Applet Example
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Transcript of Applet Example
Applets and applications
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An applet is a Java program that runs on a web pageApplets can be run within any modern browserTo run modern Java applets, old browsers need an
up-to-date Java pluginappletviewer is a program that can run
An application is a Java program that runs all by itself
Applet
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An applet is a program that comes from server into a client and gets executed at client side and displays the result.
An applet represents byte code embedded in a html page. (applet = bytecode + html) and run with the help of Java enabled browsers such as Internet Explorer.
An applet is a Java program that runs in a browser. Unlike Java applications applets do not have a main () method. To create applet we can use java.applet.Applet .
Packages and classes
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Java supplies a huge library of pre-written “code,” ready for you to use in your programs
Code is organized into classesClasses are grouped into packagesOne way to use this code is to import itYou can import a single class, or all the
classes in a package
The Applet class
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To create an applet, you must import the Applet classThis class is in the java.applet package
The Applet class contains code that works with a browser to create a display window
Capitalization matters! applet and Applet are different names
Importing the Applet class
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Here is the directive that you need: import java.applet.Applet;
import is a keywordjava.applet is the name of the packageA dot ( . ) separates the package from the
classApplet is the name of the classThere is a semicolon ( ; ) at the end
The java.awt package
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“awt” stands for “Abstract Window Toolkit”The java.awt package includes classes for:
Drawing lines and shapesDrawing lettersSetting colorsChoosing fonts
If it’s drawn on the screen, then java.awt is probably involved!
Importing the java.awt package
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Since you may want to use many classes from the 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
Your applet class
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public class Drawing extends Applet { … }
Drawing is the name of your classClass names should always be capitalized
extends Applet says that our Drawing is a kind of Applet, but with added capabilitiesJava’s Applet just makes an empty windowWe are going to draw in that window
The only way to make an applet is to extend Applet
The applet so far
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import java.applet.Applet;import java.awt.*;
public class Drawing extends Applet {
…we still need to put some code in here...
}
The paint method
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Our applet is going to have a method to paint some colored rectangles on the screen
This method must be named paintpaint needs to be told where on the screen it
can drawThis will be the only parameter it needs
paint doesn’t return any result
The paint method, part 2
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public void paint(Graphics g) { … }public says that anyone can use this methodvoid says that it does not return a result
A Graphics (short for “Graphics context”) is an object that holds information about a paintingIt remembers what color you are usingIt remembers what font you are usingYou can “paint” on it (but it doesn’t remember
what you have painted)
The applet so far
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import java.applet.Applet;import java.awt.*;
public class Drawing extends Applet {
public void paint(Graphics g) { …we still need to put some code in here… }}
Colors
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The java.awt package defines a class named ColorThere are 13 predefined colors—here are their fully-
qualified names:
For compatibility with older programs (before the naming conventions were established), Java also allows color names in lowercase: Color.black, Color.darkGray, etc.
Color.BLACK Color.PINK Color.GREENColor.DARK_GRAY Color.RED Color.CYANColor.GRAY Color.ORANGEColor.BLUEColor.LIGHT_GRAY Color.YELLOWColor.WHITE Color.MAGENTA
New colors
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Every color is a mix of red, green, and blueYou can make your own colors:
new Color( red , green , blue )Amounts range from 0 to 255Black is (0, 0, 0), white is (255, 255, 255)We are mixing lights, not pigmentsYellow is red + green, or (255, 255, 0)
Setting a color
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To use a color, we tell our Graphics g what color we want: g.setColor(Color.RED);
g will remember this color and use it for everything until we tell it some different color
The paint method so far
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public void paint(Graphics g) { g.setColor(Color.BLUE); …draw a rectangle,write any string… g.setColor(Color.RED); …draw another rectangle write any string… }}
Pixels
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A pixel is a picture (pix) elementone pixel is one dot on your screenthere are typically 72 to 90 pixels per inch
java.awt measures everything in pixels
Java’s coordinate system
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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 right
corner, where w is the width of the window and h is its height
(0, 0)
(0, 20)
(50, 0)
(50, 20)
(w-1, h-1)
Drawing rectangles
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There are two ways to draw rectangles:g.drawRect( left , top , width , height );
g.fillRect(left , top , width , height );
The complete applet
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import java.applet.Applet;import java.awt.*;
public class Drawing extends Applet {
public void paint(Graphics g) {
g.setColor(Color.BLUE); g.fillRect(20, 20, 50, 30); g.setColor(Color.RED); g.fillRect(50, 30, 50, 30); }}
Some more java.awt methods
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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 );
The HTML page
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You can only run an applet in an HTML pageThe HTML looks something like this:
<html> <body> <h1>DrawingApplet Applet</h1> <applet code="DrawingApplet.class" width="250" height="200"> </applet> </body></html>
BlueJ will create this HTML for you
Programme-2
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import java.applet.*; import java.awt.*; public class HelloWorld extends Applet { public void init() { resize(150,25); }//init public void paint(Graphics g) { g.setFont(new Font("Helvetica", Font.PLAIN, 8)); g.drawString("Hello world!", 50, 25); }//paint }//HelloWorld
Compiling and Executing Program:
javac HelloWorld.java
appletviewer test.HelloWorld.html
Programme-1import java.applet.*;
import java.awt.*;
public class DrawingLines extends Applet {
int width, height;
public void init()
{ width = getSize().width;
height = getSize().height;
setBackground( Color.black );
}
public void paint( Graphics g )
{
g.setColor( Color.green );
for ( int i = 0; i < 10; ++i )
{
g.drawLine( width, height, i * width / 10, 0 );
} }}
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Basic Methods(1/2)
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Methods for Milestonesinit() – initialize the appletstart() – start the applet’s executionstop() – stop the applet’s executiondestroy() – perform a final cleanup
Basic Methods(2/2)
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Typical Structure
import java.applet.Applet;import java.awt.Graphics;
public class Simple extends Applet { ... public void init() { ... } public void start() { ... } public void stop() { ... } public void destroy() { ... } public void paint(Graphics g) { ... }}
<Applet Code>
<APPLET CODE=... CODEBASE=... WIDTH=... HEIGHT=...> ...</APPLET>
<HTML Code>
Example
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Source CodeResult
import java.applet.Applet;import java.awt.Graphics;public class HelloWorld extends Applet { public void paint(Graphics g) { g.drawString("Hello", 50,20); }}
HelloWorld.java
<APPLET CODE="HelloWorld.class“ WIDTH=200 HEIGHT=140></APPLET>
HelloWorld.htm
Programme- public class SetGraphicsColorExample extends Applet{ public void paint(Graphics g) { /* * Graphic objects like lines and rectangles uses
current * foreground color. * * To change the current graphic color use * void setColor(Color c) method of Graphics Class. */ //this will create light blue color Color customColor = new Color(10,10,255); g.setColor(customColor); g.drawLine(10,10,30,30); g.setColor(Color.red); g.fillRect(40,40,40,40); g.setColor(Color.green); g.fillRect(80,80,40,40); g.draw3DRect(81,81,40,40,true); } }
ProgramProgram 1: Write an applet program with a message and display the message in paint () method./* <applet code="MyApplet.class" width = 600 height= 450></applet> */