Display dropdown in a JComboBox Component Using a Keystroke in Java

import java.awt.BorderLayout;
import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;
//from www  .  ja v a  2  s.  c  o  m
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;

public class Main {
  public static void main(String[] argv) throws Exception {
    String[] items = { "A", "A", "B", "B", "C", "C" };
    JComboBox cb = new JComboBox(items);

    // Create and register the key listener
    cb.addKeyListener(new MyKeyListener());
    JPanel panel = new JPanel();
    panel.setLayout(new BorderLayout(10, 10));

    panel.add(cb, BorderLayout.CENTER);
    JFrame f = new JFrame();
    f.add(panel);
    f.pack();
    f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    f.setVisible(true);

  }
}

class MyKeyListener extends KeyAdapter {
  public void keyPressed(KeyEvent evt) {
    JComboBox cb = (JComboBox) evt.getSource();

    // Get pressed character
    char ch = evt.getKeyChar();

    // If not a printable character, return
    if (ch != KeyEvent.CHAR_UNDEFINED) {
      cb.showPopup();
    }
  }
}