Java - Que pasa en este programa?

import java.awt.Graphics;

import javax.swing.JPanel;

public class Fons extends JPanel{

	@Override

	public void paintComponent(Graphics g) {

		g.drawLine((int)(Math.random() * 400),

				(int)(Math.random() * 400),

				(int)(Math.random() * 400),

				(int)(Math.random() * 400));

	}

}

import javax.swing.JFrame;

public class Main {

	public static void main(String[] args) {

		Fons fons = new Fons();

		JFrame finestra = new JFrame();

		finestra.setSize(400, 400);

		finestra.setLocationRelativeTo(null);

		finestra.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

		finestra.add(fons);

		finestra.setVisible(true);

		for(int i = 0; i < 10; i++) {

			fons.repaint();

			try {

				Thread.sleep(1);

			} catch (InterruptedException e) {

				e.printStackTrace();

			}

		}

	}

}

public class Main {

	public static void main(String[] args) {

		DrawA panel = new DrawA();

		JFrame mainWin = new JFrame();

		mainWin.setSize(400, 400);

		mainWin.setLocationRelativeTo(null);

		mainWin.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

		mainWin.add(panel);

		mainWin.setVisible(true);

		for(int i = 0; i < 10; i++) {

			panel.repaint();

			try {

				Thread.sleep(1);

			} catch(InterruptedException e) {

				e.printStackTrace();

			}

		}

	}

	/* */

	static class DrawA extends JPanel {

		DrawA() {

			setOpaque(false);

		}

		@Override

		public void paintComponent(Graphics g) {

			g.drawLine((int)(Math.random() * 400),

					   (int)(Math.random() * 400),

					   (int)(Math.random() * 400),

					   (int)(Math.random() * 400));

		}

	}

}