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An applet is a Java program that runs on a webpage Applets can be run within any modern browser

To run modern Java applets, old browsers need anup-to-date Java plugin

appletviewer is a program that can run

An application is a Java program that runs all byitself

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Java supplies a huge library of pre-writtencode, ready for you to use in yourprograms

Code is organized into classes Classes are grouped into packages

One way to use this code is to import it

You can import a single class, or all the

classes in a package

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To create an applet, you must import the Appletclass This class is in the java.applet package

TheAppletclass contains code that works with abrowser to create a display window

Capitalization matters! applet and Applet are different names

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Here is the directive that you need:

import java.applet.Applet;

import is a keyword

java.applet is the name of the package A dot ( .) separates the package from the

class

Applet is the name of the class

There is a semicolon ( ; ) at the end

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awt stands for Abstract Window Toolkit

The java.awt package includes classes for: Drawing lines and shapes

Drawing letters Setting colors

Choosing fonts

If its drawn on the screen, then java.awt is

probably involved!

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Since you may want to use many classes fromthe java.awt package, simply import them all:

import java.awt.*;

The asterisk, or star (*), means all classes The import directives can go in any order, but

must be the first lines in your program

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import java.awt.*;

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public class Drawing extends Applet { }

Drawing is the name of your class Class names should always be capitalized

extends Applet says that our Drawing is a kindofApplet, but with added capabilities Javas Applet just makes an empty window We are going to draw in that window

The onlyway to make an applet is to extendApplet

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import java.awt.*;

public class Drawing extends Applet {

we still need to put some code in here...

}

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Our applet is going to have a method to paintsome colored rectangles on the screen

This method must be named paint

paint needs to be told where on the screen itcan draw This will be the only parameter it needs

paintdoesnt return any result

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public void paint(Graphics g) { } public says that anyone can use this method

void says that it does not return a result

A Graphics(short for Graphics context) is an

object that holds information about a painting It remembers what color you are using

It remembers what font you are using

You can paint on it (but it doesnt remember what

you have painted)

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import java.awt.*;


public void paint(Graphics g) {

we still need to put some code in here}

}

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The java.awtpackage defines a class named Color There are 13 predefined colorshere are their

fully-qualified names:

For compatibility with older programs (beforethe naming conventions were established), Javaalso allows color names in lowercase: Color.black,Color.darkGray, etc.

Color.BLACK Color.PINK Color.GREENColor.DARK_GRAY Color.RED Color.CYAN

Color.GRAY Color.ORANGE Color.BLUE

Color.LIGHT_GRAY Color.YELLOW

Color.WHITE Color.MAGENTA

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Every color is a mix of red, green, and blue

You can make your own colors:new Color(red,green,blue)

Amounts range from 0 to 255 Black is (0, 0, 0), white is (255, 255, 255)

We are mixing lights, not pigments

Yellow is red + green, or (255, 255, 0)

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To use a color, we tell our Graphicsgwhat colorwe want:

g.setColor(Color.RED);

g will remember this color and use it foreverything until we tell it some different color

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g.setColor(Color.BLUE);

draw a rectangle

g.setColor(Color.RED);draw another rectangle

}

}

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Java uses an (x, y) coordinate system

(0, 0) is the top left corner

(50, 0) is 50 pixels to the right of (0, 0)

(0, 20) is 20 pixels down from (0, 0)

(w - 1, h - 1) is just inside the bottom rightcorner, where w is the width of the window andh is its height

(0, 0)

(0, 20)

(50, 0)

(50, 20)

(w-1, h-1)

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There are two ways to draw rectangles: g.drawRect(left,top,width,height);

g.fillRect(left,top,width,height);

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import java.awt.*;



g.setColor(Color.BLUE);g.fillRect(20, 20, 50, 30);

g.setColor(Color.RED);

g.fillRect(50, 30, 50, 30);

}

}

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g.drawLine(x1 ,y1,x2 ,y2 ); g.drawOval( left,top,width,height ); g.fillOval( left,top,width,height); g.drawRoundRect( left,top,width,height ); g.fillRoundRect( left,top ,width,height); g.drawArc( left,top,width,height,

startAngle,arcAngle ); g.drawString(string,x,y);

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You can only run an applet in an HTML page The HTML looks something like this:

DrawingApplet Applet

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public class TrivialApplet extends Applet { }

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import java.awt.*;


public class HelloWorld extends Applet {

public void paint( Graphics g ) {

g.drawString( "Hello World!", 30, 30 );

}

}

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Running the applet Compile

javac HelloWorld.java

If no errors, bytecodes stored in HelloWorld.class

Create an HTML file Loads the applet into appletviewer or a browser

Ends in .htm or .html

To execute an applet

Create an HTML file indicating which applet thebrowser (or appletviewer) should load and execute

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Simple HTML file (HelloWorld.html) Usually in same directory as .class file

Remember, .class file created after compilation

HTML codes (tags) Usually come in pairs

Begin with < and end with >

Lines 1 and 4 - begin and end the HTML tags

Line 2 - begins tag Specifies code to use for applet

Specifies width and height of display area in pixels

Line 3 - ends tag

1 2

3 4

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appletviewer only understands tags

Ignores everything else

Minimal browser Executing the applet

appletviewer HelloWorld.html

Perform in directory containing .class file

1 2

3 4

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destroy ()

init()

start()

stop()

java.applet.Applet

java.awt.Panel

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init()

Called exactly once in an applets life.

Called when applet is first loaded, which is

after object creation, e.g., when the browservisits the web page for the first time.

Used to read applet parameters, startdownloading any other images or media files,

etc.

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start()

Called at least once.

Called when an applet is started or restarted,

i.e., whenever the browser visits the webpage.

stop()

Called at least once.

Called when the browser leaves the webpage.

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destroy()

Called exactly once.

Called when the browser unloads the applet.

Used to perform any final clean-up.

init

destroystart stop

start

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

public class ab extends Applet{

public void paint(Graphics g){int a=10, b=20;

int d=a+b;

String s= sum:+String.valueOf(d);

g.drawString(s,100,100);}

}

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14508 Applets