Change backround color of JPanel multiple times

Question

I am trying to make a little program that includes changing the color of a Panel time-based. Right now I am just trying to do that part without the rest. So I just wrote a little Interface with only one panel and I want to change the color within a loop multiple times. The problem is, even though the thread pauses for the correct amount of time, the color of the Panel doesn't change correctly. It changes just sometimes in the loop not every time.

my Interface Class:

import javax.swing.*;
import java.awt.*;

//creates the Interface
public class Interface extends JFrame {
    private JPanel frame1;

    public Interface (String titel) {
        super(titel);
        setSize(600, 400);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.frame1 = new JPanel();
        this.frame1.setPreferredSize(new Dimension (200, 200));
        setLayout(new FlowLayout());
        add(frame1);
        this.setVisible(true);
    }

    public JPanel getFrame1() {
        return frame1;
    }

}

my Pause Class:

import java.util.TimerTask;


//supposed to pause the thread by @pause amount of milliseconds
public class Pause extends TimerTask {
    private int pause;

    public Pause(int pause){
        this.pause = pause;
    }

    @Override
    public void run() {
        System.out.println("Timer"+ pause+" task started at:"+System.currentTimeMillis());
        pause();
        System.out.println("Timer task"+ pause+" ended at:"+System.currentTimeMillis());
    }

    public void pause() {
        try {
            Thread.sleep(this.pause);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

}

my Blink Class

import javax.swing.*;
import java.awt.*;
public class Blink {
    private JPanel frame1;
    public Blink(Interface anInterface){
        this.frame1 = anInterface.getFrame1();
    }

    // blink should change the color of the JPanel inside my Frame. 
    // Its supposed to change to red for 200 ms
    // and then to white again for 1000 ms.
    // this should be repeated 10 times.
    public void blink() {
        Pause pause1 = new Pause(200);
        Pause pause2 = new Pause(1000);
        pause2.run();
        int i = 1;
        while(i <= 10){
            i++;
            frame1.setBackground(Color.red);
            frame1.repaint();
            pause1.run();
            frame1.setBackground(Color.white);
            frame1.repaint();
            pause2.run();
        }
    }

    public static void main ( String[] args ) {
        Interface anInterface = new Interface("Title");
        anInterface.setVisible(true);
        Blink blink = new Blink(anInterface);
        blink.blink();
    }
}

score 1 · Accepted Answer · answered Nov 02 '19 at 11:22

According to Concurrency to Swing you cannot simply Thread.sleep the Thread where the GUI runs because it will freeze it, hence events cannot take place. Instead, for any kind of animation or long-heavy task (consider Thread.sleep as one), Swing Timers and Swing Workers should be used. In your case, a javax.swing.Timer fits better.

One example of its usage:

public class Blink {
    private JPanel frame1;
    private int pause1TimesRan;
    private int pause2TimesRan;

    private Timer pauser1, pauser2;

    public Blink(Interface anInterface) {
        this.frame1 = anInterface.getFrame1();
        //Create pauser 1 with delay 200ms
        pauser1 = new Timer(200, e -> {
            if (pause1TimesRan == 10) {
                pauser1.stop();
                return;
            }
            Color color = randomColor();
            frame1.setBackground(color);
            System.out.println("Pauser #1 changed background to: " + color);
            pause1TimesRan++;
        });
        //Create pauser 2 with delay 1000ms
        pauser2 = new Timer(1000, e -> {
            if (pause2TimesRan == 10) {
                pauser2.stop();
                return;
            }
            Color color = randomColor();
            frame1.setBackground(color);
            System.out.println("Pauser #2 changed background to: " + color);
            pause2TimesRan++;
        });
    }

    private static Color randomColor() {
        return new Color((int) (Math.random() * 255), (int) (Math.random() * 255), (int) (Math.random() * 255));
    }

    public void blink() {
        pauser1.start();
        pauser2.start();
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> {
            Interface anInterface = new Interface("Title");
            anInterface.setVisible(true);
            Blink blink = new Blink(anInterface);
            blink.blink();
        });
    }

    static class Interface extends JFrame {
        private JPanel frame1;

        public Interface(String titel) {
            super(titel);
            setSize(600, 400);
            setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            this.frame1 = new JPanel();
            this.frame1.setPreferredSize(new Dimension(200, 200));
            setLayout(new FlowLayout());
            add(frame1);
            this.setVisible(true);
        }

        public JPanel getFrame1() {
            return frame1;
        }

    }
}

One off-topic advice is to name your methods (and variables) properly. You called the method getFrame1(), but it is actually a JPanel and not a JFrame. So, a better name could be getPanel(). Also, about the SwingUtilities.invokeLater part, read What does SwingUtilities.invokeLater does.

Change backround color of JPanel multiple times

1 Answers1