Salut!

J'ai testé et la méthode getLineCount() me renvoie bien le nombre de lignes (en fait, le nombre de 'return' +1)

package linecounter;

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;

import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextArea;

/**
 * Created on 09-avr.-07
 * 
 * @author: HackTrack
 */

public class LineCounterDemo extends JFrame {
	private JTextArea textArea;

	private JLabel lineCount;

	public LineCounterDemo() {
		super("Demo du comptage des lignes dans un JTextArea");
		initializeComponent();
	}

	private void initializeComponent() {
		setDefaultCloseOperation(EXIT_ON_CLOSE);
		JPanel mainPanel = new JPanel();
		mainPanel.setLayout(new BorderLayout());
		lineCount = new JLabel("Nombre de lignes = ");
		mainPanel.add(lineCount, BorderLayout.NORTH);
		textArea = new JTextArea();
		textArea.setLineWrap(true);
		textArea.addKeyListener(new KeyListener() {
			public void keyTyped(KeyEvent e) {

			}

			public void keyPressed(KeyEvent e) {

			}

			public void keyReleased(KeyEvent e) {
				int count = textArea.getLineCount();
				lineCount.setText("Nombre de lignes = " + count);
			}
		});
		textArea.setPreferredSize(new Dimension(320, 200));
		mainPanel.add(textArea, BorderLayout.CENTER);
		getContentPane().add(mainPanel);
	}

	public static void main(String[] args) {
		LineCounterDemo demo = new LineCounterDemo();
		demo.pack();
		demo.setVisible(true);
	}

}