I am trying to change the background color of the JPanel every 3 seconds (3000 ms) when I click START button till I press STOP button.

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

class TimerBackground implements ActionListener
{
	JFrame frame; 	JPanel panel;
	JButton btnStart; 	JButton btnRed;
	JButton btnGreen; 	JButton btnBlue;
	JButton btnStop; 	Timer t;  
	Random num = new Random();  
	int r, g, b;
	TimerBackground()
	{
		r = g = b = 0;
		frame = new JFrame("Timer Background");
		panel = new JPanel();
		btnStart = new JButton("START");
		btnRed = new JButton("Red");
		btnGreen = new JButton("Green");
		btnBlue = new JButton("Blue");
		btnStop = new JButton("STOP");
		t = new Timer(3000, this);

		btnStart.addActionListener(this);	
		btnStop.addActionListener(this);
		btnRed.addActionListener(this);
		btnBlue.addActionListener(this);
		btnGreen.addActionListener(this);
		t.addActionListener(this);
	
		frame.setSize(800,600);
		frame.setVisible(true);
		frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
		frame.setContentPane(panel);
		panel.add(btnStart);
		panel.add(btnStop);
		panel.add(btnRed);
		panel.add(btnGreen);

		t.start();
	}
	
	public void actionPerformed(ActionEvent evnt)
	{
		if(evnt.getSource() == btnRed)
		{
			panel.setBackground(new Color(255,0,0));
		}
		else if(evnt.getSource() == btnGreen)
		{
			panel.setBackground(new Color(0,255,0));
		}
		else if(evnt.getSource() == btnBlue)
		{
			panel.setBackground(new Color(0,0,255));
		}
		else if(evnt.getSource() == btnStart)
		{
			//Are The Lines Below Correct or else What Should I Write Here??
			r = num.nextInt(255);
			g = num.nextInt(255);
			b = num.nextInt(255);
			panel.setBackground(new Color(r, g, b));
		}
		else if(evnt.getSource() == btnStop)
		{
			t.stop();
		}
	}

	public static void main(String[] args)
	{
		TimerBackground timerB = new TimerBackground();
	}
}

The above code works only once on click of START button. I don't understand how to make it run every 3 seconds.

javax.swing.Timer is what you need - documentation & samples in the usual places.

Define a timer task (a "run()" method) that you want to run repeatedly. On click of the start button start a Timer that will run that task every 3000 msec. On click of the stop button stop the timer.

But I don't know how to implement it, can u give me a sample?

PLease let me know what modifications can i make to my code??

@ JamesCherrill

hard of gold :-)

Hey! Was that deliberate? Normal phrase is "heart of gold", but you wrote "hard" as in tough. difficult etc. Neat pun!
I still stand by what I said.

also implement a loop if that operation is to keep on being repeated

thank you, my fast hands and still problems wrote English words correctly, nor ...

but my reaction == this forum is long times one man show throught EU working hours, with my respect man

also implement a loop if that operation is to keep on being repeated

No no no.

The javax.swing.Timer handles the repeating. If you tell a newbie to use a loop in this context you will get a loop with a sleep in it that blocks the EDT and leads inevitably to the post that goes "why isn't by background updating?".

So, no, no loop. Just a swing Timer.

Hey I used TimerTask one as suggested by James Cherrill... Thanks.. I had to dig deep, but I found out..