Re: Can a jPanel's backgr

"Andrew Thompson" <andrew.thompson@THRWHITE.remove-dii-this>
Wed, 27 Apr 2011 15:35:18 GMT
JT wrote:

In any case, it is possible to *extend* JPanel (or jPanel,
I am guessing) to provide a method that will do that.

Sometimes the fonts on my machine make jPanel and JPanel look the same,
especially if it's late at night.

Please either get a better font, or better excuses..

...But, to answer your question, I am
looking for a way to make my "flashing green" light flash. I looked in
the API but could not find anything which would do what I wanted,

I find that completely unsurprising. I cannot imagine
it is often that a GUI requires *flashing* panels.
(muses) Perhaps slightly more common than a
requirement for panels with polka dots, but not
much so. I
thought someone might have done something similar in the past and would
be willing to share their ideas.

I haven't, but a slight tweak of my earlier code should
point *a* way to go about it. Note that this is still using
AWT (just because I'm feeling perverse) and uses the
centred circle (rather than the background) to change
color. Further, no logic is performed to ensure that only
one signal is flashing at any instant.

Adapt as needed ('batteries not included').

import java.awt.*;
import java.awt.event.*;

class TrafficLight {

  public static void main(String[] args) {
    Frame f = new Frame("Lights");
    Panel lightboard = new Panel( new GridLayout(0,1) );

    lightboard.add( new TrafficSignal( );
    lightboard.add( new TrafficSignal(Color.yellow) );
    lightboard.add( new TrafficSignal( );

    f.add( lightboard );
    f.addWindowListener( new WindowAdapter() {
      public void windowClosing(WindowEvent we) {
    } );

class TrafficSignal extends Panel implements Runnable {

  Color on;
  int radius = 55;
  int border = 10;
  boolean active;

  boolean flashing = false;
  long startFlash = 0;
  /** It should flash for 10 seconds. */
  long lengthFlash = 10000;

  TrafficSignal(Color color) {
    on = color;
    active = true;
    this.addMouseListener( new MouseAdapter() {
      public void mouseClicked(MouseEvent me) {
    } );
    Thread t = new Thread(this);

  public Dimension getPreferredSize() {
    int size = (radius+border)*2;
    return new Dimension( size, size );

  public void run() {
    while (true) {
      if ( flashing ) {
        active = !active;
        if ( System.currentTimeMillis()-startFlash>lengthFlash ) {
          flashing = false;
      try {
      } catch (InterruptedException ie) {
        // wake and continue

  public void setFlashing(boolean flash) {
    flashing = flash;
    startFlash = System.currentTimeMillis();

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

    if (active) {
      g.setColor( on );
    } else {
      g.setColor( on.darker().darker().darker() );
    g.fillOval( border,border,2*radius,2*radius );


Andrew Thompson

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