Download A Simple Applet

Applet Example
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Applets and applications
 An applet is a Java program that runs on a web page
 Applets can be run within any modern browser
 To run modern Java applets, old browsers need an up-to-date Java
plugin
 appletviewer is a program that can run
 An application is a Java program that runs all by itself
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Applet
 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 .
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Applet
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Packages and classes
 Java supplies a huge library of pre-written “code,” ready for you
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to use in your programs
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
The Applet class
 To create an applet, you must import the Applet class
 This 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
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Importing the Applet class
 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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The java.awt package
 “awt” stands for “Abstract Window Toolkit”
 The java.awt package includes classes for:
 Drawing lines and shapes
 Drawing letters
 Setting colors
 Choosing fonts
 If it’s drawn on the screen, then java.awt is probably
involved!
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Importing the java.awt package
 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
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The applet so far
import java.applet.Applet;
import java.awt.*;
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Your applet class
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 kind of Applet,
but with added capabilities
 Java’s Applet just makes an empty window
 We are going to draw in that window
 The only way to make an applet is to extend Applet
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The applet so far
import java.applet.Applet;
import java.awt.*;
public class Drawing extends Applet {
…we still need to put some code in here...
}
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The paint method
 Our applet is going to have a method to paint some colored
rectangles on the screen
 This method must be named paint
 paint needs to be told where on the screen it can draw
 This will be the only parameter it needs
 paint doesn’t return any result
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The paint method, part 2
 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 doesn’t remember what you have
painted)
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The applet so far
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…
}
}
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Colors
 The java.awt package defines a class named Color
 There are 13 predefined colors—here are their fully-qualified
names:
Color.BLACK
Color.DARK_GRAY
Color.GRAY
Color.LIGHT_GRAY
Color.WHITE
Color.PINK
Color.RED
Color.ORANGE
Color.YELLOW
Color.MAGENTA
Color.GREEN
Color.CYAN
Color.BLUE
 For compatibility with older programs (before the naming
conventions were established), Java also allows color names in
lowercase: Color.black, Color.darkGray, etc.
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New colors
 Every color is a mix of red, green, and blue
 You can make your own colors:
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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)
Setting a color
 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
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The paint method so far
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…
}
}
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Pixels
 A pixel is a picture (pix) element
 one pixel is one dot on your screen
 there are typically 72 to 90 pixels per inch
 java.awt measures everything in pixels
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Java’s coordinate system
(0, 0)
(50, 0)
(0, 20)
(50, 20)
(w-1, h-1)
 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
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Drawing rectangles
 There are two ways to draw rectangles:
 g.drawRect( left , top , width , height );
 g.fillRect(left , top , width , height );
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The complete applet
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);
}
}
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Some more java.awt methods
 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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The HTML page
 You can only run an applet in an HTML page
 The 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
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Programme-2
 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
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Compiling and Executing Program:
javac HelloWorld.java
appletviewer test.HelloWorld.html
Programme-1
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Programme-1
import 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)
 Methods for Milestones
 init() – initialize the applet
 start() – start the applet’s execution
 stop() – stop the applet’s execution
 destroy() – perform a final cleanup
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Basic Methods(2/2)
 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) {
...
}
}
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<Applet Code>
<APPLET CODE=... CODEBASE=...
WIDTH=... HEIGHT=...>
...
</APPLET>
<HTML Code>
Example
 Source Code
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
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 Result
Programme
public class SetGraphicsColorExample extends Applet{
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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.
*/
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//this will create light blue color
Color customColor = new Color(10,10,255);
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g.setColor(customColor);
g.drawLine(10,10,30,30);
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g.setColor(Color.red);
g.fillRect(40,40,40,40);
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g.setColor(Color.green);
g.fillRect(80,80,40,40);
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g.draw3DRect(81,81,40,40,true);
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}
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}
Program
Program 1: Write an applet program with a message and display the message in paint () method.
/* <applet code="MyApplet.class" width = 600 height= 450>
</applet> */
Program