Question

I have the code

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

public class MondrianPanel extends JPanel
{
    public MondrianPanel()
    {
        setPreferredSize(new Dimension(200, 600));
    }

    public void paintComponent(Graphics g) {
        for(int i = 0; i < 20; i++) {
            paint(g);
        }
    }

    public void paint(Graphics g)
    {
        Color c = new Color((int)Math.random()*255, (int)Math.random()*255, (int)Math.random()*255);
        g.setColor(c);
        g.fillRect((int)Math.random()*200, (int)Math.random()*600, (int)Math.random()*40, (int)Math.random()*40);
    }

}

What I’m trying to get it to do is draw a bunch of randomly colored rectangles at random places on the screen. However, when I run it, I just get a gray box. I was reading this question Drawing multiple lines with Java Swing and I saw that you should have a single paintComponent that calls paint a bunch of times and I tried adapting my code to this, but it still doesn’t work.

Answer 1

The biggest problem here is that the (int) Math.random() * something is always 0. That’s because the cast is executed first and is 0 and then multiplied by something is still 0.

It should be something like this: (int) (Math.random() * something).

Then, you should rename paint(Graphics g) to draw(Graphics g), otherwise you are overwriting paint in a wrong way.

The code below is working as you need:

public class TestPanel extends JPanel {
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        for (int i = 0; i < 20; i++) {
            draw(g);
        }
    }

    public void draw(Graphics g) {
        Color c = new Color((int) (Math.random() * 255), (int) (Math.random() * 255), (int) (Math.random() * 255));
        g.setColor(c);
        g.fillRect((int) (Math.random() * 400), (int) (Math.random() * 300), (int) (Math.random() * 40), (int) (Math.random() * 40));
    }

    public static void main(String[] args) {
        JFrame f = new JFrame();
        f.getContentPane().add(new TestPanel(), BorderLayout.CENTER);
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setSize(400, 300);
        f.setVisible(true);
    }
}