Java repaint() method doesn't always work

Question

There is a problem with the repaint() method in Java. I made a new thread that constantly repaints the screen. When I release the spacebar I want my player to fall smoothly by setting its position and then waiting for 50 milliseconds and looping that 20 times. Instead, it waits the whole amount of time in the loop, then repaints. I am wondering why it doesn't constantly repaint the changes in the players co-ordinates. Thank you.

(Edit) Thanks everyone for the help. This is my first time using stack overflow, and I am only 13 and still learning java, so I probably will go back to the tutorials again.

My 'a' class (main):

public class a {
    public static void main(String[] args) {
        JFrame frame = new JFrame("StickFigure Game");
        frame.setSize(740, 580);
        frame.setDefaultCloseOperation(frame.EXIT_ON_CLOSE);
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
        b board = new b();
        frame.add(board);
        frame.addKeyListener(board);
    }
}

My 'b' class (JPanel/drawing):

public class b extends JPanel implements KeyListener {
    c player = new c();

    public class MyRunnable implements Runnable {

        public void run() {
            while (true)
                repaint();
        }
    }

    MyRunnable run = new MyRunnable();

    public void paint(Graphics g) {
        super.paint(g);
        Graphics2D g2d = (Graphics2D) g;
        g2d.drawImage(player.getImage(), player.getX(), player.getY(), 80, 140,
                null);
    }

    public b() {
        Thread thread = new Thread(new MyRunnable());
        thread.start();
    }

    public static void slow(int n) {
        long t0, t1;
        t0 = System.currentTimeMillis();
        do {
            t1 = System.currentTimeMillis();
        } while (t1 - t0 < n);
    }

    public void keyPressed(KeyEvent e) {
        if (e.getKeyCode() == KeyEvent.VK_D) {
            player.setPos(player.getX() + 6, player.getY());
        }
        if (e.getKeyCode() == KeyEvent.VK_A) {
            player.setPos(player.getX() - 6, player.getY());
        }
        if (e.getKeyCode() == KeyEvent.VK_SPACE) {
            player.setPos(player.getX(), player.getY() - 60);
        }
    }

    public void keyReleased(KeyEvent e) {
        if (e.getKeyCode() == KeyEvent.VK_SPACE) {

            for (int i = 0; i < 20; i++) {
                slow(50);
                player.setPos(player.getX(), player.getY() + 2);
            }
        }
    }

    public void keyTyped(KeyEvent e) {
    }
}

my 'c' class (player):

public class c {
    private ImageIcon i = new ImageIcon("guy.png");
    private Image img = i.getImage();
    private int x = 0;
    private int y = 100;

    public void wait(int what) {
        try {
            Thread.sleep(what);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

    public c() {
    }

    public Image getImage() {
        return img;
    }

    public int getX() {
        return x;
    }

    public int getY() {
        return y;
    }

    public void setPos(int mx, int my) {
        x = mx;
        y = my;
    }
}

Answer 1

I haven't gone through all the code here but here are some pointers:

• Swing has its own concurrency mechanisms which allow you to handle UI updates. You can use a Swing Timer rather than a raw Thread . Related is the use of Thread.sleep - don't do this, it only blocks the EDT and prevents UI updates.
• The Swing paint chain mechanism requires you to override paintComponent rather than paint .
• Always use Key Bindings rather than KeyListeners in Swing. KeyListeners require component focus to work to interact with the KeyEvents . Key Bindings do not have this limitation.

Answer 2

"There is a problem with the repaint() method in java." Did you consider that perhaps the problem is with your code instead? You are blocking the event thread and giving the system no time to do the intermediate repaints. In particular, this method:

public static void slow (int n){
    long t0,t1;
    t0=System.currentTimeMillis();
    do{
        t1=System.currentTimeMillis();
    }
    while (t1-t0<n);
}

and this loop:

for(int i = 0;i<20;i++){
    slow(50);
    player.setPos(player.getX(), player.getY()+2);
}

do not relinquish control to the system so that repaints can actually happen. Rewrite those using Swing timers. Look at this tutorial for an introduction on how to use these.

Also, your thread that constantly calls repaint() in a tight loop:

public void run(){
  while(true) repaint();
}

is a terrible idea. You don't need to call repaint() at full CPU speed. Once every 30 milliseconds or so is fine for animation. Again, consider using Swing utilities to do this rather than writing your own looping thread.

Answer 3

The repaint is only a "request" to paint as soon as possible. so when you call it it causes a call to the paint method as soon as possible.

from here

So basically you just flooding the scheduled calls of paint or update with while(true) repaint(); .

Oracle's stance on painting in AWT and Swing

One way you could do it, or should I say how I would do it, is to make your c class implement KeyListener , so that when a key is pressed (and only when it is pressed) you update it's location.

So move your KeyListener methods to class c , in your class b constructor you can add the call this.addKeyListener(player) or make a method void addPlayer(c player) that adds it.