Question

In Java, let’s say I have a GUI with 2 buttons, Go and Pause.

When I press Go, “Hello” gets printed out over and over again. When I press Pause, “Hello” no longer gets printed to the screen.

Example: User presses Go button. “Hello” gets printed out for 1 minute until the user presses “Pause.”

What is the proper way to express this approach in Java? Is it equivalent to my commented pseudocode within the goButton source?

public void actionPerformed(ActionEvent e) {
    if(e.getSource() == goButton)
    {
        // while user has not pressed the pause button
        printHello();
    }
    else if(e.getSource() == pauseButton)
    {
        pause();
    }
}

Thanks

Answer 1

In order to get this to work, in reasonable fashion, you will need a Thread. This is executed in the background until such time as you decide to cancel/pause it.

This is an EXTREMELY basic example. Normally I’d wrap the task and the GUI up in appropriate classes rather then accessing static references, but it gives a basic idea

public class TestHello {

    private static HelloTask task;

    public static void main(String[] args) {

        Thread thread = new Thread((task = new HelloTask()));
        thread.setDaemon(true);
        thread.start();

        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setLayout(new GridBagLayout());
        frame.setSize(200, 200);
        frame.setLocationRelativeTo(null);

        JButton goButton = new JButton("Go");
        JButton stopButton = new JButton("Stop");

        goButton.setActionCommand("Go");
        stopButton.setActionCommand("Stop");

        ActionHandler handler = new ActionHandler();

        goButton.addActionListener(handler);
        stopButton.addActionListener(handler);

        frame.add(goButton);
        frame.add(stopButton);

        frame.setVisible(true);

    }

    public static class ActionHandler implements ActionListener {

        @Override
        public void actionPerformed(ActionEvent e) {

            if (e.getActionCommand().equals("Go")) {
                task.start();
            } else if (e.getActionCommand().equals("Stop")) {
                task.pause();
            }

        }

    }

    public static class HelloTask implements Runnable {

        private static final Object WAIT_LOCK = new Object();
        private boolean dump = false;

        public void start() {
            synchronized (WAIT_LOCK) {
                dump = true;
                WAIT_LOCK.notify();
            }
        }

        public void pause() {
            synchronized (WAIT_LOCK) {
                dump = false;
                WAIT_LOCK.notify();
            }
        }

        @Override
        public void run() {
            while (true) {
                while (dump) {
                    System.out.println("Hello");
                }
                try {
                    synchronized (WAIT_LOCK) {
                        WAIT_LOCK.wait();
                    }
                } catch (Exception e) {
                }
            }
        }
    }
}

Some further read:

• Java Concurrency
• Concurrency in Swing

Caveats

NEVER try and modify the GUI from any thread other then the Event Dispatching Thread.