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
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 DrawPanel is-a Jpanel
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 < 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