Applets

The hierarchy

   Object -> Component -> Container -> Panel -> Applet ->  your program
                              \
                                 -->  Window -> Frame : for applications

Words Applet

 import java.applet.*;
 import java.awt.*;

public class Words extends Applet 
 {
   int random(int i)
 {
   return Math.round(Math.random()*i) ;  // calls to class methods
 }

   public void paint(Graphics g)       // overriding paint
 {
   String fontName = getParameter("font");  // taking parameters from html file
   Font f = new Font(fontName, Font.BOLD,18)
   g.setFont(f);
   for (int i = 0; i<10; i++)
      g.drawString(random(200),random(100),"Don't touch"); // graphics upside down
 }  
}

Corresponding html file

 < HTML >
    < HEAD >
    < TITLE > Whatever < /TITLE >
    < /HEAD >
    < BODY >
    < APPLET CODE="Words.class" WIDTH=283 HEIGHT=190 >
    < PARAM NAME=font VALUE="Helvetica" > 
    < /APPLET >
    < /BODY >
< /HTML >

Applet Syntax

• Just like an application an applet is a collection of classes.
• Rules for execution and definition of classes are the same, except for the driver class and reading of html files.
• You also need to define an associated html file.
• The browser first reads the html file.
• Parameters can be passed from the html file to the applet via getParameter(...).
• After reading the html file, loading the applet and executing any static expressions and blocks, then the applet is executed.
• The driver class for an applet is a class which extends applet and is called first by the browser or appletviewer.
• Typically applets respond to events. We will discuss event-driven programming later, as they have another model of interpretation.
• Browsers have security managers which place various restrictions on Applets for obvious reasons. Typically you are not allowed to read or write files.

Applet Methods

• void init() Trigger Event: first visit to html page.
  This is the first method that is executed. Typically one creates all the needed visual objects and other objects here. This method is only executed once and behaves much like a constructor.
• void start() Trigger Event: every visit to html page.
  This method is called after init and everytime the user revisits the page. This resumes the computation.
• void stop() Trigger Event: moving off the page.
  The default suspends the applet from running. So this can be executed multiple times, just like start().
• void destroy() Trigger Event: Browser closes applet.
  This behaves somewhat like a destructor in C++ in that all resources associated with the applet are now released. Before it is executed, stop() is called for the last time.

Graphic Methods inherited from Component

• public void paint(Graphics g):
  This method must be executed to render anything on the display. It does the drawing.
• public void repaint():
  This schedules update(), i.e. puts update into the task list. Multiple calls to repaint() may yield only one call to update().
• public void update(Graphics g):
  The default method redraws the background and calls paint() on each of the visual components. By overriding this method you can selectively update the display, which is useful for animation.

Another Applet Example

/*
   Simple fractal program that illustrates recursion.
   Recall that the vector <-y,x> is perpendicular to the vector .
   Check the dot product.
 */

import java.awt.*;
import java.applet.*;

public class Fractal extends Applet {

	public void init()
	{
           resize(600,400);
	}	
	
	Point addPoints(Point one, Point two)
	{
	    return new Point (one.x+two.x,one.y+two.y);
	}
	
	Point up(Point one, Point two, double scale)
	{
	     int x = (int) (scale*(two.x-one.x));
	     int y = (int) (scale*(two.y-one.y));
	     return addPoints(one, new Point(x,y));
	}
	
	Point rotate(Point one,Point two)
	{
	    Point p = up(one,two, 1.0/2);
	    int y = (int)((one.x - two.x)/4.0 );
	    int x = (int) ((two.y- one.y)/4.0);
	    return addPoints( p, new Point(x,y));
	}   
	    
	public void drawFractal(Point b,Point e, int d, Graphics g)
	{
	    if (d == 0) g.drawLine(b.x,b.y,e.x,e.y);
	      else
	     {
	        Point next = up(b,e,1.0/4);
	        Point mid = rotate(b,e);
	        Point last = up(b,e,3.0/4);
	        drawFractal(b,next, d-1,g);
	        drawFractal(next,mid,d-1,g);
	        drawFractal(mid,last,d-1,g);
	        drawFractal(last,e,d-1,g);
	   	 }
	}
	
	public void paint(Graphics g)
	{
	  g.setColor(Color.blue);
	  Point begin = new Point(100,200);
	  Point end = new Point(500,200);
	  int depth = 4;     // How deep is too deep?
	  drawFractal(begin,end,depth, g );
	}
}