MainWindow.java

import java.awt.Color;
import java.awt.event.FocusEvent;
import java.awt.event.FocusListener;
import java.awt.event.MouseEvent;
import javax.swing.JFrame;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;
import javax.swing.event.DocumentEvent;
import javax.swing.event.DocumentListener;
import javax.swing.event.MouseInputListener;
import javax.swing.text.BadLocationException;
import javax.swing.text.Element;
import javax.swing.text.Highlighter;
import java.awt.Color;
import javax.swing.text.DefaultHighlighter;

public class MainWindow extends JFrame implements MouseInputListener, DocumentListener, FocusListener{
    private JTextArea jta;
    private JTextArea lines;
    private Highlighter highlighter;

    public MainWindow(){
        super("Line Numbering & Highlighter Example");
    }
    public void createAndShowGUI(){
        /* Set up frame */
        JFrame frame = new MainWindow();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        /* Set up JtextArea */
        jta = new JTextArea();
        jta.getDocument().addDocumentListener(this);

        /* Set up Highlighter */
        highlighter = jta.getHighlighter();

        /* Set up line numbers */
        lines = new JTextArea("1");
        lines.setBackground(Color.LIGHT_GRAY);
        lines.setEditable(false);
        lines.addMouseListener(this);
        lines.addFocusListener(this);

        /* Set up scroll pane */
        JScrollPane jsp = new JScrollPane();
        jsp.getViewport().add(jta);
        jsp.setRowHeaderView(lines);
        jsp.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);

        jta.setText("Hello world how are you today!");

        /* pack and show frame */
        frame.add(jsp);
        frame.pack();
        frame.setSize(500,500);
        frame.setVisible(true);
    }

    /* Document Listener Events */
    public void changedUpdate(DocumentEvent de) {
        lines.setText(getText());
    }
    public void insertUpdate(DocumentEvent de) {
        lines.setText(getText());
    }
    public void removeUpdate(DocumentEvent de) {
        lines.setText(getText());
    }
    public String getText(){
        int caretPosition = jta.getDocument().getLength();
        Element root = jta.getDocument().getDefaultRootElement();
        String text = "1\n";
        for(int i = 2; i < root.getElementIndex( caretPosition ) + 2; i++)
            text += i + "\n";
        return text;
    }

    /* Mouse Listener Events */
    public void mouseClicked(MouseEvent me) {
        if(me.getClickCount() == 2){
            try {
                int caretPos = lines.getCaretPosition();
                int lineOffset = lines.getLineOfOffset(caretPos);
                if(lines.getText().charAt(caretPos-1) == '\n')
                    lineOffset--;
                highlighter.addHighlight(jta.getLineStartOffset(lineOffset),
                                         jta.getLineEndOffset(lineOffset),
                                         new MyHighlighter(Color.cyan));
            } catch (BadLocationException e) {
                e.printStackTrace();
            }
        }
    }
    public void mouseEntered(MouseEvent me) {}
    public void mouseExited(MouseEvent me) {}
    public void mousePressed(MouseEvent me) {}
    public void mouseReleased(MouseEvent me) {}
    public void mouseDragged(MouseEvent me) {}
    public void mouseMoved(MouseEvent me) {}

    /* Focus Listener Events for line numbers*/
    public void focusGained(FocusEvent fe) {}
    public void focusLost(FocusEvent fe) {
        highlighter.removeAllHighlights();
    }
    public static void main(String[] args){
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
                                                   public void run() {
                                                              new MainWindow().createAndShowGUI();
                                                   }
                                           });
    }
}



class MyHighlighter extends DefaultHighlighter.DefaultHighlightPainter {
    public MyHighlighter(Color c) {
        super(c);
    }
}