Java GUI >> Basic Animation 2 (super.paintComponent(), repaint())

gistfile1.java

/*1. JPanel has an opaque property.
2. By default it is true.
3. JPanel's paintComponent method contains the code the draws the background color, when it is opaque.
4. When you subclass JPanel and override paintComponent, it's up to you to remember to call super.paintComponent; if you don't call the code it wouldn't be called otherwise.
5. That's it. "Losing opaqueness" should really be thought of as "subclassing and making a mistake.
 */

import java.awt.Color;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.Timer;
public class MyGUI extends JPanel implements ActionListener 
{
        Timer tm = new Timer(5, this); //this refers to ActionListener's event?
	int x=0,y=0,vel=1;
	
	public void paintComponent(Graphics g) 
	{
		/*If your paintComponent() method wants to clear the background area all it has
		to do is call the super paintComponent() method and make the component opaque when it is initialized.*/
		super.paintComponent(g); 
		g.setColor(Color.RED);
		g.fillRect(x, y, 50, 30);
		
		tm.start(); //this starts ActionEvent after 5 miliseconds
		
	}
	     
	public static void main(String[] args)
       {
    	   MyGUI gui=new MyGUI();
    	   JFrame frame = new JFrame();
    	   frame.setTitle("Basic Animation");
    	   frame.setSize(500,500);
    	   frame.setVisible(true);
    	   frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    	   frame.add(gui); 	   
       }

	public void actionPerformed(ActionEvent arg0)
	{
		x+=vel;
		y+=vel;
		repaint();
	}     
 }