Java TimerTick event for game loop

Question

I tried making a game loop in Java using the Timer from java.util.Timer. I am unable to get my game loop to execute during the timer tick. Here is an example of this issue. I am trying to move the button during the game loop, but it is not moving on the timer tick event.

import java.util.Timer;
import java.util.TimerTask;
import javax.swing.JFrame;
import javax.swing.JButton;

public class Window extends JFrame {

    private static final long serialVersionUID = -2545695383117923190L;
    private static Timer timer;
    private static JButton button;

    public Window(int x, int y, int width, int height, String title) {

        this.setSize(width, height);
        this.setLocation(x, y);
        this.setTitle(title);
        this.setLayout(null);
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setVisible(true);

        timer = new Timer();
        timer.schedule(new TimerTick(), 35);

        button = new JButton("Button");
        button.setVisible(true);
        button.setLocation(50, 50);
        button.setSize(120, 35);
        this.add(button);
    }

    public void gameLoop() {

        // Button does not move on timer tick.
        button.setLocation( button.getLocation().x + 1, button.getLocation().y );

    }

    public class TimerTick extends TimerTask {

        @Override
        public void run() {
            gameLoop();
        }
    }
}

Answer 1

First of all, Timer.schedule schedules the task for one execution, not for repeated executions. So this program can only make the button move once.

And you have a second problem : all the interactions with swing components should be done in the event dispatch thread, and not in a background thread. Read http://download.oracle.com/javase/6/docs/api/javax/swing/package-summary.html#threading for more details. Use a javax.swing.Timer to perform swing actions at repeated intervals.

Answer 2

Since this is a Swing application, don't use a java.util.Timer but rather a javax.swing.Timer also known as a Swing Timer.

e.g.,

private static final long serialVersionUID = 0L;
private static final int TIMER_DELAY = 35;

in the constructor

  // the timer variable must be a javax.swing.Timer
  // TIMER_DELAY is a constant int and = 35;
  new javax.swing.Timer(TIMER_DELAY, new ActionListener() {
     public void actionPerformed(ActionEvent e) {
        gameLoop();
     }
  }).start();

and

   public void gameLoop() {
      button.setLocation(button.getLocation().x + 1, button.getLocation().y);
      getContentPane().repaint(); // don't forget to repaint the container
   }