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Java Notes

Introduction

Java(tm) is an Object Oriented platform independent programming language from Sun. It was designed to be a "simple, object-orientated, distributed, interpreted, robust, secure, architecture neutral, portable, high-performance, multithreaded, and dynamic language". At a low level the source looks rather like C++. Unlike C++ however, it has standard graphical and internet routines. Also it's more purely object-orientated - it's a new language that didn't have to try to be compatible with C.

Jargon

JavaBeans - a reusable software component architecture, like VBX (Visual Basic Extensions) or OCX (OLE Controls). JavaBeans components - beans - are separate code modules, written at the source level, that an application developer can use to combine into larger applications. You can make any Java class into a bean just by changing the class to adhere to the JavaBeans specification.
Java Server Pages - it embeds Java code in HTML.
Applets - java designed to be run in a WWW browser.
Servlets - Java's replacement for CGI scripts (embeds HTML in java).
Swing - a set of libraries for making GUIs
Packages - namespaces for classes (sort of)

Language basics

keywords:

       abstract    default     if          private     throw
       boolean     do          implements  protected   throws
       break       double      import      public      transient
       byte        else        instanceof  return      try
       case        extends     int         short       void
       catch       final       interface   static      volatile
       char        finally     long        super       while
       class       float       native      switch
       const       for         new         synchronized
       continue    goto        package     this

All primitive values belong to one of eight primitive types: int, float, boolean, char, byte, short, long, and double.

Program Structure

As you may already have guessed from the keyword list, Java has many features in common with C++. Every Java program consists of a collection of classes - nothing else. A class is a template for creating a particular form of object. Each object created by the template contains the same members, each of which is either a field or a method. A field is a "container" that holds a value. A method is an operation on the fields of the object and any values that are passed as arguments to the method. The objects created by a particular class template are called the instances or objects of that class. Each instance contains the members specified in the class template.

Every complete Java program must contain a root class where execution can begin. A root class must contain a main method defined with the header

       public static void main(String[] args)

A simple example

Put the following into a file called Euro.java.

class Euro {
         public static void main(String[] args) {
           System.out.println("99 euros = " + 99. *(61./100.) + " pounds");
         }
       }

This has a root class which prints out 2 strings and the result of a calculation by calling the println method.

With C++, the source code is compiled to produce a file in the native language of the CPU. With interpreted languages like BASIC the source is converted at run-time (almost line-by-line) by an interpreter. Java is both compiled and interpreted.

To execute this it first needs to be compiled into a byte-code file. The Java compiler, javac, takes your source file and translates its text into the native language of the Java Virtual Machine.

   javac euro.java

This produces a file called Euro.class which can be fed to the java interpreter by typing

   java Euro.class

Applets and Inheritance

As in C++, classes can be derived from other classes. A simple example of this is applet creation. An applet is a piece of java that can be run inside a java-enabled browser. To give a class access to the functions that an applet needs, it's derived from an Applet class. Put the following into a file called EuroApplet.java somewhere on the WWW.

import java.applet.Applet;
import java.awt.Graphics;

public class EuroApplet extends Applet {
    public void paint(Graphics g) {
       g.drawString("Hello world!", 50, 25);
       }
}

Then create a WWW page with the following code on it

  <applet codebase="http://the.folder.where.your.EuroApplet.is"
        code="EuroApplet.class" width=400 height=75 align=center >
  <param name="text" value="This is the Applet Viewer.">
  <blockquote>
  <hr>
  If you were using a Java-enabled browser,
  you would see dancing text instead of this paragraph.
  <hr>
  </blockquote>
  </applet>

When you view the page you should see the applet's output too.

Java Utilities

The suite of java utilities includes

Java Compiler (javac)
Java Interpreter (java)

Further Information

JavaSoft
CUED's material
Notes on java from computer labs (Dr A C Norman)

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