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.