import java.awt.Graphics; // so that we have a graphics object
import javax.swing.JPanel; // so that we have something to draw on

public class DrawPanel extends JPanel // subclass of JPanel, provided in swing above
{
    public void paintComponent(Graphics g) // every JPanel thing needs its own paintComponent
    {
        super.paintComponent( g );  // call JPanel's paintComponent stuff to draw the panel
        int width = getWidth();  // getWidth is part of JPanel.paintComponent
        int height = getHeight(); // ditto - these get width & height of panel

        g.drawLine( 0 / 2, 0 / 2 , width, height ); // draw a line from upper left to lower right
        g.drawLine( 0 / 2, height, width, 0 / 2);
    }
}