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Applets, Graphical User Interfaces, and Threads

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Interface ?

• an agent between two entities

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Interface

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Interface

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Interface

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Interfaces

• ATM machine

• remote control

• bank teller

• browser (Netscape Navigator or Internet Explorer)

real interfaces

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Interfaces

• are designed to handle events initiated by user

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Events

• single click

• double click

• dragging

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Events

• An applet has mechanisms to handle events

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Applet

• A Java programs designed to run in a browser

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Applet

• applet class files are embedded in a HTML document

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all applets are subclasses of the applet class

Applet

public class SomeApplet extends Applet{ … }

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Applets differ from applications in that a browser controls the applet’s execution

Applet

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the browser calls your applet's init() method when the

applet is started. An applet's

start() and stop() methods are called when the page where the applet

is running gains or loses focus.

Applet

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Why Restrict Applets ?

• An applet is code from another machine that you are not familiar

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Applet Restrictions

• cannot automatically send information to a user’s printer

• prevented from performing file operations

• prevented from certain network activities

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Applet Restriction Relaxed

• can be relaxed by changing the browser’s security settings

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Applet Syntax

public class appletName extends Applet{ … }

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Sample Applet

import java.applet.Applet;

import java.awt.Graphics;

public class Welcome extends Applet {

public void paint(Graphics g){

g.drawString("Java is fuuuuuuuuuun :) ...",25,25);

}

}

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Applet’s HTML File

<applet code=Welcome.class width=215 height=100>

</applet>

•the HTML file name does not have to match the name of the class name as in the java file

•however, the class name in the applet code tag must match the class name

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Applet’s HTML File

<applet code=Welcome.class width=215 height=100>

</applet>

the area of the applet is determined by the width and height parameters

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Applet Template

import java.awt.*;import java.applet.*;

public class SampleApplet extends Applet{ public void init(){

System.out.println("SampleApplet.init() executed"); } public void paint(Graphics g){ }

public void start(){ }

public void stop(){ }

}

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Applet’s Life

•the browser manages the life cycle of the applet•the life cycle includes the following

› init› start› stop› destroy

•paint

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Applet Methods(init)

•called when an applet begins•called once during the applet’s execution

•code to initialize the applet is placed in init()

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Applet Methods(start)

•called immediately after init() •when the page where the applet is running gains focus

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Applet Methods(stop)

•called when the page where the applet is running loses focus

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Applet Methods(destroy)

•called when the browser terminates

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Applet Methods(paint)

•refreshes the browser display