Question

I’m trying to edit my timer so that every 25 times repaint() is called the timer firing speed cuts in half. So the first 25 times it’s 500; then the next 25 times its 250; and so on.

Two ‘EASY FOR THE EXPERIENCED’ questions:

1) Why is Eclipse making me make the variables static (or otherwise not compiling)?

2) The program doesn’t seem to reach the function where I divide the speed in half and set the delay to that new speed. Why is that? How do I fix it?

public class MovingCircle extends JFrame implements ActionListener {

    Ellipse2D.Double myEllipse;
    Rectangle2D.Double backgroundRectangle;
    private static int paintCount = 0;
    private static int speed = 500;

    public MovingCircle() {

        //Make the ellipse at the starting position
        myEllipse = new Ellipse2D.Double( 30, 30, 20, 20 );

        //Make the background rectangle to "erase" the screen
        backgroundRectangle = new Rectangle2D.Double( 0, 0, 400, 300 );
    }

    public static void main(String[] args ) {

        MovingCircle b = new MovingCircle();
        b.setSize( 400, 300 );
        b.setVisible(true);
        b.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );

        Timer t = new Timer(500, b );
        t.start();

        if(paintCount % 25 == 0) {

            t.setDelay((int)(speed / 2));
            speed = (int)(speed / 2);
            System.out.println(speed);
        }
  }

    public void actionPerformed( ActionEvent ae ) {

        //This will be called by the Timer
        myEllipse.setFrame( myEllipse.getX()+1, myEllipse.getY()+1, myEllipse.getWidth(), myEllipse.getHeight());  
        //Move 1 x-pixel and 1 y-pixel every 50 milliseconds ^
        repaint();
    }

    public void paint(Graphics g) {

        paintCount++;     // Incremenets by one for every repaint().
        System.out.println(paintCount);
        int isPaintTen = (int)(paintCount / 10);  // Divid current count by 10.
        Graphics2D g2 = (Graphics2D)g;

        if((isPaintTen % 2) == 0){      // Take modulus to set if #/10 is odd or even.

            g2.setColor( Color.YELLOW );
            g2.fill( backgroundRectangle );
            g2.setColor( Color.RED );
            g2.draw( myEllipse );
        }

        else if((isPaintTen % 2) == 1) {

            g2.setColor( Color.RED );
            g2.fill( backgroundRectangle );
            g2.setColor( Color.YELLOW);
            g2.draw( myEllipse );  
        }
   }

}

Answer 1

1. In your example, paintCount and speed have to be static because you are using them without an instance, from within a method, main(), which is itself static. To avoid having to make them static, you could have referenced them as b.paintCount and b.speed.

2. The code that modifies your timer needs to move into your paint() method. This means your Timer instance will need to become an instance variable, and you should probably create and start the timer within the constructor. Incidentally, these changes also require that paintCount and speed also be made “non-static”.

You should end up with something like this:

public class MovingCircle extends JFrame implements ActionListener{
    Ellipse2D.Double myEllipse;
    Rectangle2D.Double backgroundRectangle;
    private int paintCount = 0;
    private int speed = 500;
    private Timer tmr;

    public MovingCircle() {
        //Make the ellipse at the starting position
        myEllipse = new Ellipse2D.Double( 30, 30, 20, 20 );

        //Make the background rectangle to "erase" the screen
        backgroundRectangle = new Rectangle2D.Double( 0, 0, 400, 300 );

        this.tmr = new Timer(500, this);
        tmr.start();
    }

    public static void main(String[] args ) {
        MovingCircle b = new MovingCircle();
        b.setSize( 400, 300 );
        b.setVisible(true);
        b.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
    }

    public void actionPerformed( ActionEvent ae ) {
        //This will be called by the Timer
        myEllipse.setFrame( myEllipse.getX()+1, myEllipse.getY()+1, myEllipse.getWidth(), myEllipse.getHeight());   //Move 1 x-pixel and 1 y-pixel every 50 milliseconds
        repaint();
    }

    public void paint(Graphics g) {
        paintCount++;     // Incremenets by one for every repaint().
        System.out.println(paintCount);

        if(paintCount % 25 == 0){
            tmr.setDelay((int)(speed / 2));
            speed = (int)(speed / 2);
            System.out.println(speed);
        }

        int isPaintTen = (int)(paintCount / 10);  // Divid current count by 10.
        Graphics2D g2 = (Graphics2D)g;
        if((isPaintTen % 2) == 0){       // Take modulus to set if #/10 is odd or even.
            g2.setColor( Color.YELLOW );
            g2.fill( backgroundRectangle );
            g2.setColor( Color.RED );
            g2.draw( myEllipse );

        } else if((isPaintTen % 2) == 1) {
            g2.setColor( Color.RED );
            g2.fill( backgroundRectangle );
            g2.setColor( Color.YELLOW);
            g2.draw( myEllipse );    
        }
    }
}