Re: repainting a JPanel from that panel's MouseLIstener

From:

Knute Johnson <nospam@rabbitbrush.frazmtn.com>

Newsgroups:

comp.lang.java.programmer

Date:

Fri, 18 Aug 2006 15:17:33 -0700

Message-ID:

<f%qFg.40193$k54.29676@newsfe11.phx>

Ted wrote:

Hello all. This is a problem some of my students are experiencing and
I'm not sure of the best way to solve it.

They have written a program to play othello. They have extended JPanel
to create an Othello board which paints the 8x8 grid. They have added a
Mouselistener so when the user click on a spot, it makes the move and
updates.

The problem comes in when a student wanted to have the computer PAUSE
before making its move so the human could see the effect of his/her own
move. The code looks kind of like:

class X extends JPanel implements MouseListener {

paintComponent(Graphics g) { ... }

mousePressed() {
   makeHumanMove();
   repaint();
   Thread.sleep(1000); // wait 1 second...
   makeComputerMove();
   repaint();
}
}

The problem is that since all this code is happening from within the
JPanel, the sleep takes effect before the repaint occurs, so the user
doesn't see any change until after the computer has moved and the
humans move appears to be skipped.

Thanks for your consideration.

Ted Ward

This question has probably been asked more than any other question.
Fortunately it is a very easy problem to solve.

The Swing GUI that the JPanel is part of, does its update from the Event
Dispatch Thread (EDT). Events, such as the mouse press also are
processed on the EDT. So you can't sleep in listener methods if you
want your GUI to stay lively. What you need to do is run your event
code in another thread so that the EDT is never blocked.

public void mousePressed(MouseEvent me) {
     Runnable r = new Runnable() {
         public void run() {
             // sleep, dream or speak (as the case may be)
         }
     };
     EventQueue.invokeLater(r);
}

--

Knute Johnson
email s/nospam/knute/