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From: elharo metalab.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: 1997/10/06

Version: 1.6

URL:

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

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copy and is likely to be out of date.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.

reproduction, redistribution and other such rights please see the end of

this file.What s New

Question 4.10: How do I load and display formatted HTML.

I ve added some info about Sun s HotJava HTML bean.

Question 8.4: How do I format numbers like C s printf.

I ve added some info about the java.text package.

I ve also added the following two questions:

For the most part, this FAQ list covers Java 1.0.2.

I have begun the slow 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

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 bug free 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. Oak was running its

first programs in August of 1991. Joy got his first demos of the system that winter, when

Gosling and Naughton 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 off-load 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

JDK available for Sparc and X86

Solaris, Windows NT and Windows 95 Intel. The JDK is free, and most third party ports

are based on this.

Sun also published a 1.0.2

JDK for MacOS 7.5 on PowerMacs

and 68030 25 MHz and faster and 68040 Macs. However it s dropped development for Macs in favor

of Apple s own Macintosh Runtime for Java MRJ

which is a better option for Mac users. Currently MRJ

supports Java 1.0. Java 1.1 support is promised soon.

SGI s port of Sun s JDK 1.1.x to IRIX 5.3, 6.2, 6.3, and 6.4 is available at

An IRIX

port of JDK 1.0 has mostly been completed by Simon Leinen.

A Linux port is in fairly good shape. See

IBM has ported Java to Windows 3.1, OS/2, and AIX. It is working on

ports for MVS and OS/400. See

The OSF has ported the JDK 1.0.2 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

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 1.0 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 1.0. Netscape 4.0 and earlier

and Internet Explorer

3.0 and earlier do not support Java 1.1 to any significant extent,

only Java 1.0. The only web browser that really supports Java 1.1 is Sun s

HotJava.

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

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,

COBOL, and Basic.

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 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

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

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

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

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 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 APIcomp.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.

2.4: 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 1.0 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,

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: 1-56592-262-X, 19.95

the tiger book.

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.

None of these books cover Java 1.1. In fact as of this writing, few books

truly cover Java 1.1 though several claim to. The best of the lot as of this writing

seems to be Rogers Cadenhead s Teach Yourself

Java 1.1 Programming in 24 Hours from Sams.net, ISBN 1-57521-270-6.

This is

a decent book introducing Java 1.1 programming to non-programmers. However it s quite basic,

and coverage of the AWT is very limited.

For details about these and many more books see the Cafe au Lait books page at

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. The boolean type

has been added. However the implementation of the data types has been

substantially cleaned up in several ways.

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. Java prevents casting between arbitrary variables.

Only casts between numeric variables and between sub and superclasses of the same object

are allowed. All numeric variables in Java are signed.

Here are the detailed primitive 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 32,767int

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.

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.

Strings are a reference or object type, that is a

instances of the class java.lang.String.

They are not null terminated and are not the same as an array

of chars.

Arrays are also objects. Multidimensional arrays are created via arrays of arrays.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 adds 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:

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

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 JDK 1.0 or the Java Native Interface in JDK 1.1.

Chapter 19 of my book Java

Secrets discusses the Java Native Interface in depth.

IDG Books, 1997, ISBN

0-764-58007-8, 0764580078/cafeaulaitA/

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 examplepublic 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

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.

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. 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.

Recently Sun released the 195 payware

HotJava HTML Component.

This is a Java bean which parses and renders HTML. In requires Java 1.1 or later.

There is at least one semi-free third party solution. Data Technology s Ice Browser

can handle most HTML 3.2 constructs, though I haven t had a chance

to test it personally yet. Ice Browser also requires Java 1.1.

4.11: How do I play a sound in an application.

Java s audio support comes from the AppletContext class and the AudioClip interface.

Since applications don t have applet contexts, they have no easy way to play audio clips.

This is supposed to be fixed in Java 1.2. In the meantime you have to use undocumented

classes in the sun packages. The following example demonstrates:import sun.audio. ;

import java.awt. ;

import java.io. ;

public class SoundPlayer extends Frame implements FilenameFilter

Button openButton new Button Open ;

Button playButton new Button Play ;

Button loopButton new Button Loop ;

Button stopButton new Button Stop ;

Label filename new Label ;

File theFile null;

AudioData theData null;

InputStream nowPlaying null;

public SoundPlayer

super Sound Player ;

resize 300, 200 ;

Panel north new Panel ;

north.setLayout new FlowLayout FlowLayout.LEFT ;

north.add new Label File: ;

north.add North, filename ;

add North, north ;

Panel south new Panel ;

south.add openButton ;

south.add playButton ;

south.add loopButton ;

south.add stopButton ;

add South, south ;

SoundPlayer sp new SoundPlayer ;

sp.show ;

public void open

FileDialog fd new FileDialog this, Please select a. au file: ;

fd.setFilenameFilter this ;

fd.show ;

try

theFile new File fd.getDirectory / fd.getFile ;

if theFile. null

filename.setText theFile.getName ;

FileInputStream fis new FileInputStream theFile ;

AudioStream as new AudioStream fis ;

theData as.getData ;

catch IOException e

System.err.println e ;

public void play

stop ;

if theData null open ;

if theData. null

AudioDataStream ads new AudioDataStream theData ;

AudioPlayer.player.start ads ;

nowPlaying ads;

public void stop

if nowPlaying. null

AudioPlayer.player.stop nowPlaying ;

nowPlaying null;

public void loop

stop ;

ContinuousAudioDataStream cads new ContinuousAudioDataStream theData ;

AudioPlayer.player.start cads ;

nowPlaying cads;

public boolean action Event e, Object what

if e.target playButton

play ;

return true;

else if e.target openButton

open ;

else if e.target loopButton

loop ;

else if e.target stopButton

return false;

public boolean accept File dir, String name

name name.toLowerCase ;

if name.endsWith. au return true;

if name.endsWith. wav return true;

This example is taken from Chapter 10 of Java

Secrets,

available from amazon

0764580078/cafeaulaitA/

and various independent bookstores. It s published by IDG Books, ISBN

number 0-764-58007-8. Chapters 8-17 cover various of the sun packages

including sun.audio. Chapters 1-7 cover the internals of Java like byte

code and class files. Chapter 19 and 20 cover platform dependent Java

including native methods. 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

web 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 Java Web Server 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.5.5: How do I make my applets work well on

multiple browsers and virtual machines.

Java is cross-platform, but that doesn t mean all platforms, browsers,

and virtual machines operate identically. However,

there are a number of

steps a developer can take to ensure that their applet works reasonably

well on most browsers.

Use Java 1.0 only. Use a Java 1.0 compiler and a Java 1.0 environment

to test in. Do not use Java 1.1.From day 1, run your tests in Netscape Navigator 3.0 and earlier. Of

browsers that support Java, Navigator s probably the buggiest so if you

can get something to work there it s more likely to run elsewhere. In

particular, do not develop your applets using the appletviewer. The

appletviewer s too reliable and too stable to accurately model real user

experience.From day 1, include multiple platforms in your tests and development.

You may not be able to test on every platform Java supports, but Windows 95

and the Mac are a must. The Mac VMs in Navigator are some of the worst

around so it s important to write for them. Windows NT is also a nice

test, but I d stay away from Solaris unless I had lots of time. It s

too stable and reliable. However, Linux makes a very nice test since it s a stable OS

which you re not testing against combined with a buggy VM and a strange

GUI which you are testing against. If

you ve got multiple people working on the project, have them work on

different platforms and report bugs to each other. Better yet have them

switch development environments daily so programmers

are forced to make sure their code works in all browsers.Learn to love layout managers. Provide plenty of extra white space in

your user interfaces. Learn to hate absolute positioning. If you must

use it, be sure to check font metrics. Don t just eyeball it.

Avoid filename filters, multiple window interfaces, and other

GUI features that don t translate well across platforms.

I know this sounds a little perverted. I m telling you to work with the

worst tools rather than the best. But right now Sun s promise of write

once/run anywhere translates into write once/debug everywhere. And the

fact is, users are far more likely to be using the buggy platforms like

Mac/Netscape rather than stable ones like Solaris/appletviewer. Working

around a bug in one VM generally doesn t cause problems on other VMs. In

fact it will probably make your code more portable to platforms you

haven t tested. However, assuming that your development environment is

bug-for-bug compatible with users runtime environments does cause

problems. It is much easier to work with multiple platforms from the

beginning, rather than developing a great app on Windows or Solaris and

then porting it to all the other platforms.

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.

Fields 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. out

is a static field in the java.lang.System class.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

or Chapter 14 of my book, Java Network Programming, from

O Reilly Associates.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.

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 doint 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

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

dontChangePoint 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 is partially

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