Java Programming - Applets and Graphics in Java, Slides of Java Programming

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Java-Programming

Day-

Outlines

• Applets

 Creation

 Applets and Html

 Graphics in Applets

 Displaying Pictures

 Applet life Cycle

 Creating Applets in JBuilder

Writing Your First Applet

• Required Packages/Classes

 java.applet.*

 java.applet.Applet

 java.awt.*

 javax.swings.*

• Few steps to write an applet

 Import the required package into your program

import java.applet.*;

import java.awt.*;

 Create a class of your choice

public class HelloApplet{ }

 Extend the class from java.applet.Applet class

public class helloApplet extends Applet{ }

Helloapplet..cont..

• Add the following method

 public void paint(Graphics g) { }

public class HelloApplet extends Applet{

public void paint(Graphics g){ } }

• Add g.drawString(“Hello World”,25,25)

public class HelloApplet extends Applet{

public void paint(Graphics g){ g.drawString(“Hello World”,25,25); } }

• Save the program to a file HelloWorld.java
• Compile it

Complete HelloWorld Applet

• HelloWorld.java

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

public class HelloWorld extends Applet{ public void paing(Graphics g){ g.drawString(“Hello World”,25,25); } }

• HelloWorld.html

<applet codebase=“.” code=“HelloWorld.class” height= width=100 >

HelloWorld Eplained

• Few questions about HelloWorld program

 Why I need a html file

 Applet are to be executed in a web browser

 Why extending from Applet class

 Each web browser require specific method to be in an applet  There are numerious (one book say about 200 ) methods required by the browser  java.applet.Applet have the core functionality required by the browser

 Why using public void paint(Graphics g) method

 Inherited from the parent  Empty implementation  Child creates its own version according to requirements

 What is the parameter (Graphic g)

 A java class for rendering graphics on the screen  Browser provides the implementation according to the local system

Graphics Class

• Contains extensive set for free hand drawing

 Drawing Lines, Circles, rectangles etc

 Drawing polygons, 3D rectangles, spheres etc

 Color filling in drawings

• Contains method for drawing images and playing audioclips
• Contains method for displaying different fonts with different colors
• Few of such methods are

 drawLine();

 drawRect();

 setFont();

 drawImage();

 drawOval();

Changing font color

public void paint(Graphics g){ g.setColor(Color.green); g.drawString("The changed world",25,25); }

• setColor(color)

 Require a color class object

 Used to change the current drawing color

• The awt package contain extensive set of classes for color and fonts

 Color class

 Can be uses to define colors  Color c=new Color(12,34,20);  Contains predefined colors  Color.red  Color.green

Loading images to you applet

public void paint(Graphics g){ Image img=getImage(getCodeBase(),"test.gif"); g.drawImage(img,40,40,50,50,this); }

 Image Class

 An abstract class in awt package  Provide image manipulation method such as controlling height width etc..

 GetImage

 A method provided by Applet parent class  Use to load images from local system and servers as well

 getCodeBase()

 Returns a url pointing to the location where the class file is saved

 Can also use getDocumentBase()

 Returns a url pointing to the location where the html file of the applet is saved

Applet Life Cycle

• Browser calls specific methods from your applets at specific events
• The life cycle of an applet

 init()  The first method called by the browser  Can be used to do some initialization work  start()  Called by the browser when init() method has finished execution  Called to restart the applet when it is stopped by the user or browser  paint()  The paint method is called after start to draw graphics on the screen  re-called each time when there is something to draw on the screen or there is some changing in the drawing  If the applet has GUI components it follow a different path  Stop()  Called by the browser when the applet is not being displayed  This method stops the execution of the applet thread  destroy()  Called by the browser just before destroying the applet  This method can be used to do some wrap up work

Creating Applets in JBuilder

• Step-
• Open file menu

 select new  Select applet  Click OK

Step-

• Create package

 Write your package name  Write class name for the applet  Select one of the two base file  Choose Applet  If checked unchecked generate standard methods  Used to create all standard method e.g. init ,start stop…  Click next

Step-

• The HTML file

 Choose a file name for your html file  Choose a title  Choose height and width parameter values for your applet  You can ignore hspace and vspace  Click next

Step-

• Applet Configuration

 Ignore it and click finish