How to create a multi-layer JComboBox

Question

I'm creating a simulator using Java Swing. I used JComboBox to display units of utilities such as "KW, KL, KM" etc to measure Power, Water and distance. It's simple to add bunch of items to a JComboBox. User select a unit and the JFrame will save the selection when a "save" button is clicked.

    JComboBox comboBox = new JComboBox();
    for(ValueUnits u: ValueUnits.values()){
        comboBox.addItem(u.returnUnits());
    }

    comboBox.setSelectedIndex(-1);
    unitColumn.setCellEditor(new DefaultCellEditor(comboBox));

Now I want to create an multi-layer JComboBox (perhaps JMenu?). The function of such should behave as a multi-layer JMenu. When the JComboBox is clicked, it will show the first layer - category such as "Electricity, Water, Distance...", Then when mouse hover over Electricity, a list of Electricity units such as "KW, MW, W ..." will show. These collections are fetched from Enumerations. I wonder what's the most correct way to create such component.

Thank you so much world!

Answer 1

Maybe use 2 combo boxes? That is you select a value in the first and the units are displayed in the second:

import java.awt.*;
import java.awt.event.*;
import java.util.*;
import javax.swing.*;

public class ComboBoxTwo extends JPanel implements ActionListener
{
    private JComboBox<String> mainComboBox;
    private JComboBox<String> subComboBox;
    private Hashtable<String, String[]> subItems = new Hashtable<String, String[]>();

    public ComboBoxTwo()
    {
        String[] items = { "Select Item", "Color", "Shape", "Fruit" };
        mainComboBox = new JComboBox<String>( items );
        mainComboBox.addActionListener( this );

        //  prevent action events from being fired when the up/down arrow keys are used
        mainComboBox.putClientProperty("JComboBox.isTableCellEditor", Boolean.TRUE);
        add( mainComboBox );

        //  Create sub combo box with multiple models

        subComboBox = new JComboBox<String>();
        subComboBox.setPrototypeDisplayValue("XXXXXXXXXX"); // JDK1.4
        add( subComboBox );

        String[] subItems1 = { "Select Color", "Red", "Blue", "Green" };
        subItems.put(items[1], subItems1);

        String[] subItems2 = { "Select Shape", "Circle", "Square", "Triangle" };
        subItems.put(items[2], subItems2);

        String[] subItems3 = { "Select Fruit", "Apple", "Orange", "Banana" };
        subItems.put(items[3], subItems3);
    }

    public void actionPerformed(ActionEvent e)
    {
        String item = (String)mainComboBox.getSelectedItem();
        Object o = subItems.get( item );

        if (o == null)
        {
            subComboBox.setModel( new DefaultComboBoxModel() );
        }
        else
        {
            subComboBox.setModel( new DefaultComboBoxModel( (String[])o ) );
        }
    }

    private static void createAndShowUI()
    {
        JFrame frame = new JFrame("SSCCE");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.add( new ComboBoxTwo() );
        frame.setLocationByPlatform( true );
        frame.pack();
        frame.setVisible( true );
    }

    public static void main(String[] args)
    {
        EventQueue.invokeLater(new Runnable()
        {
            public void run()
            {
                createAndShowUI();
            }
        });
    }
}