Applets

1
Applets Applications

• CSC 171 FALL 2001
• LECTURE 15

2
History Grace Hooper

• 1952 Grace Hopper working on the UNIVAC, "The
  Education of a Computer", (Proc. ACM Conference)
• The use of selected pre-written code segments to
  be assembled into programs written in a high
  level language
• The concept of a compiler, and the general
  concept of language translation.

3
Applets Applications

• Applets
• Generally run over networks
• Code download
• Client side execution
• Extend Applet
• Applications
• Generally run on hosts
• Resident host
• Local host execution
• Extend Frame

4
GUIs in general

• Sometimes, it is useful to have a program that
  runs as either an applet or an application

5
Applet/App Architecture
Button Handler
actionListener
Applet
actionPerformed()
DrawShapes
JPanel
setWidth()
init()
DrawPanel
setHeight()
main()
setcurrentchoice()
paintComponent()
6
Relationships

• DrawShapes
• is-a Applet
• has-a Jframe (inside of main)
• Has-a DrawPanel
• Has-a ButtonHandler
• DrawPanel is-a Jpanel
• ButtonHandler implements ActionListener

7
What Happens In an Applet?

• JVM calls DrawShapes.init()
• Builds DrawPanel
• Builds ButtonHandler
• BuildsButtons
• Adds components

8
What Happens In an Application?

• JVM calls DrawShapes.main()
• Builds Jframe application window
• Builds DrawShapes Applet (see previous)
• Calls DrawShapes.init()
• Calls DrawShapes.start.
• Adds applet to application window
• I.E. (har) Frame takes place of browser

9
Using Command Line Args

• public static void main( String args )
• int width, height
• // check for command-line arguments
• if ( args.length ! 2 )
• width 300
• height 200
• else
• width Integer.parseInt( args 0 )
• height Integer.parseInt( args 1 )

10
Build the Application

• // create window in which applet will execute
• JFrame applicationWindow
• new JFrame( "An applet running as an
  application")
• applicationWindow.setDefaultCloseOperation(
• JFrame.EXIT_ON_CLOSE )

11
Build the applet from the application

• // create one applet instance
• DrawShapes appletObject new DrawShapes()
• appletObject.setWidth( width )
• appletObject.setHeight( height )
• // call applet's init and start methods
• appletObject.init()
• appletObject.start()

12
Get it going

• // attach applet to center of window
• applicationWindow.getContentPane().add(
  appletObject )
• //note nested method calls
• applicationWindow.setSize( width, height )
• applicationWindow.setVisible( true )

13
Architecture of Applet

• Separate class for button handler
• Separtae class for drawing region
• Separate button panel Draw panel
• Applet is in BorderLayout
• NORTH
• Panel containing grid layout with 3 buttons
• CENTER
• Drawing region

14
ButtonHandler

• private class ButtonHandler implements
  ActionListener
• // can access choices because it is an inner
  class
• public void actionPerformed( ActionEvent
  event )
• for ( int count 0 count lt
  choices.length count )
• if ( event.getSource() choices
  count )
• drawingPanel.setCurrentChoice(
  count )
• break
• // end private inner class ButtonHandler

15
DrawPanel

• Using a separate panel allows painting without
  interfering with other components
• Loosely coupled with Applet
• No shared data

16
When ButtonHandler invokes setCurrentChoice()

• // specify current shape choice and repaint
• public void setCurrentChoice( int choice )
• currentChoice choice
• repaint()

17
DrawPanel

• public void paintComponent( Graphics g )
• super.paintComponent( g )
• switch( currentChoice )
• case 0
• g.drawLine( randomX(), randomY(),
• randomX(),
  randomY() )
• break //etc. . . .
• // end method paintComponent