CODEWITHSUNDEEP
javaswingjlabelimageicon
Mads Justesen
Mads Justesen

Reputation: 17

For loop stalls my swing application, until the loop completes

*Problem solved - thanks for all your answers, they were very helpful!

I am making a small dice game for a school assignment, and i came across this problem. I want to simulate the rolling of a die, by quickly cycle trough a number of die-icons. This by itself is not what is causing the problem though. If i make the "animation" directly in a JFrame it displays correctly. I have done that in the code below:

public class Example{

        private static ImageIcon die1 = new ImageIcon("terning1.jpg");
        private static ImageIcon die2 = new ImageIcon("terning2.jpg");
        private static ImageIcon die3 = new ImageIcon("terning3.jpg");
        private static ImageIcon die4 = new ImageIcon("terning4.jpg");
        private static ImageIcon die5 = new ImageIcon("terning5.jpg");
        private static ImageIcon die6 = new ImageIcon("terning6.jpg");

        private static JLabel die = new JLabel(die1);
        private static Random generator = new Random();

    public static void main(String[] args) {
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        frame.add(die);

        frame.pack();
        frame.setVisible(true);

        for (int i = 0; i < 100; i++) {
            int x = generator.nextInt(6) + 1;

            switch(x){
                case 1 : die.setIcon(die1);
                    break;
                case 2 : die.setIcon(die2);
                    break;
                case 3 : die.setIcon(die3);
                    break;
                case 4 : die.setIcon(die4);
                    break;
                case 5 : die.setIcon(die5);
                    break;
                case 6 : die.setIcon(die6);
                    break;
            }

            //Make the loop wait for 50 millis
            long a, b;
            a = System.currentTimeMillis();
            do {
                b = System.currentTimeMillis();
            } while ((b-a) < 50);           
        }

    }       
}

Now that works fine, but obviously it only works when i first open the JFrame. So i want to add a button, that makes the die roll. But if i add a JButton with a actionlistener, and put the for-loop in the actionPerformed method, it stalls the program until the loop has finished, and only shows the last die in the loop. Example:

public static void main(String[] args) {
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        frame.setLayout(new FlowLayout());

        frame.add(button);
        frame.add(die);

        button.addActionListener(new ButtonListener());

        frame.pack();
        frame.setVisible(true);
    }

    private static class ButtonListener implements ActionListener {

        public void actionPerformed(ActionEvent event) {
            for (int i = 0; i < 100; i++) {
                int x = generator.nextInt(6) + 1;

                switch (x) {
                    case 1:
                        die.setIcon(die1);
                        break;
                    case 2:
                        die.setIcon(die2);
                        break;
                    case 3:
                        die.setIcon(die3);
                        break;
                    case 4:
                        die.setIcon(die4);
                        break;
                    case 5:
                        die.setIcon(die5);
                        break;
                    case 6:
                        die.setIcon(die6);
                        break;
                }

                //Make the loop wait for 50 millis
                long a, b;
                a = System.currentTimeMillis();
                do {
                    b = System.currentTimeMillis();
                } while ((b - a) < 50);
            }

Any tips on how to solve this? Thanks beforehand!

Upvotes: 1

Views: 180

Answers (4)

Sage
Sage

Reputation: 15418

Swing's event handling code runs in EDT(even dispatch thread). The ActionEvent is also no exception. You can't do any long running task inside EDT. Put your dice rolling loop inside a new thread:

new Thread(){
   public void run(){
         // your dice rolling code
    }
  }.start();

and then update the GUI using SwingUtilities.invokdeLater().

Note: Instead of conditioning with switch-case you just could use an array of ImageIcon and access it with random generated index.

int x = generator.nextInt(6) + 1;
die.setIcon(imageIconArr[x]); // array of image icon

See this answer for details.

Upvotes: 0

alex2410
alex2410

Reputation: 10994

You run your actionPerformed(ActionEvent event) method in EDT because of that you can't update UI. For updating UI from your code try to use SwingWorker, it can update UI while background process running. You can found a lot of examples of it in the internet.

Or you can try to use Executors for background process and updating UI from EDT.

Upvotes: 0

Dawood ibn Kareem
Dawood ibn Kareem

Reputation: 79877

What's happening is that the "event dispatch thread", where all the Swing events happen, is having to wait for your code. Don't do long-running stuff on the event dispatch thread. This is a famous anti-pattern.

You should read the lesson from the Java tutorials, that starts at http://docs.oracle.com/javase/tutorial/uiswing/concurrency/index.html, which describes this.

Two small points that are unrelated to your problem:

  1. Your code will be much more manageable if you use an array of ImageIcon variables, instead of six separate variables.
  2. You could use the sleep method of the Thread class instead of the "busy sleep" that you're using.

Upvotes: 3

Todoy
Todoy

Reputation: 1266

Your UI is locked because everything is in the same thread so it has to wait until the for loop is done executing.Your going to need to run your logic in a separate thread.

Upvotes: 1

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