JLabel beim Klick ändern

import java.awt.FlowLayout;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;

import javax.swing.BorderFactory;
import javax.swing.ImageIcon;
import javax.swing.JFrame;
import javax.swing.JLabel;

public class Memory extends JFrame {

	boolean a_1 = true;

	boolean b_1 = true;

	ImageIcon a1 = new ImageIcon("a1.jpg");

	ImageIcon a2 = new ImageIcon("a2.jpg");

	ImageIcon b1 = new ImageIcon("b1.jpg");

	ImageIcon b2 = new ImageIcon("b2.jpg");

	JLabel l1 = new JLabel(a1);

	JLabel l2 = new JLabel(b1);

	public void example() {
		l1.setBorder(BorderFactory.createEtchedBorder());
		l2.setBorder(BorderFactory.createEtchedBorder());

		setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		setLayout(new FlowLayout());
		add(l1);
		add(l2);
		pack();

		MouseListener mouse1 = new MouseListener() {
			public void mouseClicked(MouseEvent e) {
				if (a_1) {
					l1.setIcon(a2);
					a_1 = false;
				} else {
					l1.setIcon(a1);
					a_1 = true;
				}
			}

			public void mousePressed(MouseEvent e) {
			}

			public void mouseReleased(MouseEvent e) {
			}

			public void mouseEntered(MouseEvent e) {
			}

			public void mouseExited(MouseEvent e) {
			}
		};
		l1.addMouseListener(mouse1);
		MouseListener mouse2 = new MouseListener() {
			public void mouseClicked(MouseEvent e) {
				if (b_1) {
					l2.setIcon(b2);
					b_1 = false;
				} else {
					l2.setIcon(b1);
					b_1 = true;
				}
			}

			public void mousePressed(MouseEvent e) {
			}

			public void mouseReleased(MouseEvent e) {
			}

			public void mouseEntered(MouseEvent e) {
			}

			public void mouseExited(MouseEvent e) {
			}
		};
		l2.addMouseListener(mouse2);

		setVisible(true);
	}

	public static void main(String[] args) {
		Memory test = new Memory();
		test.example();
	}
}