private void updateReviewers() {
        jComboBox_reviewer.addMouseListener(new MouseListener() {

        @Override
        public void mouseClicked(MouseEvent e) {
            System.out.println("clicked");
        }

        @Override
        public void mousePressed(MouseEvent e) {
            System.out.println("pressed");
        }

        @Override
        public void mouseReleased(MouseEvent e) {
            System.out.println("released");
        }

        @Override
        public void mouseEntered(MouseEvent e) {
            System.out.println("entered");
        }

        @Override
        public void mouseExited(MouseEvent e) {
            System.out.println("exited");
        }

    }
    );

}

jcomboBox.addActionListener(new ActionListener() {

        public void actionPerformed(ActionEvent e)
        {
            JComboBox comboBox = (JComboBox) event.getSource();
            Object o = comboBox.getSelectedItem();
            //Any extra code
        }
    });

package JComboBox;

    import java.awt.*;

    import java.awt.event.*;

    import javax.swing.*;

    /*<applet code="JComboBoxDemo" width=200 height=120></applet>
     */

    public class JComboBoxDemo extends JApplet
    {

        JLabel jlab;
        ImageIcon hourglass, digital, analog, stopwatch;
        JComboBox <String> jcb;
        String timepieces[] = {"Digital", "Analog", "Hourglass",  "Stopwatch"};
        String s;

        public void init()
        {
            try {
                SwingUtilities.invokeAndWait(new Runnable() {

                    public void run() {

                        makeGUI();
                    }
                });
            }

            catch(Exception exc)
            {
                System.out.println("Program can't run because of "+exc);
            }
        }

        private void makeGUI()
        {
            setLayout(new FlowLayout());
            jcb = new JComboBox<String>(timepieces);
            add(jcb);

            jcb.addActionListener(new ActionListener() {

                public void actionPerformed(ActionEvent ae) {

                    s = (String) jcb.getSelectedItem();
                    jlab.setIcon(new ImageIcon(s + ".jpg"));
                }
            });

                jlab = new JLabel(new ImageIcon());
                add(jlab);
        }
    }