applet paint() refresh problems

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foo - 27 May 2005 21:48 GMT

the following applet is supposed to draw random lines on the screen, but
doesn't. does anyone know how to make it refresh?

thanks in advance

import java.awt.*;
import java.awt.event.*;
import java.applet.*;
import java.util.*;
import java.text.*;
import java.lang.*;

public class UI2 extends Applet{
private int x1,y1,x2,y2;

public void init() {
for(;;) {
x1=(int)(Math.random()*100);
y1=(int)(Math.random()*100);
x2=(int)(Math.random()*100);
y2=(int)(Math.random()*100);
repaint();
}
}

public void paint(Graphics g) {
g.setColor(Color.black);

g.drawLine(x1, y1, x2, y2);
}

public void update(Graphics g) {
paint(g);
}

}

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sanjay manohar - 02 Jun 2005 02:08 GMT

try doing your for loop in a new thread. at the moment, it blocks the
initialisation of the applet.

public void init(){
Thread t = new Thread(this);
t.start();
}
public void run(){
for(;;){
// etc....
}
}

PS - do you want lines that have been drawn to remain on the screen or
not?
if not, call super.paint(g) in the paint method before you draw.
if so, you may want to store the coordinates of the lines and repaint
them.

Reply to this Message

Dave - 23 Jun 2005 18:46 GMT

Adding a few more details to Sanjay's response...

Like Sanjay said, the endless loop in your init() method prevents init()
from completeing. This prevents a number of things from happening,
including preventing your paint() method from being called by the Applet
code.

Your Applet is not fully instantiated until init() runs to completion.
Until the Applet is instantiated, paint() can not be called. Once
init() is done, the Applet class code will call your start() method.
Only you do not have a start method (you should have one).

Rename your init() method to start(), and it will work.

Without understanding how Applets work this is a hard problem to
understand. Try reading the file java.applet.Applet.html (included in
your JDK). Grab some Applet example code from the web to learn about
other things you need to know and do. Or a book. Whatever.

To debug things like this yourself, try adding println() calls to the
begining and end of every method (when you have a small number of
methods), like this:
System.out.println("-> UI2 init"); // Going into init()
System.out.println("<- UI2 init"); // Leaving init()

Carefully reading the output can help you understand what went wrong, so
you can debug code faster on your own.

Add as many println() calls as you need to help figure it out.

--Dave

Java__Dave@NOSPAM_Hotmail.com

Remove NOSPAM_ to reply...
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applet paint() refresh problems

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