Core Swing Components - The Definitive Guide to Java Swing - page 108

When you want to create a new component, subclass JPanel and override the public void
paintComponent(Graphics g) method. Although you can subclass JComponent directly, it seems
more appropriate to subclass JPanel . Listing 4-8 demonstrates a simple component that draws
an oval to fit the size of the component; it also includes a test driver.
Listing 4-8. Oval Panel Component
import java.awt.*;
import javax.swing.*;
public class OvalPanel extends JPanel {
Color color;
public OvalPanel() {
this(Color.black);
}
public OvalPanel(Color color) {
this.color = color;
}
public void paintComponent(Graphics g) {
int width = getWidth();
int height = getHeight();
g.setColor(color);
g.drawOval(0, 0, width, height);
}
public static void main(String args[]) {
Runnable runner = new Runnable() {
public void run() {
JFrame frame = new JFrame("Oval Sample");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setLayout(new GridLayout(2, 2));
Color colors[] = {Color.RED, Color.BLUE, Color.GREEN, Color.YELLOW};
for (int i=0; i<4; i++) {
OvalPanel panel = new OvalPanel(colors[i]);
frame.add(panel);
}
frame.setSize(300, 200);
frame.setVisible(true);
}
};
EventQueue.invokeLater(runner);
}
}