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

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

import javax.swing.JFrame;
import javax.swing.JComponent;
import javax.swing.Timer;

class BouncingBall extends JComponent implements ActionListener {

	private int x = 0;
	private int y = 0;
	private int size = 50;

	private int horizontalStep = 1;
	private int verticalStep = 1;

	private int delay = 2;

	private Color color = Color.BLUE;

	private BouncingBall() {
		
		// set canvas to an initial size,
		// otherwise it won't show up
		this.setSize(1, 1);
		
		// create timer to redraw the circle
		// after the delay time has expired
		Timer timer = new Timer(delay, this);
		
		// start the timer
		timer.start();
	}

	public void actionPerformed(ActionEvent e) {

		// boundaries
		int west = 0;
		int south = this.getHeight() - 1 - size;
		int east = this.getWidth() - 1 - size;
		int north = 0;

		// increment step for x coordinate,
		// check for boundaries and invert
		// direction if necessary
		this.x = this.x + this.horizontalStep;
		if (this.x < west || this.x > east) {
			this.horizontalStep = this.horizontalStep * -1;
			this.x = this.x + 2 * this.horizontalStep;
		}

		// dito for y coordinate
		this.y = this.y + this.verticalStep;
		if (this.y < north || this.y > south) {
			this.verticalStep = this.verticalStep * -1;
			this.y = this.y + 2 * this.verticalStep;
		}

		// fancy gimmick for changing the color
		// when the circle hits a boundary
		if (this.x == west || this.x == east || this.y == north || this.y == south) {
			this.color = color.equals(Color.BLUE) ? Color.RED : Color.BLUE;
		}

		// call for repainting the canvas
		this.repaint();
	}

	@Override
	public void paintComponent(Graphics g) {

		// fit canvas size to window size
		this.setSize(this.getParent().getSize());
		
		// fill background
		g.setColor(Color.WHITE);
		g.fillRect(0, 0, this.getWidth(), this.getHeight());

		// draw circle shape
		g.setColor(this.color);
		g.fillOval(x, y, size, size);
	}

	public static void main(String[] args) {

		JFrame window = new JFrame("My Bouncing Ball");
		window.setLocation(100, 100);
		window.setSize(800, 600);
		window.getContentPane().add(new BouncingBall());
		window.setVisible(true);
	}

}