Timer control for fading button

Question

Hi
I'm trying to control transparency of dialog and its components when hovering the mouse over. Also, I want a smooth transition between the two transparency (initial and final) levels by fading in and out. To do that I'm using a timer. The code (below) works but it sometimes throws a "java.lang.NullPointerException
" at "alphaChanger.stop();". This only happens if the cursor enters and exits the dialog very quickly. I think adding "alphaChanger.stop();" inside the "new ActionListener()" is not a good idea. Any thoughts will be really appreciated.
   public void mouseEntered(MouseEvent e) {
        alphaChanger = new Timer(delay,new ActionListener() {
            float newAlpha = alpha;
            public void actionPerformed(ActionEvent e) {
                newAlpha += alphaIncrementer;
                if(newAlpha > alphaOn) {
                    newAlpha = alphaOn;
                    alphaChanger.stop();
                    alphaChanger = null;
                    alpha = newAlpha;
                alpha=newAlpha;
                repaint();
                updateComponentsAlpha(newAlpha);
        alphaChanger.start();
    }

Answer 1

Thanks for you prompt response Maxideon. I completely agree with you. There is still something I dont understand:
Consider the following code and result:
    public void mouseEntered(MouseEvent e) {
        System.out.println ("alphaChangerIn= " + alphaChangerIn);
        //if (alphaChanger == null){
        alphaChangerIn = new Timer(delay,new ActionListener() {
            float newAlpha = alpha;
            public void actionPerformed(ActionEvent e) {
                newAlpha += alphaIncrementer;
                if(newAlpha > alphaOn) {
                    newAlpha = alphaOn;
                    System.out.println ( "alphaChangerIn to stop " + alphaChangerIn);
                    alphaChangerIn.stop();
                    alphaChangerIn = null;
                    alpha = newAlpha;
                alpha=newAlpha;
                repaint();
                updateComponentsAlpha(newAlpha);
        alphaChangerIn.start();
        System.out.println (alphaChangerIn.toString());
(20 cycles with not Exception...)
(cycle 21)
alphaChangerIn= null
javax.swing.Timer@15b7986
alphaChangerIn to stop javax.swing.Timer@15b7986
(cycle 22)
alphaChangerIn= null
javax.swing.Timer@87816d
alphaChangerIn to stop javax.swing.Timer@87816d
(cycle 23)
alphaChangerIn= null
javax.swing.Timer@93dcd
alphaChangerIn= javax.swing.Timer@93dcd
javax.swing.Timer@b89838
alphaChangerIn to stop javax.swing.Timer@b89838
alphaChangerIn to stop null
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
As you can see during the first 22 cycles everything went as expected. The mouse event is fired the timer at first is null the it is created and the same timer is destroyed. However, in cycle 23 timer javax.swing.Timer@93dcd
is never destroyed and a new one is created javax.swing.Timer@b89838. However, the timer class is trying to stop the first timer which as you anticipated has been stopped by the creation of the second timer. So, I guess my question is: if the first timer has been destroyed and the second timer has been created, why does the timer class try to stop the first timer when throughout the previous cycles it stops the last timer created?
I hope this makes sense