Which objects do I attach the Timer class event listener to, in Java?

Question

I am trying to create a whack a mole game. I have used swing to create background and add mole images with event listeners which increment a score each time they are clicked, but I am having problems setting whether they should be visible or not. I thought the best way to do this would be to use a timer to set/reset a boolean (vis). Randomizing the period for which the images are visible would be ideal. I have tried using a swing timer several times but doesn't seem to be working. Where do I instantiate the timer, and to what do I attach the event listener which executes the code after the timer has counted down?

package whackmole;

import java.awt.Dimension;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.Timer;

public class WhackAMole extends JFrame {

public WhackAMole() {
    createAndShowGUI();
}

static int score = 0;
public static JLabel scoreDisplay;
boolean vis;

public static void main(String[] args) throws Exception {
    // run asynchronously
    javax.swing.SwingUtilities.invokeLater(new Runnable() {
        public void run() {
            createAndShowGUI();
        }
    });
}

private static void createAndShowGUI() {

    JFrame frame = new JFrame();
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setMinimumSize(new Dimension(600, 600));
    Holes holes = new Holes(frame);
    frame.getContentPane().add(holes);
    holes.setLayout(null);
    frame.pack();
    frame.setVisible(true);

    scoreDisplay = new JLabel("Score: " + score);
    scoreDisplay.setBounds(239, 11, 84, 38);
    holes.add(scoreDisplay);

    Mole mole = new Mole(68, 92, true);
    mole.addMouseListener(new MouseAdapter() {
        @Override
        public void mouseClicked(MouseEvent e) {
            score++;
            scoreDisplay.setText("Score: " + score);
        }
    });
    holes.add(mole);

    Mole mole2 = new Mole(181, 320, false);
    mole2.addMouseListener(new MouseAdapter() {
        @Override
        public void mouseClicked(MouseEvent e) {
            score++;
            scoreDisplay.setText("Score: " + score);
        }
    });
    holes.add(mole2);

    Mole mole3 = new Mole(414, 439, true);
    mole3.addMouseListener(new MouseAdapter() {
        @Override
        public void mouseClicked(MouseEvent e) {
            score++;
            scoreDisplay.setText("Score: " + score);
        }
    });
    holes.add(mole3);

    Mole mole4 = new Mole(297, 203, false);
    mole4.addMouseListener(new MouseAdapter() {
        @Override
        public void mouseClicked(MouseEvent e) {
            score++;
            scoreDisplay.setText("Score: " + score);
        }
    });
    holes.add(mole4);
 }
}

Answer 1

In this context, you can instantiate your Timer with a fixed rate and a class that implements ActionListener .

public class Example extends JPanel implements ActionListener {
    private static final int RATE = 1000 / 8; // ~8 Hz
    private final Timer timer = new Timer(RATE, this);
}

In this complete example , GameButton is a subclass of JToggleButton , and the implementation of ActionListener simply toggles the state of a randomly selected GameButton .

private final List<GameButton> buttons = new ArrayList<GameButton>(MAX);
public void actionPerformed(ActionEvent e) {
    Object src = e.getSource();
    if (src == timer) {
        int index = random.nextInt(game.max());
        GameButton gb = buttons.get(index);
        gb.setSelected(!gb.isSelected());
    }
    ...
}

To distinguish states, the example uses Unicode glyphs, but you can use setIcon() and setSelectedIcon() .