[hardwired]

Code:

import java.awt.*;
import java.awt.geom.*;
import javax.swing.*;

public class HitDetection extends JPanel implements Runnable {
    Rectangle[] blocks = {
        new Rectangle(100,100,30,30), new Rectangle(140,160,30,30),
        new Rectangle(240,270,30,30), new Rectangle(200,300,30,30)
    };
    Ellipse2D.Double ball = new Ellipse2D.Double(200,10,20,20);
    int dx = 3;
    int dy = 2;

    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D)g;
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                            RenderingHints.VALUE_ANTIALIAS_ON);
        for(int j = 0; j < blocks.length; j++)
            g2.draw(blocks[j]);
        g2.setPaint(Color.red);
        g2.fill(ball);
    }

    public void run() {
        boolean running = true;
        while(running) {
            try {
                Thread.sleep(50);
            } catch(InterruptedException e) {
                running = false;
            }
            checkForCollision();
            checkBoundries();
            double x = ball.x + dx;
            double y = ball.y + dy;
            ball.setFrame(x, y, ball.width, ball.height);
            repaint();
        }
    }

    private void checkForCollision() {
        Ellipse2D.Double e = new Ellipse2D.Double();
        e.setFrame(ball.x+dx, ball.y+dy, ball.width, ball.height);
        for(int j = 0; j < blocks.length; j++) {
            if(e.intersects(blocks[j])) {
                bounce(blocks[j], e);
                break;
            }
        }
    }

    private void bounce(Rectangle r, Ellipse2D.Double e) {
        double cx = e.getCenterX();
        double cy = e.getCenterY();
        int outcode = r.outcode(cx, cy);
        switch(outcode) {
            case Rectangle.OUT_TOP:
            case Rectangle2D.OUT_BOTTOM:
                dy *= -1;
                break;
            case Rectangle2D.OUT_TOP + Rectangle2D.OUT_LEFT:
            case Rectangle2D.OUT_LEFT + Rectangle2D.OUT_BOTTOM:
            case Rectangle2D.OUT_BOTTOM + Rectangle.OUT_RIGHT:
            case Rectangle2D.OUT_RIGHT + Rectangle2D.OUT_TOP:
                dx *= -1;
                dy *= -1;
                break;
            case Rectangle2D.OUT_LEFT:
            case Rectangle2D.OUT_RIGHT:
                dx *= -1;
                break;
            default:
                System.out.println("CENTER: " + outcode);
        }
    }

    private void checkBoundries() {
        if(ball.x + dx < 0 || ball.x + ball.width + dx > getWidth())
            dx *= -1;
        if(ball.y + dy < 0 || ball.y + ball.height + dy > getHeight())
            dy *= -1;
    }

    private void start() {
        while(!isVisible()) {
            try {
                Thread.sleep(25);
            } catch(InterruptedException e) {
                break;
            }
        }
        Thread thread = new Thread(this);
        thread.setPriority(Thread.NORM_PRIORITY);
        thread.start();
    }

    public static void main(String[] args) {
        HitDetection test = new HitDetection();
        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.getContentPane().add(test);
        f.setSize(400,400);
        f.setLocation(200,200);
        f.setVisible(true);
        test.start();
    }
}