How do I re run the paint method so the JPanel is animated?

Question

I think I need to put some code where the comment is (or maybe use non static method but I am not sure). The main method creates the window and then starts the graphics method. I would like the blue square to flash.

import java.awt.Color;
import java.awt.Graphics;
import javax.swing.JFrame;
import javax.swing.JPanel;


public class paintTest extends JPanel{
private static JFrame theWindow = new JFrame("Window");
static boolean blueSqr = false;

public void paint(Graphics g) {
    g.setColor(Color.RED);
    g.fillRect(10, 10, 10, 10);

    if(blueSqr){
        g.setColor(Color.BLUE);
        g.fillRect(10, 10, 10, 10);
    }
}

public static void main(String[] args){
    createWindow();
    theWindow.getContentPane().add(new paintTest());
    while(true){
        blueSqr = false;

        System.out.println("off");

        try {Thread.sleep(1000);} catch (InterruptedException e) {e.printStackTrace();}

        blueSqr = true;
        // Needs something here
        System.out.println("on");

        try {Thread.sleep(1000);} catch (InterruptedException e) {e.printStackTrace();}
        }
}

public static void createWindow(){
    theWindow.setSize(500, 500);
    theWindow.setLocationRelativeTo(null);
    theWindow.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    theWindow.setVisible(true);
}
}

Any help would be really good.

Answer 1

Use a Swing Timer to call repaint(). Also, override paintComponent() in a JPanel, rather than paint().

Something like this:

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

public class PaintTest extends JPanel{

    boolean blueSqr = false;

    PaintTest() {
        setPreferredSize(new Dimension(100,25));
        ActionListener al = new ActionListener() {
            public void actionPerformed(ActionEvent ae) {
                blueSqr = !blueSqr;
                repaint();
            }
        };
        Timer timer = new Timer(1000,al);
        timer.start();
    }

    public void paintComponent(Graphics g) {
        Color c = (blueSqr ? Color.BLUE : Color.RED);
        g.setColor(c);
        g.fillRect(10, 10, 10, 10);
    }

    public static void main(String[] args){
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                JFrame theWindow = new JFrame("Window");
                theWindow.getContentPane().add(new PaintTest());
                createWindow(theWindow);
            }
        });
    }

    public static void createWindow(JFrame theWindow){
        theWindow.pack();
        theWindow.setLocationByPlatform(true);
        theWindow.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        theWindow.setVisible(true);
    }
}

There were other improvements I could not be bothered documenting (code speaks louder than words). If you have any questions (check the docs first, then) ask.

Answer 2

your issues are

1) by calling Thread.sleep(int) in the Swing related code, never do that, for delaying in Swing (there are lots of topics about why not use sleep in programing languages ...) use Swing Timer

2) your JPanel doesn't returns any XxxSize

3) for Swing use paintComponent(), only if you have got really important reasons then use method paint() more about repaint and animating Graphics in the 2D Graphics tutorial

4) Swing GUI should be built in the Event Dispatch Thread