Java Swing How to - Make JComboBox behave like a menu with subdivisions

Question

We would like to know how to make JComboBox behave like a menu with subdivisions.

Answer

import java.awt.GridLayout;
/*  w  w  w  .jav  a 2 s .  c  o m*/
import javax.swing.ComboBoxModel;
import javax.swing.DefaultComboBoxModel;
import javax.swing.JComboBox;
import javax.swing.JOptionPane;
import javax.swing.JPanel;

public class Main {

  public static void main(String[] args) {
    JPanel gui = new JPanel(new GridLayout(0, 1, 5, 5));

    String[] speciesName = { "1", "2", "3" };
    String[][] breedName = { { "A", "P", "A" },
        { "B", "P", "S" },
        { "DDo", "A", "P" } };
    JComboBox<String> petSpecies = new JComboBox<>(speciesName);
    JComboBox<String> petBreed = new JComboBox<>();
    petSpecies.addItemListener(e->{
      int ii = petSpecies.getSelectedIndex();
      ComboBoxModel cbm = new DefaultComboBoxModel(breedName[ii]);
      petBreed.setModel(cbm);
      petBreed.requestFocusInWindow();
    });
    gui.add(petSpecies);
    gui.add(petBreed);

    JOptionPane.showMessageDialog(null, gui);
  }
}