How to make the paintComponent method work?

I’m now studying Head First Java book. And I’ve encountered an example that shows how to override paintComponent in order to draw a random color circle over black color rectangle having width and height of the whole JPanel.

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

public class MyDrawPanel extends JPanel {
    public void paintComponent(Graphics g) {
        g.fillRect(0, 0, this.getWidth(), this.getHeight());

        int red = (int)(Math.random() * 255);
        int green = (int)(Math.random() * 255);
        int blue = (int)(Math.random() * 255);

        Color randomColor = new Color(red, green, blue);
        g.setColor(randomColor);
        g.fillOval(70, 70, 100, 100);
    }
}

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

public class SimpleGui3 {
    public static void main(String[] args) {
        JFrame frame = new JFrame();


        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(300, 300);
        frame.setVisible(true);
    }
}

But I am not achieving the expected results: nothing is painted on the panel.

What am I doing wrong? How to make this work?

Answer

You need two additions in main method for this to work. You have to instantiate the MyDrawPanel() class and then add it to the frame.

MyDrawPanel p = new MyDrawPanel();

and

frame.add(p);

So the main method should be:

    public static void main(String[] args) {
         JFrame frame = new JFrame();
         MyDrawPanel p = new MyDrawPanel();
    
         frame.add(p);
         frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
         frame.setSize(300, 300);
         frame.setVisible(true);
    }