Opening Microsoft files (winzip, word, excel, powerpoint) on this page may require you to download a special viewer. Or you can download and save the files and use your preferred office applications to view and edit them.

The Java Programming Language

 http://www.csci.csusb.edu/dick/samples/java.html

This page is designed to be a step by step introduction to Java for a competent C++ programmer. Getting your first Java programs to run is unusually hard because
Net

Names of files have to precisely match the identifiers used inside the files.
Names of classes in Applets must match the names of classes in Java.
It is hard to keep all these names in synchronization as the code develops.
Some books do not follow the newer scoping rules.

(End of Net)

Overview

This page also has many links to more in depth information about Java.

Overview

Java

It has inheritance and dynamic polymorphism but not generics and/or templates. It is designed with concurrent programming in mind. It has most of the expressions and control structures of C plus exceptions but without the "go to". So a C++ programer finds Java programs easy to read but verbose. It is slightly more work to write, but the result is nearly always clearer than the abbreviated C++ code.

Java is compiled into a special machine code that is then interpreted. Java code does not perform as fast as C++. The interpreter however protects the machine on which it is running from errors that can break operating systems in C++. Furhter the same (compiled) code can run on many different systems: Compile-Once-Run-Anywhere. So a Java program can be transmitted across a network to a machine of a different type, with a different operating system, and different graphic user interface. There it will run safely and securely (in theory) and look-and-feel as if it was programmed for that system. Doing this with C++ is extremely expensive.

Java can be used to write applications and applets. A Java application is similar to any other high-level language program: It can only be compiled and then run on the same machine. An applet is compiled on one machine, stored on a server in binary, and can be sent to a another machine over the Internet to be interpretted by a Java-aware browser.

Java comes with a large library of ready made classes and objects. The key difference betwen Java 1.0 and 1.1 was in this library. - the AWT. Similarly, Java 2.0 has a very much larger library for handling user interfaces (Swing by name) but only small changes to the core of the language.

Applications

public static main(String[])

function

class

Example Application

Hello

 	import java.lang.*;

 	public class Hello {

 	    public static void main(String argv[]){

 	        System.out.println("Hello, World!");

Hello.java

The import statement in a Java file allow it to refer to other classes in other files and directories. The compiler uses these other classes's definitions to check your code, and the interpreter loads the binary bytecode into the running program.

The word public means that the thing following it can be accessed by anyone. A public class can be imported over the internet into any other class and so can be sent to a browser. A public function can be called by statements and expressions in different classes.

static indicates that a function or object belongs to the class rather than to each individual object in the class. The function is called like this 'Hello.main(...)' even when there are no objects constructed in class Hello.

Notice that Java code was designed to be interpreted by a machine independent virtual machine (JVM) that runs a special machine code called bytecode.

. . . . . . . . . ( end of section Applications) <<Contents | End>>

Tools

Hello

 		 javac Hello.java

 		 java Hello

file

class

One nice feature of the Sun JDK is the documentation tool javadoc. The above application Hello has the following documentation: [ Hello.html ] which was generated by javadoc. javadoc scans source code files (*.java) and creates one HTML file per class in the files. Classes and parts of classes can have special documentary comments (starting "/**" ) added to them that are reformatted and incorporated into the Hypertext produced by javadoc. This should be a boon for realistic projects. The full syntax of the special comments can be found at [ Doc_Comment in java.syntax ] on this web site.

The javap program disassembles bytecode (*.class files) and outputs a description of the what is in them.

Applets

Applet

You can not run a Java applet unless it is also an application or you have a WWW page that refers to it.

The page needs HTML like the following to call the compiled code, in the same directory:

          <APPLET CODE="ClassName.class" HEIGHT=h WIDTH=w>

		Alternate text

		</APPLET>

alternate text

ClassName

comp.html.syntax.html

Example

HelloWorld

 		<head><title>Test</title></head><body>

          <APPLET CODE="HelloWorld.class" HEIGHT=150 WIDTH=150>

 		You can not see this brilliant Java Applet.

		</APPLET>

 		</body>

 		test.HelloWorld.html

The code for the HelloWorld applet has to be a public class called "HelloWorld" that extends Applet and is in in a Java file called:

 		HelloWorld.java

HelloWorld

 	import java.applet.*;

 	import java.awt.*;

	public class HelloWorld extends Applet {

	      public void init() {

	          resize(150,25);

	      }//init

	      public void paint(Graphics g) {

	          g.setFont(new Font("Helvetica", Font.PLAIN, 8));

	          g.drawString("Hello world!", 50, 25);

	      }//paint

	  }//HelloWorld

The Applet is compiled just like any other program:

 		 javac HelloWorld.java

bytecode

          appletviewer test.HelloWorld.html

HelloWorld

class

test.HelloWorld.html

On the Suns in the Sun lab you can tell 'Netscape' to "Open local File..." and select

 		test.HelloWorld.html

problems

Public Classes and Pages

Normally you should put applet code (compiled and source) in the same directory as the public page that refers to them. This is simple and works.

There are more complicated ways of organizing the files. It is said that if the page is in directory with path name D on the server and your operating system uses S to separate directories in path names

	( S.DOS=`\`, S.UNIX=`/`, S.Mac=`:`)

class

CODEBASE=classes

There is also an interaction between packages and subdirectories that goes beyong the basic knowledge covered here.

Debugging Problems with Browsers

problems

You may have to change the options/properties of your browser to be able to run Java. This is a security issue.
Your browser must trust the compiler you use. If it doesnot recognize the compiler version (hidden in the compiled file) it may reject the applet.
To be sure of a browser finding all the necessary class files they have to be in the same directory as the pages that refer to them. You can not keep the bytecode .class file private and the page public. To put .class files in a subdirectory you may have to place them in a package with the same name as well.
If the Applet fails, use the Options menu to open the Java Console... and look for errors.
The Older Netscape running in background does not make a good development tool because it is hard to make the program reload the code of Applets. You can change the code, recompile it, publish all the files perfectly, and hit the reload button in Netscape and it will not reload the new bytecode. I've wasted hours looking for bugs that I've fixed. Instead use Sun's appletviewer and/or debuggers. You can also include a main function in applets that make them applications. See [ Applications Can Run Applets ] next.
If it is absolutely necessary to debug the applet inside an html page then (1) use the Sun Appletviewer, (2) find a machine with Netscape 4.0 and use Shift+Reload, or (3) execute Netscape directly on a local copy of the page and Exit from it before working on the applet code. In vi you can use something like this:
```
 		:!java %
```
```
 		:!netscape test.page.name.html
```
to suspend the editor and run Netscape on the test page. Exit Netacape and tap Return/Enter to continue editting. On Orion, if you use Q to compile a program Stuff.java and you have a readable file called test.Stuff.java then it will automagically execute netscape for you. This is in Beta -- EMail dick if you don't like it.

Applications Can Run Applets

function

	java classname

method

function

main

Create a new window to display the applet.
Create a new object of the correct class.
Tell the applet to initialize itself.
Tell the applet to start running.
Add the running applet into the window.
Resize the window.
Show the window to you.

applet_test_harness

 	   private final static int SIZE = 300;//size of window

     public static void main(String args[]) {

 		HelloWorld aHelloWorldObject = new HelloWorld();

 			//The aHelloWorld object exists

 		   aHelloWorldObject.init();

 			//aHelloWorld has now initialized it self

 		   aHelloWorldObject.start();

 			//aHelloWorld has now started running

 		Frame aWindow = new Frame("HelloWorld");

 			//A window called "HelloWorld" will hold the Applet

 		   aWindow.add("Center", aHelloWorldObject);

 			//aWindow now has aHelloWorld in it's center

 		   aWindow.resize(SIZE, SIZE);

 		   aWindow.show();

 			//aWindow is now shown to you (until you CTRL/C)

     }//main

class

HelloWorld.java

I have just developed a new tool that will generate an HTML test page for a new applet and also a suitable *.java file ready for the details to be filled in It is in [ instantapplet ] Please download and try out -- Beta Testing.

Another Example

test.Goodbye.html

Caveat

. . . . . . . . . ( end of section Applets) <<Contents | End>>

Summary

To get	write this
Application:	public class C ... { ... public static void main(String[])...}
Applet:	import java.applet.*; .... public class C extends Applet ... { ... }

A class with name C
is best put in a file C.java
and compiled by javac C.java
producing Bytecode C.class
executed by java C arguments
and documented by javadoc C.java
producing C.html


A class with name	C
is best put in a file	C.java
and compiled by	javac C.java
producing Bytecode	C.class
executed by	java C arguments
and documented by	javadoc C.java
producing	C.html

. . . . . . . . . ( end of section Overview) <<Contents | End>>

Glossary

java.glossary.html

abstract::methods=A method that must exist for objects in a class but is fully defined only in subclasses, -- an abstract method can not be private, final, native, or synchronized.
abstract::classes=A class with one or more abstract methods.
Applet::AWT=Class of objects that are embedded in an HTML page and initialized and run by a browser over the WWW
applet::=A small program that can be sent across a network and interpreted safely on the receiving machine.
application::Java=A class that defines a public static void main(String args[]) method
AWT::=awt.
awt::=Abstract windowing toolkit, another windowing toolkit. A set of machine independent classes that make it easier to create graphic user interfaces and output.
bytecode::= [ byte_code] .
byte_code::=a way of describing classes as a stream of byte oriented machine code for the Java Virtual Machine.
classes::=plural of class.
class::=A set of objects with similar behaviors and intelligence. A class defines a collection of knowledge and know how. Classes are defined by declaring variables and and functions.. [ class in objects.glossary ]
constant::=any final field -- a piece of data about an object or class that can not be varied once it is initialized.
concurrency::=faking multiple processors so that the programmer can pretend that many processes or threads are executing at one time and interacting together. [ Synchronized_statement in java.syntax ]
extends::=indicates that a new class has all the properties of the class it extends. In Java all classes ultimately extend the class Object.
exception::=a way of exitting gracefully from expressions and statements when something unusual occurs, [ Try_block in java.syntax ] [ Throw_statement in java.syntax ]
exceptions::=plural of exception.
field::java=A variable or constant associated with a class or object - a piece of data or knowledge that that object knows about and controls.
file::=A collection of data. A Java source code file defines one or more classes, interfaces that is placed in a particular package.
final::modifier=a final class can not be extended, a final variable can not have its value changed, and a final function can not be overridden.
function::computing=A named piece of code that returns a value and may also do something
function::java=A piece of know-how attached to a class or accessed via an object
ISO::="International Standards Organization".
implements::=indicates that a class defines all the public functions listed in an interface.
interface::=The way that something is accessed from outside. The parts of an object that are shared with the clients that use that object.
interface::java=A collection of abstract public function headers that can be implemented in different ways by different classes, and extended to give more interfaces..
interface::=Describes a set of classes in terms of what they can do for you, but allows each class to implement these methods in any way that you wish.
Java::=A language developed by Sun for developing software that is distributed over the Internet to special purpose hardware like a thermostat or (latterly) to browsers via the WWW.
JDK::=Java Development Kit( compiler=> javac, interpreter=> java, classes=> AWT.
JVM::=Java Virtual Machine, the virtual_machine provides the operational semantics of Java and so defines the hardware+software environment needed to run Java bytecode.
methods::=plural of method.
method::=A piece of "know-how". A procedure or function that is associated with an object or a class.
modifier::=public | static | final | private | protected | abstract | ..., a word placed in front of a declaration that changes its semantics.
object::=An instance of a class, in Java an object is always handled via a reference that is created by the 'new' operation and a constructor and can be assigned to field of that class.
procedure::ComputerScience=`A named piece of code that does something for a caller'.
procedure::java=a void function.
public::modifier=indicates that a class, field, or function can be used by anything and anywhere on the internet.
private::modifier=indicates that a class, field, or function can only be used locally inside a class.
protected::modifier=A degree of hiding lie between public and private, subclasses and classes in the same package have access to these items. -- unwise since any class can claim to be a member of the same package! (Thanks to Andrew for correcting this definition)
private_protected::modifier="deprecated in 1.5".
package::=a collection of classes, encode as files in a common directory, that has the same name as the package and are allowed special access privileges. The default scope/access for a function or class is to its package. If no package is declared at the start of a file then the classes etc belong in the deafult global package.
static::=something associated with a class rather than an object.
variable::java=A piece of knowledge associated with a class or an object. Any non-final field.
virtual_machine::=A hypothetical machine that can be emulated on many different actual machines.
void::=word used in place of a type to indicate that a function does not return a value. -- introduced in ANSI C and still confusing people 10 years later.
WWW::="World Wide Web".
URL::="Universal Resource Locator'.
Also see [ java.glossary.html ]

. . . . . . . . . ( end of section Glossary) <<Contents | End>>

Syntax

Introduction

syntax::= See http://www.csci.csusb.edu/dick/samples/java.syntax.html.
In outline a java file starts with an optional package declaration and some import statements:
```
 	package whatever;
```
```
  import package.subpackage.....class
```
```
  import package.subpackage.*
```
These are followed by a sequence of class and interface declarations:
```
 		modifiers name extends class possible_interfaces{
```
```
 			field and function definitions
```
```
 		}
```
Field definitions have two forms:
```
 		modifiers type name;
```
and
```
 		modifiers type name = initial value;
```
where a type is either a classname or a simple data type like 'int' or 'char' or 'double'. If the modifiers include 'final' then the field is a constant, othewise it is a variable. The following is a very common and useful type of declaration:
```
 		modifiers type name = new type(arguments);
```
Arrays are declared like this:
```
 		modifiers type name[] = new type[size];
```
You can omit the initializer -- and name will be NIL!
```
 		modifiers type name[];
```
The array must not be used until it is initialised in a staement like this:
```
 		 name = new type[size];
```
In either case, if the 'type' is a class(like Object or Applet), the individual objects must be constructed and put in the array before they are used.
```
 		name[0] = new type;
```
```
 		name[1] = new type;
```
perhaps.
Function definitions have form
```
 		modifiers return_type name(arguments){
```
```
			local definitions and statemants
```
```
 		}
```
the return_type above is "void" or the name of a class or simple data type. modifiers are listed in the glossary. In older Java only variables (fields) can be defined as local.
Statements have following main forms:
```
 		variable.function_name(arguments)...;
```
```
 		class.function_name(arguments)...;
```
```
 		variable = expression;
```
```
 		while(expression) statement
```
```
 		if(expression) statement
```
```
 		if(expression) statement else statement
```
Expressions are typical arithmetic, relational, conditional, expressions of C or C++ plus
```
 		variable.function_name(arguments)...;
```
```
 		class.function_name(arguments)...;
```
Java allows you to declare local variables and constants inside a function. You can declare a new variable or constant at any place in a function but you can not define the same identifier in different blocks. This causes a common error with for statements like this:
```
 		for(int i=0; i<array.size; ++i) {...}
```
You must not declare i anywhere else in the same function! This constraint makes the interpreter much simpler and possibly faster since all space for local data is allocated when the function is called rather than when it is declared.
For a detailed description of the syntax of Java see [ java.syntax.html ] at this site. The following definitions define the syntax of UNIX and HTML related to Java.
Meta-Notation
O(X)::=Optional X. #(X)::=any number of X's including none.
quoted(X)::= double_quotes X double_quotes.
tagged(X)::= less_than X greater_than.
number::=one or more digits.

UNIX
CLASSPATH::UNIX_variable=Tells java tools where to to look for classes.
UNIX_path::=directory #(":" directory).
appletviewer::command=Scan HTML file for Applet tag and display its appearance.
binary::=A file that can be loaded and run as a program which uses the machine's own language.
java::command=run compiled class name (_). -- not the name of a class file!
javac::command=compile Java source code file named (_) generating one or more *.class files.
javadoc::command=Extract HTML documentation from Java Source code file named (_).
javap::command=Disassemble a bytecode class (_).

(filenames):
name_of_class_file::= class_name ".class". -- watch out for case sensitivity.
name_of_java_file::= class_name ".java".
class_name::=identifier, appearing as name of class in class definition.
name_of_HTML_page::=UNIX_file_name ".html".

HTML
applet_in_html::= applet_tag #(applet_parameter_tag) O(alternative_html_text) end_applet_tag.
end_applet_tag::=tagged("/APPLET").
applet_tag::= tagged( "APPLET" applet_tag_attributes ).
applet_tag_attributes::=O(alignment) O(codebase) code width height O(alternative) O(name) O(vertical_spacing) O(horizontal_spacing).
code::="CODE=" quoted(name_of_class_file), -- relative to [ code_URL ] & [ codebase ] below.
width::="WIDTH=" number. -- in pixels
height::="HEIGHT="number. -- in pixels
alignment::= "ALIGN=" ( "LEFT" | "MIDDLE" | "RIGHT" ).
codebase::= "CODEBASE=" quoted(code_URL).
code_URL::=the URL of the directory that contains the applet's code.
alternative::= "ALT=" quoted(text).
name::="NAME=" applet_instance_name.
vertical_spacing::="VSPACE=" pixels.
horizontal_spacing::="HSPACE=" pixels.
applet_parameter_tag::=tagged("PARAM" "NAME=" quoted(name) "VALUE="quoted(value)
app_in_html::=tagged( "APP CLASS="name_of_class) ), --obsolete
future_HTML::=tagged("embed" arguments) alternative tagged("/embed").

. . . . . . . . . ( end of section Syntax) <<Contents | End>>

Predefined Classes

For 1.4.2 API see [ index.html ] and for 1.5.0 [ index.html ] (Summer of 2004).

Also see my rough introduction to some commonly used classes and interfaces [ java.classes.html ]

In theory at least the library includes all the public classes that have been put on the WWW as well!

Javascript is Not Java

Javascript was once called LiveScript. It is beeing standardized by ECMA (European Standards...) as ECMAScript. Meanwhile MicroSoft has produced its own version called JScript! Even Netscape has 3 or 4 versions of Javascript. The common syntactic ancestor (C++) with Java is clear but the details are quite different to Java. The semantics is closer to Smalltalk than anything else.

Both Java and Javascript let you produce more complex pages. Both Java and Javascript run on the user's system. A CGI [ CGI in comp.html.syntax ] is executed on the server and sends the results to the client.

Pointers to news and Google groups: comp.lang.javascript

Specification of Java

Standardization

ISO

Microsoft would prefer its own version of Java and libraries to be the standard. 10,000 developers who want Java's to be a cross-platform language have formed the Java Lobby.

Sun sued MicroSoft for abusing its licence of Java by giving out a version that will work with only MicroSoft software rather than on all platforms. So far(Winter 1998-1999) Microsoft has been ordered to stop giving out their polluted version and have been forced to distribute an upgrade to the "Pure Java" version.

Now MicroSoft is promoting .NET and C# as rival tecnologies to Java.

Other Specifications

java.glossary.html

The syntax (1.0alpha) is summarized at [ java.syntax.html ] in my documentation samples.

Java has its semantics defined by a compiler that translates the Java source code for a virtual machine (the JVM), There is a detailed specification of the virtual machine. My own notes on Java semantics are under construction [ java.semantics.html ] in my documentation samples.

(File Formats): A specification of the format of a .class file was held at http://www.javasoft.com/1.0alpha3/doc/vmspec/vmspec_38.html but I haven't found the replacement.....

Java Documentation

Java reference

http://java.sun.com/reference/

(Java Specification): For the the Official Java(TM) 2 Platform, Standard Edition. [ http://java.sun.com/j2se/ ]

API::="Application Programmers Interfaces".
APIs::= See http://java.sun.com/reference/api/index.html, and pick the version of Java that you are using.

. . . . . . . . . ( end of section Specification of Java) <<Contents | End>>

Trivia

CAFEBABE

class

0xCAFEBABE

The Naming of Java

The name "Java" is not an acronym. The original name was "Oak", after the tree outside the developers window, but it turned out that "Oak" was already copyrighted/trademarked/in-use. The descriptions of the next stage vary but all indicate some kind of brainstorming session and/or a local coffee shop, during which name "Java" was proposed and accepted.

. . . . . . . . . ( end of section The Java Programming Language) <<Contents | End>>

Opening Microsoft files (winzip, word, excel, powerpoint) on this page may require you to download a special viewer. Or you can download and save the files and use your preferred office applications to view and edit them.

Contents

The Java Programming Language

Overview

Applications

Example Application

Tools

Applets

Example

Public Classes and Pages

Debugging Problems with Browsers

Applications Can Run Applets

Another Example

Caveat

Summary

Glossary

Syntax

Introduction

Meta-Notation

UNIX

HTML

Predefined Classes

Javascript is Not Java

Specification of Java

Standardization

Other Specifications

Java Documentation

See Also

Information on the Java Language

The Source Code

News and Announcements

Tutorials

Books

Source Code Examples

Trivia

The Naming of Java

End