JTextArea中显示LineNumber

//package tryLineNumber;

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Point;

import javax.swing.JFrame;
import javax.swing.JTextArea;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
//import javax.swing.JTextPane;
import javax.swing.text.BadLocationException;
import javax.swing.text.Document;

/**
  * A class illustrating running line number count on JTextPane. Nothing
is painted on the pane itself,
  * but a separate JPanel handles painting the line numbers.<br>
  *
  * @author Daniel Sj?blom<br>
  * Created on Mar 3, 2004<br>
  * Copyright (c) 2004<br>
  * @version 1.0<br>
  */
public class LineNr extends JPanel
{
        // for this simple experiment, we keep the pane + scrollpane as members.
        //JTextPane pane;
        JTextArea pane;
        JScrollPane scrollPane;

        public LineNr()
        {
                super();
                setMinimumSize(new Dimension(30, 30));
                setPreferredSize(new Dimension(30, 30));
                setMinimumSize(new Dimension(30, 30));
                //pane = new JTextPane() // we need to override paint so that the linenumbers stay in sync
                pane = new JTextArea()
                {
                        public void paint(Graphics g)
                        {
                                super.paint(g);
                                LineNr.this.repaint();
                        }
                };
                scrollPane = new JScrollPane(pane);