CODEWITHSUNDEEP
javaswing
Vexx
Vexx

Reputation: 9

.fillOval coordinates not updating during repaint()

I'm following along in a (dated) Java book, and this project is supposed to "animate" a circle moving across the screen. However, when the program is run, the circle remains in one spot. My code looks identical to the book's. Am I forgetting something? Am I calling repaint() at the wrong time?

public class Animation 
{
JFrame f;
int x, y;
public static void main(String [] args)
{
    Animation a = new Animation();
    a.go();
}

public void go()
{
    f=new JFrame();
    myPanel p=new myPanel();

    f.getContentPane().add(p);
    f.setSize(300, 300);
    f.setVisible(true);
    for(int i=0; i<=50; i++)
    {
        p.repaint();
        x++;
        y++;
    }

}

class myPanel extends JPanel
{
    public void paintComponent(Graphics g)
    {
        super.paintComponent(g);
        g.setColor(Color.GREEN);
        g.fillOval(x, y, 40, 40);
    }
}
}

Upvotes: 0

Views: 39

Answers (1)

MadProgrammer
MadProgrammer

Reputation: 347234

So, immediately, two things jump out at me.

  1. It's possible that the loop which is updating the values is NOT running on the Event Dispatching Thread, which could be leading to dirty read/writes
  2. repaint can consolidate requests to reduce the amount of work placed on the Event Queue/Event Dispatching Thread. Since there is not "artificial" delay in the loop, it's possible that all your requests are been reduced to a single update pass by the RepaintManager

The first thing I would do is isolate the responsibility for the management of the position of the oval, because in your current code, it could be updated from anywhere, which is just a mess

class MyPanel extends JPanel {

    private Point posy = new Point(0, 0);

    public Point getPosy() {
        return posy;
    }

    public void move() {
        Point posy = getPosy();
        posy.x++;
        posy.y++;
        repaint();
    }

    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.setColor(Color.GREEN);
        g.fillOval(posy.x, posy.y, 40, 40);
    }
}

Next, I'd ensure that the context of the UI is been modified from within the EDT...

public static void main(String[] args) {
    EventQueue.invokeLater(new Runnable() {
        @Override
        public void run() {
            Animation a = new Animation();
            a.go();
        }
    });
}

and finally, I'd make use of Swing Timer to act as a pseudo loop for the animation...

public void go() {
    f = new JFrame();
    MyPanel p = new MyPanel();

    f.getContentPane().add(p);
    f.setSize(300, 300);
    f.setVisible(true);

    Timer timer = new Timer(5, new ActionListener() {
        @Override
        public void actionPerformed(ActionEvent e) {
            p.move();
        }
    });
    timer.start();

}

There are a number of reasons for using a Swing Timer in this context. First, the "ticks" are executed ON the EDT, making it safe to update the UI from within and it won't block the UI while it's "waiting" between ticks

I would recommend having a look at:

Upvotes: 2

Related Questions