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Introduction to Java. Yin-Hsong Hsu and Yen-Cheng Chen 1998. History of Java. Green project funded by Sun for intelligent consumer electronic devices announced at May 23, 1995 experiences from CE so many kinds of machines, portability simple no hard disk => network download security.

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introduction to java

Introduction to Java

Yin-Hsong Hsu and Yen-Cheng Chen 1998

history of java
History of Java
  • Green project funded by Sun for intelligent consumer electronic devices
  • announced at May 23, 1995
  • experiences from CE
    • so many kinds of machines, portability
    • simple
    • no hard disk => network download
    • security

- “Write Once, Run Everywhere & Reuse Everywhere”

  • Java: An Object-Oriented Programming Language for the Internet.
  • A Better C++-Like Language
    • object-oriented, distributed, interpreted, multi-threaded, secure, no pointers, garbage collection,...
  • Platform-independent
    • Java programs are compiled into bytecodes, which can be run on Java Virtual Machine.
  • Java Applets/Servlets: small Java programs executed in WWW browsers/servers.
  • Simple
  • Object-oriented
  • Distributed
  • Interpreted
  • Robust
  • Secure
  • Architecture-neutral
  • Portable
  • High performance
  • Multithreaded
  • Dynamic
java applet vs java application
Java Applet vs. Java Application
  • Applet
    • A Java program that runs inside a Java-enabled Web Browser
  • Application
    • A stand-alone Java program
how java program being executed

Source Code

Compile javac

Byte Code

Interpret by a virtual machine


How Java Program Being Executed
  • Application
how java program being executed1


Source Code




Virtual Machine

Byte Code

Load Bytecode

JIT: Just In Time Compiler

How Java Program Being Executed
  • Applet
programming environment
Programming Environment
  • Sun’s JDK (public)
    • javac
    • java
    • jdb
  • Symantec Visual Café
  • Microsoft Visual J++
  • Borland JBuilder
  • IBM VisualAge
  • Sybase PowerJ
first java application hello world
First Java Application - Hello World
  • Code :
  • compile to bytecode “HelloWorldApp.class”
  • execute the bytecode

class HelloWorldApp {

public static void main(String[] args) {

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



    • Compile to HelloWorldApp.class
  • java HelloWorldApp
    • Interpret HellowWorldApp.class
the anatomy of a java application
The anatomy of a Java application
  • Comments in Java Code
    • C, C++, and /** … */
  • defining a class
  • the main() method
    • modifiers: public static and void
    • arguments to the main method
  • using classes and objects
    • class variables versus instance variables
first java applet hello world
First Java Applet - Hello World
  • Code:
  • compile to bytecode : HelloWorld.class



public class HelloWorld extendsApplet {

public void paint(Graphics g) {

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



running an applet
Running an Applet
  • HTML file : xx.html

<APPLET CODE="HelloWorld.class" WIDTH=150 HEIGHT=25>

  • The Applet tag

< appletcode=class width=nn height=nn codebase=dir name=n align=str vspace=nn hspace=nn>

<paramname=pn1 value=pv1>

<paramname=pn2 value=pv2>



the anatomy of a java applet
The anatomy of a Java applet
  • importing classes and packages
  • defining an Applet subclass
  • implementing Applet methods
what applets can do
What Applets Can Do?
  • Applets can usually make network connections to the host they came from.
  • Applets running within a Web browser can easily cause HTML documents to be displayed.
  • Applets can invoke public methods of other applets on the same page.
  • Applets that are loaded from the local file system (from a directory in the user's CLASSPATH) have none of the restrictions that applets loaded over the network do.
what applets can t do
What Applets Can’t Do?
  • An applet cannot load libraries or define native methods.
  • It cannot ordinarily read or write files on the host that's executing it.
  • It cannot make network connections except to the host that it came from.
  • It cannot start any program on the host that's executing it.
  • It cannot read certain system properties.
  • Windows that an applet brings up look different than windows that an application brings up.
java core api
Java Core API
  • Included in JDK
  • Java classes/packages:
    • Data manipulation & processing.
    • AWT (Abstract Windowing Toolkit).
    • I/O.
    • Networking.
    • Security.
    • JDBC (Java Database Connectivity)
    • JavaBeans
    • RMI (Remote Method Invocation).
    • Object Serialization