Java code for Traffic Control System

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
import javax.swing.border.*;
public class TrafficLight extends JFrame implements ActionListener {
    JButton b1, b2, b3;
    Signal green = new Signal(Color.green);
    Signal yellow = new Signal(Color.yellow);
    Signal red = new Signal(Color.red);
public TrafficLight(){
        super("Traffic Light");
        getContentPane().setLayout(new GridLayout(2, 1));
        b1 = new JButton("Red");
        b2 = new JButton("Yellow");
        b3 = new JButton("Green");
        b1.addActionListener(this);
        b2.addActionListener(this);
        b3.addActionListener(this);        
        green.turnOn(false);
        yellow.turnOn(false);
        red.turnOn(true);
        JPanel p1 = new JPanel(new GridLayout(3,1));
        p1.add(red);
        p1.add(yellow);
        p1.add(green);
        JPanel p2 = new JPanel(new FlowLayout());
        p2.add(b1);
        p2.add(b2);
        p2.add(b3);
        getContentPane().add(p1);
        getContentPane().add(p2);
        pack();
        }
public static void main(String[] args){
        TrafficLight tl = new TrafficLight();        
        tl.setVisible(true);
    }    
public void actionPerformed(ActionEvent e){        
        if (e.getSource() == b1){
            green.turnOn(false);            
            yellow.turnOn(false);
            red.turnOn(true);
        } else if (e.getSource() == b2){
            yellow.turnOn(true);            
            green.turnOn(false);
            red.turnOn(false);
        } else if (e.getSource() == b3){
            red.turnOn(false);            
            yellow.turnOn(false);
            green.turnOn(true);
        }
    }
}     
class Signal extends JPanel{
    Color on;
    int radius = 40;
    int border = 10;
    boolean change;
Signal(Color color){
        on = color;
        change = true;
    }
public void turnOn(boolean a){
        change = a;
        repaint();        
    }
public Dimension getPreferredSize(){
        int size = (radius+border)*2;
        return new Dimension( size, size );
    }
public void paintComponent(Graphics g){
        g.setColor( Color.black );
        g.fillRect(0,0,getWidth(),getHeight());
        if (change){
            g.setColor( on );
        } else {
            g.setColor( on.darker().darker().darker() );
        }
        g.fillOval( border,border,2*radius,2*radius );
    }
}

Output-1:

Output-2:

Output-3: