graphics有关问题，求高手啊弄了好久还是不会。如果能详细点，就好，多谢

import java.awt.*;import javax.swing.*;public class text extends JFrame {    private JLabel panelNorth;    private JPanel panelSouth;    private JButton but_start;    private JButton but_out;    private JPanel pan;    private BorderLayout lay;    public text() {        setTitle("面板");        lay = new BorderLayout();        setLayout(lay);        lay.setVgap(15);        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);        setBounds(400, 100, 500, 550);        init();        setVisible(true);    }    void init() {        panelNorth = new JLabel("画线！");        panelNorth.setFont(new Font("楷体", 30, 24));        panelNorth.setForeground(Color.blue);        this.getContentPane().add(panelNorth, BorderLayout.NORTH);        panelSouth = new JPanel();        but_start = new JButton("开始");        but_out = new JButton("退出");        but_start.setFont(new Font("楷体", 10, 16));        but_out.setFont(new Font("楷体", 10, 16));        panelSouth.add(but_start);        panelSouth.add(but_out);        this.getContentPane().add(panelSouth, BorderLayout.SOUTH);        pan = new JPanel() {            @Override            public void paintComponent(Graphics g) {                super.paintComponent(g);                g.setColor(Color.black);                g.drawLine(50, 50, 100, 100);            }        };        this.getContentPane().add(pan, BorderLayout.CENTER);    }    public static void main(String args[]) {        new text();    }}