How to repeatedly change colors of a circle using a timer in a Java GUI?

Question

I am trying to create a traffic light using a Java GUI, where it displays only one circle and it changes colours from red, to yellow, to green. There should be a timer and only

Answer 1

Here is one approach. The lights stay on for 3 seconds. To change their duration the code must be modified accordingly. This would be done inside the timer.

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
import javax.swing.Timer;

public class TrafficLight extends JPanel {
    JFrame frame = new JFrame();
    
    int colorIdx = 0;
    Color[] colors = { Color.green, Color.yellow, Color.red };
    
    public static void main(String[] args) {
        // Ensure the program is started on the Event Dispatch Thread
        SwingUtilities.invokeLater(() -> new TrafficLight().start());
    }
    
    public void start() {
        // set up
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        // set the preferred size. Okay in this instance but best
        // practice dictates to override getPreferredSize()
        setPreferredSize(new Dimension(500, 500));
        
        frame.add(this);
        frame.pack();
        
        // center on screen
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
        
        Timer t = new Timer(3000, ae -> {
            colorIdx = colorIdx >= 2 ? 0 : colorIdx + 1;
            repaint();
        });
        t.start();
    }
    
    
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        
        g.setColor(colors[colorIdx]);
        g.fillOval(150, 150, 200, 200);
        
    }
    
}