Introduction To Applets

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Introduction To Applets

• (Finally!)

2
Overview

• Applet Skeleton
• Callback methods
• HTML Overview
• The Graphics class
• 2D coordinate system
• Drawing primitive shapes
• Colors
• Text
• Images and Sounds

3
What you should know(before starting applets)

• Good grasp of classes
• Accessors
• Modifiers
• Calling methods of classes (dot operator)
• Functions
• Data Passing

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Notes about Applets

• Security
• Live in a sandbox
• Cant read/write files
• No communication over the net
• Web-based
• No main to start things off (callback based)

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The Applet Skeleton

• import java.awt.
• public class MyApplet extends java.applet.Applet
  

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Remember

• the class must be a public class!
• you inherit from the Applet class
• put in a file called MyApplet.java
• The import statement pulls in a graphics package
• Graphics, Buttons, Textfields
• Applets have attributes too!

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Callback Methods

• Event-based programming
• Mouse clicked
• Button pressed
• Mouse moved
• Window resized
• Web-browser looks for certain functions in your
  applet to handle certain situations
• init (used for initializing attributes)
• paint (guess what this function does)

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public void init ( )

• Inherit this method from java.applet.Applet
• Called exactly once
• Called at the beginning of the lifetime of the
  applet (a lot like a constructor)
• Used to initialize attributes
• Load large files (audio/images)
• Bring attributes to life
• Set layout of Buttons, Textfields

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public void paint (Graphics g)

• Inherit this method from java.applet.Applet
• Used any time the applet needs to redraw itself
• The first time the applet is run
• The applet is resized, minimized, maximized
• Graphics object passed as a parameter
• Tell the graphics object what to do

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Graphics Methods

• void drawLine (int x1, int y1, int x2, int y2)
• void drawOval (int x1, int y1, int width, int
  height)
• void drawRect (int x1, int y1, int width, int
  height)
• void setColor (Color c)
• void setFont (Font f)
• void drawString (String s, int x, int y)
• .
• .
• .

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2D Coordinate System(x over, y up)
y a x i s
(, )
(-, )
x axis
(-, -)
(, -)
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Javas Coordinate System
(0, 0)
x axis
y a x i s
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Colors

• There is a Color class
• Two ways to initialize a color
• Use new (call to constructor with RGB)
• Use static colors (Color.green, Color.blue)
• Constructor has ranges of (0-255) or (0.01.0)
• Example
• Color myColor, yourColor
• myColor Color.red //myColor is red
• yourColor new Color (255, 0, 0) //yourColor
  is red

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Example(What does this do?)

• import java.awt.
• public class MyApplet extends java.applet.Applet
  
• Color myColor // An attribute
• public void init ( )
• myColor new Color (255, 0, 255) // purple
• public void paint (Graphics screen)
• screen.setColor (myColor)
• screen.drawLine (0, 0, 200, 200)

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Output
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Basic HTML

• Necessary to learn a little HTML
• Applets run in web pages, remember?
• Embed the applet into the web page
• This is the language you make web pages with
  (HyperText Markup Language)
• Things between the lt and the gt are called tags
• There are four tags you need to know

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The Most Simple Web Page

• ltHTMLgt // Means begin the web page
• lt/HTMLgt // Means end the web page
• This is a pair of tags
• It is not case-sensitive (unlike Java)
• lthTMlgt is the same as ltHtMlgt
• Put things you want in the web page between
  lthtmlgt and lt/htmlgt
• Save as a .html file

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Our Applet Web Page

• lthtmlgt
• ltapplet codeMyApplet.class width300
  height300gt
• lt/appletgt
• lt/htmlgt

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Skeleton of a Web Page

• lthtmlgt
• ltapplet codeMyApplet.class width300
  height300gt
• lt/appletgt
• lt/htmlgt

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Include the Applet

• lthtmlgt
• ltapplet codeMyApplet.class width300
  height300gt
• lt/appletgt
• lt/htmlgt

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Which code to run

• lthtmlgt
• ltapplet codeMyApplet.class width300
  height300gt
• lt/appletgt
• lt/htmlgt

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How Big the Applet Is

• lthtmlgt
• ltapplet codeMyApplet.class width300
  height300gt
• lt/appletgt
• lt/htmlgt

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Images and Sounds

• Very similar in behavior
• Declare attribute (either Image or AudioClip)
• Load the information (getImage or getAudioClip)
• Play or display
• Remember to load the information in init ( )
  method
• Keeps from playing/displaying incomplete info
• init is guaranteed to be called

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Images

• Draw the image in the paint method
• Can draw .jpg and .gif format
• No .bmp (bitmaps)
• Two Formats
• public void paint (Graphics g)
• // Draw normal size
• g.drawImage (picture, 10, 10, this)
• // Force pic to be 100 x 100
• g.drawImage (picture, 10, 10, 100, 100, this)

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Audio

• Need to import java.applet.AudioClip
• File Formats
• .wav
• .au (good for web)
• .mid
• Different methods of AudioClip
• play ( ) // play one time
• loop( ) // repeat over and over
• stop ( ) // stop playing
• Can also use the play( ) command for one-shot
  deal
• play (getCodeBase( ), mySound.wav)

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Start With Skeleton

• import java.awt.
• public class MediaApplet extends
  java.applet.Applet

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Put in init( ) and paint( )

• import java.awt.
• public class MediaApplet extends
  java.applet.Applet
• public void init ( )
• public void paint (Graphics g)

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Declare Attributes

• import java.awt.
• public class MediaApplet extends
  java.applet.Applet
• Image thePicture
• AudioClip myClip
• public void init ( )
• public void paint (Graphics g)

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Remember to import!

• import java.awt.
• import java.applet.AudioClip
• public class MediaApplet extends
  java.applet.Applet
• Image thePicture
• AudioClip myClip
• public void init ( )
• public void paint (Graphics g)

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Load the Media

• import java.awt.
• import java.applet.AudioClip
• public class MediaApplet extends
  java.applet.Applet
• Image thePicture
• AudioClip myClip
• public void init ( )
• thePicture getImage (getCodeBase( ),
  thePic.gif)
• myClip getAudioClip (getCodeBase( ),
  scream.au)
• public void paint (Graphics g)

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Produce the Media

• import java.awt.
• import java.applet.AudioClip
• public class MediaApplet extends
  java.applet.Applet
• Image thePicture
• AudioClip myClip
• public void init ( )
• thePicture getImage (getCodeBase( ),
  thePic.gif)
• myClip getAudioClip (getCodeBase( ),
  scream.au)
• public void paint (Graphics g)
• g.drawImage (thePicture, 10, 10, this)
• myClip.play ( )

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Summary

• Applets have skeletons (go ahead and memorize)
• Applets have callback functions (no main)
• The paint method is used for drawing images and
  primitive shapes
• Applets are very secure
• Applets run in a web page, so you need to make an
  .html file for them

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