How does frame.repaint and paintComponent work?

[fatabass]

Hi,

Here is my code:

import java.awt.BorderLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;

public class SimpleGui3C implements ActionListener 
{
	JFrame frame;
	public static void main(String[] args) 
	{
		SimpleGui3C gui = new SimpleGui3C();
		gui.go();
	}
	
	public void go()
	{
		frame = new JFrame();
		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		JButton button = new JButton("Change colors!");
		button.addActionListener(this);
		
		MyDrawPanel drawPanel = new MyDrawPanel();
		
		frame.getContentPane().add(BorderLayout.SOUTH,button);
		frame.getContentPane().add(BorderLayout.CENTER,drawPanel);
		frame.setSize(300,300);
		frame.setVisible(true);
	}

	@Override
	public void actionPerformed(ActionEvent arg0) 
	{
		frame.repaint();
	}
}

import java.awt.Color;
import java.awt.Graphics;
import javax.swing.JPanel;

public class MyDrawPanel extends JPanel
{
	public void paintComponent(Graphics g)
	{
		int i;
		i = (int) (Math.random()*100);
		
		if (i<50)
		{
			g.setColor(Color.red);
			g.fillRect(i,i,100,100);
		}
		else
		{
			g.setColor(Color.orange);
			g.fillRect(i,i,100,100);
		}
		System.out.println(i);
	}
}

This code just works fine, every time I click the button the drawpanel gets either an orange color or red color depending on the random number i.

This is in method paintComponent as you can see. But my actionPerformed on button clicked is just frame.repaint.

How is paintComponent triggered in this code, can anyone explain me?

Thanks.

[pbrockway2]

Painting in AWT and Swing

(And remember that your JFrame instance is also an instance of Component.)