Introduction to Java

Yin-Hsong Hsu and Yen-Cheng Chen 1998

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History of Java

Green project funded by Sun for intelligent consumer electronic devices

announced at May 23, 1995experiences from CE

so many kinds of machines, portability simple no hard disk => network download security

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Java

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.

- “Write Once, Run Everywhere & Reuse Everywhere”

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Java

SimpleObject-orientedDistributedInterpretedRobustSecureArchitecture-neutral

PortableHigh performanceMultithreadedDynamic

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Java Applet vs. Java Application

Applet A Java program that runs inside a Java-enabled

Web Browser

Application A stand-alone Java program

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How Java Program Being Executed

ApplicationSource Code

Byte Code

Interpretation

Compile javac prog.java

Interpret by a virtual machine

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How Java Program Being Executed

Applet

Source Code

Byte Code

HTML

compile

Browserwith

Virtual Machine Load Bytecode

JIT: Just In Time Compiler

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Programming Environment

Sun’s JDK (public) javac java jdb

Symantec Visual CaféMicrosoft Visual J++Borland JBuilderIBM VisualAgeSybase PowerJ

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First Java Application - Hello World

Code : HelloWorldApp.java

compile to bytecode “HelloWorldApp.class”execute the bytecode

class HelloWorldApp { public static void main(String[] args) { System.out.println("Hello World!"); }}

javac HelloWorldApp.java Compile HelloWorldApp.java to HelloWorldApp.class

java HelloWorldApp Interpret HellowWorldApp.class

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The anatomy of a Java application

Comments in Java Code C, C++, and /** … */

defining a classthe main() method

modifiers: public static and void arguments to the main method

using classes and objects class variables versus instance variables

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First Java Applet - Hello World

Code: HelloWorld.java

compile to bytecode : HelloWorld.class

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

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

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Running an Applet

HTML file : xx.html <APPLET CODE="HelloWorld.class" WIDTH=150 HEIGHT=25>

The Applet tag< applet code=class width=nn height=nn codebase=dir name=n align=str vspace=nn hspace=nn>

<param name=pn1 value=pv1><param name=pn2 value=pv2>...</applet>

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The anatomy of a Java applet

importing classes and packagesdefining an Applet subclassimplementing Applet methods

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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.

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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.

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Java Core API

Included in JDKJava classes/packages:

Data manipulation & processing. AWT (Abstract Windowing Toolkit). I/O. Networking. Security. JDBC (Java Database Connectivity) JavaBeans RMI (Remote Method Invocation). Object Serialization

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JDK (Java Development Kit)

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Java Foundation Classes (JFC, swing)

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Java Family

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Java Security

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