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发信人: icefire (35.5), 信区: Java
标 题: JAVA FAQ
发信站: 深大荔园晨风站 (Thu Apr 9 11:29:44 1998), 转信
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is not a sunsite, then this is an illegal mirror copy and is likely to be out
of date.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.html.
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/~leachbj/clj.announce/guidelines.html for 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[i], o[i+1]) < 0) {
swap(o[i], 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.html or 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[i]);
// 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[i]);
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[i], "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:
Email:
Name:
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/cgi-bin/getform.pl?email=elharo%40sunsite.unc.edu
;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[i] = 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.
[ Cafe Au Lait | Books | Course | Examples | Exercises | Trade Shows | Links |
Tutorial | User
Groups ]
Copyright 1995-1997 Elliotte Rusty Harold
elharo@sunsite.unc.edu
Last Modified June 15, 1997
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