Introduction to Java Programming

Contents

1. Java, etc.

2. Java's Advantages

3. Java's Disadvantages

4. Types of Java Code

5. Java Bytecodes

6. Steps in Writing a Java Application

7. Steps in Writing a Java Applet

1. Java J, 2 SE, JSDK, JDK, JRE

There are several 'Java' names: Java is the name of the language

Java 2 is the current version

the language + tools (e.g. compiler) is called J2SE, the Java 2 Standard Edition J2SE 1.6.0. is the current version its also known as J2SE 6.0

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the samething

JSDK stands for "Java Software Development Kit JDK is the old name for JSDK don't be surprised to also see J2SDK

or Java SDK

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JSDK contains all the libraries (packages), compiler, and other tools for writing/running/debugging Java code.

JRE = "Java Runtime Environment" a cut-down version of JSDK with only the pack

ages/tools needed for running Java code most often used by Web browsers

More New Names

Sun is trying to get people to drop the "2" from the Java platform names. e.g. J2SE becomes Java Standard Edition, abbre

viated as "Java SE" not really popular, yet

2. Java’s Advantages

Productivity object orientation many standard libraries (packages)

Simpler/safer than C, C++ no pointer arithmetic, has automatic garbage

collection, has array bounds checking, etc.

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GUI features mostly located in the Swing and Abstract

Windowing Toolkit (AWT) packages

Multimedia 2D and 3D graphics, imaging, animations,

audio, video, etc.

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Network support communication with other machines/apps variety and standards:

sockets, RMI, CORBA security, resource protection

Multithreading / concurrency can run several ‘threads’ at once

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Portablility / Platform Independence “write once; run anywhere” only one set of libraries to learn

J2SE is free

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Good programming environments: Eclipse, Blue J, JBuilder, NetBeans, Sun One Studio do not use them when first learning Java http://java.coe.psu.ac.th/Tool.html

Applets (and Java Web Start) eliminates the need for explicit software installation.

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3. Java’s Disadvantages

Java/J2SE is still being developed many changes between versions

Sun has not guaranteed backward compatibility of future versions of Java. at the moment, when old-style code is compiled,

the compiler gives a “deprecation” warning, but will still accept it

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Java compilation/execution was slow, but ... not any more: J2SE 1.5 is the same speed as C

(perhaps a tiny bit slower for some things)

there are compilers to native code, but they destroy the “write one; run anywhere” idea

the first version of Java, back in 1995, was about 40 times slower than C

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Slow Internet connections makes it difficult (and irritating) to download

medium/large size applets e.g. GIF89a/flash files have replaced Java

animations

Lots to learn Java language (small) and Java libraries

(very, very large)

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4. Types of Java Code

There are two kinds of Java code:

1. Java applications ordinary programs; stand-alone they don’t run inside a browser

(but they can use Java’s GUI libraries)

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We will seeexamples inthe next part.

2. Java applets they run in a Web browser

they are attached to Web pages, so can be downloaded easily from anywhere

applets have access to browser features

5. Java Bytecodes

The Java compiler (javac) generates bytecodes a set of instructions similar to machine code not specific to any machine architecture

A class file (holding bytecodes) can be run on any machine which has a Java runtime environment.

The Bytecode Advantage

Java code(.java file)

javac (Windows)

javac (Mac)

javac (Linux)

Java bytecode(.class file)

Java runtime (Windows)

Java runtime (Mac)

Java runtime (Linux)

6. St eps i n Wri t i ng a J ava Appl ication

Foo.javatext file holding the application

javac Foo.java call the Java compiler

Foo.class class file holding Java bytecodes

java Foo execute the class usingthe Java runtime system(the JVM)

.

import java.io.*;

public class Hello {

public static void main(String args[]) { System.out.println(“Hello Andrew”); }

} // end of class

Compile & Run

7. St eps i n Wri t i ng a J ava Applet

AFoo.java text file holding the applet

javac AFoo.java call the Java compiler

AFoo.class class file holding Java bytecodes

appletviewer AFoo.html

AFoo.html

Web page that calls AFoo.class

execute the applet using the Java runtime system (the JVM)

execute the applet usingthe Java runtime system(the JVM)

AFoo.classAFoo.html

Web page that callsAFoo.class

browser downloads Web pageand Java class

Using a browser

For Java 2, the Java Plug-in is required,

or use the Opera browser

WelcomeApplet.java

import javax.swing.JApplet;import java.awt.Graphics;

public class WelcomeApplet extends JApplet {

public void paint(Graphics g) { g.drawString(“Welcome Andrew”, 25,25); }

}

WelcomeApplet.html

<html><head><title>Applet Testing</title></head><body><applet code=“WelcomeApplet.class”

width=300 height=30></applet></body></html>

Compile & Run

$ javac WelcomeApplet.java

$ appletviewer WelcomeApplet.html

Browser Execution

Microsoft IE and Netscape do not directly support Java 2.

A common solution is to use the Java plugin, available from Sun a drawback is that it requires the Web page conta

ining the applet to contain more complicated tags (and JavaScript code) so that the applet can run inside Netscape and Microsoft IE

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A better solution is to use the Opera browser: free from http://www.opera.com

it comes with JRE 1.5, the latest version of the Java Runtime Environment, or it can be linked to the JRE already on your machine

there is no need for a Java plugin

Opera is very fast, small-size, and supports many networking standards

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Load WelcomeApplet.html