IntersectTest.java

import java.awt.Frame;
import java.awt.Graphics;
import java.awt.Rectangle;

public class IntersectTest extends Frame implements Runnable {

    private Rectangle firstBox = new Rectangle(50, 50, 100, 100);
    private Rectangle secondBox = new Rectangle(50, 400, 100, 100);

    public IntersectTest() {
        setSize(800, 600);
        setVisible(true);
    }

    public static void main(String[] args) {
        Thread thread = new Thread(new IntersectTest());
        thread.start();
    }

    public void paint(Graphics g) {
        g.drawRect(50, firstBox.y, 100, 100);
        g.drawRect(50, 400, 100, 100);
        repaint();
    }

    @Override
    public void run() {
        try {
            while (true) {
                firstBox.y++;
                if (firstBox.intersects(secondBox)) {
                    break;
                }
                paint(getGraphics());
                Thread.sleep(10);
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}