Java SE (Java Platform, Standard Edition)

Hi Experts;

How do I add a caret listener on this code so that it will just add a number
when the user goes to the next line.

import java.awt.*;
import javax.swing.*;
 
public class LineNumber extends JComponent
{
	private final static Color DEFAULT_BACKGROUND = new Color(213, 213, 234);
	private final static Color DEFAULT_FOREGROUND = Color.white;
	private final static Font DEFAULT_FONT = new Font("arial", Font.PLAIN, 11);
	
	// LineNumber height (abends when I use MAX_VALUE)
	private final static int HEIGHT = Integer.MAX_VALUE - 1000000;
	
	// Set right/left margin
	private final static int MARGIN = 5;
	
	// Line height of this LineNumber component
	private int lineHeight;
	
	// Line height of this LineNumber component
	private int fontLineHeight;
	
	// With of the LineNumber component
	private int currentRowWidth;
	
	// Metrics of this LineNumber component
	private FontMetrics fontMetrics;
	
	/**
	 * Convenience constructor for Text Components
	 */
	public LineNumber(JComponent component)
	{
		if (component == null)
		{
			setBackground( DEFAULT_BACKGROUND );
			setForeground( DEFAULT_FOREGROUND );
			setFont( DEFAULT_FONT );
		}
		else
		{
			setBackground( DEFAULT_BACKGROUND );
			setForeground( DEFAULT_FOREGROUND );
			setFont( component.getFont() );
		}
		
		setPreferredSize( 99 );
	}
	
	public void setPreferredSize(int row)
	{
		int width = fontMetrics.stringWidth( String.valueOf(row) );
		
		if (currentRowWidth < width)
		{
			currentRowWidth = width;
			setPreferredSize( new Dimension(2 * MARGIN + width, HEIGHT) );
		}
	}
	
	public void setFont(Font font)
	{
		super.setFont(font);
		fontMetrics = getFontMetrics( getFont() );
		fontLineHeight = fontMetrics.getHeight();
	}
	
	/**
	 * The line height defaults to the line height of the font for this
	 * component. The line height can be overridden by setting it to a
	 * positive non-zero value.
	 */
	public int getLineHeight()
	{
		if (lineHeight == 0)
			return fontLineHeight;
		else
			return lineHeight;
	}
	
	public void setLineHeight(int lineHeight)
	{
		if (lineHeight > 0)
			this.lineHeight = lineHeight;
	}
	
	public int getStartOffset()
	{
		return 4;
	}
	
	public void paintComponent(Graphics g)
	{
		 int lineHeight = getLineHeight();
		 int startOffset = getStartOffset();
		 Rectangle drawHere = g.getClipBounds();

		 g.setColor( getBackground() );
		 g.fillRect(drawHere.x, drawHere.y, drawHere.width, drawHere.height);

		 g.setColor( getForeground() );
		 int startLineNumber = (drawHere.y / lineHeight) + 1;
		 int endLineNumber = startLineNumber + (drawHere.height / lineHeight);

		 int start = (drawHere.y / lineHeight) * lineHeight + lineHeight - startOffset;

		 for (int i = startLineNumber; i <= endLineNumber; i++)
		 {
		 String lineNumber = String.valueOf(i);
		 int width = fontMetrics.stringWidth( lineNumber );
		 g.drawString(lineNumber, MARGIN + currentRowWidth - width, start);
		 start += lineHeight;
		 }

		 setPreferredSize( endLineNumber );
	}
} 

Thanks for your time . . .
The_Developer