand it by teaching it. The
result should be interesting at least. Since I'm very new to Java and clearly do not yet understand
the language, I'll have a perspective on the issues that should be helpful to newcomers.
Are there mistakes here? Absolutely. I hope people here and elsewhere will correct me gently and
help lead me on the path to true Java enlightenment when I err. And if they aren't so gentle, well,
I've developed a pretty thick skin after six years of Usenet.
My companion tutorial in how to write Java is going to reflect the programs I'm actually writing.
As I learn to do a Hello World program, I'll write about Hello World. (OK, I've actually already
gotten beyond Hello World but not much beyond.) How fast the FAQ list grows will depend in
large part on how fast my skills grow. You'll get to watch me learn, make mistakes, correct
mistakes and so on.
There may be an advantage to this approach. People who want to learn Java will be coming from
almost identical places as me. I always hate having to write down or talk down to people, and I
don't do it very well. Similarly I find it frustrating to try to communicate with people who are
way beyond me (as obviously some people on the Java mailing lists are). This means that I can
write for people without much prior experience without boring myself or, hopefully, my audience,
to tears.
Finally I'll emphasize that I'm not trying to prevent anyone else or any other team of people from
taking on a similar project. I hope someone does. I'll probably learn from what they do, and they'll
learn from what I do, even if it's only to learn the sorts of things novices are confused about. And
of course not everyone shares the same learning style so while some people may enjoy my
approach to Java I am sure others will find it dense, incoherent and ultimately incomprehensible.
More choices are better.
Ultimately I'm doing this because I think Java is very, very cool and I am convinced I need to learn
it sooner rather than later. I think this project will propel my Java knowledge forward, and if it
does that I'll consider my project a success. If I can help a few other lost souls to climb out of the
pit with me at the same time, so much the better.
16.2: Why don't you include X in your FAQ?
Please do make suggestions about topics for FAQ list inclusion. You can email them to
elharo@sunsite.unc.edu. I do follow the various comp.lang.java groups as best I can, and have a
pretty good idea of what's discussed there.
16.3: This is really cool. Can I republish or translate it?
This FAQ list and its entire contents are copyright 1995-1997 by Elliotte Rusty Harold. If you
have specific desires to republish it in whole or part contact me at elharo@sunsite.unc.edu and tell
me exactly what you want to do. I'm generally receptive to non-profits that want to make it
available to the public at no charge and to anyone who's willing to make reasonable payment
arrangments.
However I do not allow the FAQ list, either the text or HTML version, to be placed on any web or
ftp sites but my own. Mirroring HTML files is a fundamentally bad idea that is at cross-purposes
with the design of the web. Among other problems it makes maintaining up-to-date copies
phenomenally difficult. If you think this file would be a useful addition to your web site, then link
to it. That's the way the web is designed. Not all information needs to reside in one central
location. No permission is needed to link to this or any other of my HTML files. In fact I'd prefer
it if you'd not ask me, since then I don't have to spend time answering you.
Although there's more reason to want to translate the document rather than merely mirror it, I
regret that I do not have the time or resources to support any translation efforts. This document
will be available in English only for the foreseeable future.
If you want to use the source code samples included here, please do. I consider the source code
(though not the FAQ list itself!) to be public domain. No permission is necessary to reuse, edit,
modify or publish any of the source code contained in this FAQ list.
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Copyright 1995-1997 Elliotte Rusty Harold
elharo@sunsite.unc.edu
Last Modified June 15, 1997 |
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发表于 2003-2-6 16:39