java Graphics如何使用？

package test;

import java.awt.Color;
import java.awt.Graphics;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class Hope
{
public Hope ()
{
JFrame jf = new JFrame ();
JPanel jp = new JPanel ()
{
private static final long serialVersionUID = 1L;

@Override
public void paint ( Graphics g )
{
super.paint (g);
g.drawLine (0, 0, 100, 100);
g.setColor (Color.red);
g.dispose ();
}
};
jf.getContentPane ().add (jp);
jf.setSize (300, 300);
jf.setLocationRelativeTo (null);
jf.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE);
jf.setVisible (true);
}

public static void main ( String[] args )
{
new Hope ();
}
}

代码如下：
import java.awt.*;
import javax.swing.*;

public class Hope extends JFrame {
@Override
public void paint(Graphics g) {

g.setColor(Color.red);
g.drawLine(0, 0, 100, 100);

}

public Hope() {

this.setSize(600, 400);
this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

}

public static void main(String[] args) {
new Hope().setVisible(true);
}
}