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

    public class Tube extends JPanel implements Runnable{

        private int x = 540;
        private int dx = 1;
        private int bottom_line;

        @Override
        public void run() {
            while(x!=-81){
                x -= dx;
                repaint();
                try {
                    Thread.sleep(2);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            x = 540;
            start();
        }

        public void start(){
            bottom_line = (int)(100+Math.random()*370);
            Thread thread = new Thread(this);
            thread.start();
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            g.drawRect(x,0,80,bottom_line);
            g.drawRect(x,bottom_line+150,80,600-bottom_line);
        }

    }

import javax.swing.*;

public class Window extends JFrame{

    public Window(){
        Tube tube = new Tube();
        add(tube);
        tube.start();
        setVisible(true);
        setSize(540,720);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setResizable(false);
        setLocationRelativeTo(null);

    }

    public static void main(String[] args) {
        Window window = new Window();
    }
}