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. 
  * 
  * @author Daniel Sj?blom 
  * Created on Mar 3, 2004 
  * Copyright (c) 2004 
  * @version 1.0 
  */ 
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); 


        } 
        public void paint(Graphics g) 
        { 
                super.paint(g); 


                // We need to properly convert the points to match the viewport 
                // Read docs for viewport 
                int start =pane.viewToModel(scrollPane.getViewport().getViewPosition()); // starting pos in document 
                int end = 
                        pane.viewToModel( 
                                new Point( 
                                        scrollPane.getViewport().getViewPosition().x + pane.getWidth(), 
                                        scrollPane.getViewport().getViewPosition().y + pane.getHeight())); 
                // end pos in doc 


                // translate offsets to lines 
                Document doc = pane.getDocument(); 
                int startline = doc.getDefaultRootElement().getElementIndex(start) + 1; 
                int endline = doc.getDefaultRootElement().getElementIndex(end) + 1; 


                int fontHeight = g.getFontMetrics(pane.getFont()).getHeight(); 
                int fontDesc = g.getFontMetrics(pane.getFont()).getDescent(); 
                int starting_y = -1; 


                try 
                { 
                        starting_y = pane.modelToView(start).y - 
scrollPane.getViewport().getViewPosition().y + fontHeight - fontDesc; 
                } 
                catch 

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