Object Oriented Programming

Topic to be covered today Applets

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

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

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.

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

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!

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)

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

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

Life Cycle of an Applet

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

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

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

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

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)

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

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

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

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); } }

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 );

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

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 );

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

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