1. Object Oriented Programming

2. Topic to be covered today
Applets

3. Applets and Applications
An applet is a Java program that runs on a web page
Applets can be run from:
Internet Explorer
Netscape Navigator (sometimes)
appletviewer
An application is a Java program that runs all by itself

4. Packages and Classes
Java supplies a huge library of pre-written “code,” ready for you 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

5. 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
Applet class derives from the Abstract Window Toolkit (AWT) hierarchy.

6. 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

7. 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!

8. AWT Components(Will be Covered in Next Lecture)
AWT supplies the following UI components:
Buttons (java.awt.Button)
Checkboxes (java.awt.Checkbox)
Single-line text fields (java.awt.TextField)
Menus (java.awt.MenuItem)
Containers (java.awt.Panel)
Lists (java.awt.List)

9. Importing 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

10. Your First Java Applet To try it Compile: javac Hello.java
Test: appletviewer hello.html

11. Life Cycle of an Applet

12. OUTPUT Now Press minimize button, stop( ) method will be called
init( ) method called
start( ) method called after init( )
Maximizing Screen will again call start( ) method
Closing the window will call destroy( ) method

13. Life Cycle of an Applet
init(): browser calls it when applet first loaded
start(): Called immediately after init( ) and again revisited after browser left page containing applet
stop(): stop execution (eg. after switching to different page), Called when the browser leaves the page
destroy(): Called when applet is killed (rarely used)
paint(): browser tells it it’s time to redraw
Called by the browser after init and start, and again whenever the browser redraws the screen
This method is where user-level drawing is placed
Inherited from java.awt.Container

14. Exploring Graphics class
public void paint(Graphics g) { … }
A Graphics is something 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, and it remembers what you have painted
Graphics g says g is an object of type Graphics
We can paint on g

15. Colors The java.awt package defines a class named Color
There are 13 predefined colors--here are their fully- qualified names:
Color.BLACK Color.PINK Color.GREEN Color.DARK_GRAY Color.RED Color.CYAN Color.GRAY Color.ORANGE Color.BLUE Color.LIGHT_GRAY Color.YELLOW Color.WHITE Color.MAGENTA

16. New Colors 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)

17. Setting a Color g.setColor(Color.RED);
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

18. Java Coordinate System
(0, 0)
(0, 20)
(50, 0)
(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

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

20. Example 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); } }

21. 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 );

22. Graphics Font setFont, getFont
Specifies the font to be used for drawing text
Setting the font for the Graphics object does not persist to subsequent invocations of paint
Set the font of the window (I.e., call the applet’s setFont method) for permanent changes to the Graphics object
In JDK 1.1, only 5 fonts are available: Serif (aka TimesRoman), SansSerif (aka Helvetica), Monospaced (aka Courier), Dialog, and DialogInput

23. Drawing Strings
A String is a sequence of characters enclosed in double quote marks
"Hello, World!"
A double quote mark in a String must be preceded by a backslash ( \ )
"He said, \"Please don't go.\""
g.drawString( string , x , y );

24. Graphics Behavior
Browser calls repaint method to request redrawing of applet
Called when applet first drawn or applet is hidden by another window and then re-exposed

25. Assignment
Search awt Method for loading an Image. Also find Graphics Method for drawing that image in Applet window and write instructions for loading and drawing an Image using Applet viewer
Also explore how to play an audio file