Object Oriented Programming

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 toimport 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 thejava.applet package • TheAppletclass 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.appletis the name of the package • A dot (. ) separates the package from the class • Appletis 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.awtis 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” • Theimportdirectives 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 init( ) method called Now Press minimize button, stop( ) method will be 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 • Graphicsg says gis an object of typeGraphics • We can paint ong

Colors • The java.awtpackage defines a class named Color • There are 13 predefined colors--here are their fully-qualified names: Color.BLACK Color.PINK Color.GREENColor.DARK_GRAY Color.RED Color.CYANColor.GRAY Color.ORANGE Color.BLUEColor.LIGHT_GRAY Color.YELLOWColor.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 • To use a color, we tell our Graphicsg 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

(0, 0) (50, 0) (50, 20) (0, 20) (w-1, h-1) Java Coordinate System • 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’ssetFont 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

Object Oriented Programming