Combo Box With Hidden Data

Typically a JComboBox is used to display a list of strings. There may be times when you need the items in the combo box to contain extra data so that you can do additional processing once an item is selected. What is the best way to do this?

The easiest way to accomplish this is to create a custom class that contains multiple properties. One property must be the property that will be displayed in the combo box. Other properties are added as required for additional processing. The key is to then make sure you override the toString() method of this custom class to return the display value. Why is overriding the toString() method so important?

1. JComboBox uses a renderer to display text in the combo box. The default renderer gets the text to display by invoking the toString() method on each object added to the ComboBoxModel.
2. JComboBox uses a KeySelectionManager to select items as the user types. In order to find matching items the default KeySelectionManager invokes the toString() method of each item as it searches through the ComboBoxModel.

In an application you may require several combo boxes with this type of functionality. Instead of creating a custom class for each combo box it is easier to create a general purpose class that can be used in many situations. The Item class may be class that you will consider using. It is a simple class that contains two properties:

• value – this property contains the data hidden from the user but available to be used by the application. It can be a simple object like a String or it can be a custom object containing any number of additional properties.
• description – this is a String that can be used by the renderer

Another feature of the Item class is that it implements the Comparable interface so that the SortedComboBoxModel (featured in another blog entry) can be used as the model for the JComboBox when required.

A simple example of a value/description Item might be a combo box that will contain a country code and country description. The country code would be hidden from the user and the country description will be used for display to the user. The code to create the combo box would be something like:

JComboBox<Item<String>> comboBox = new JComboBox<Item<String>>();
comboBox.addItem( new Item<String>("CA", "Canada" ) );
comboBox.addItem( new Item<String>("GB", "United Kingdom" ) );
comboBox.addItem( new Item<String>("US", "United States" ) );

In your ActionListener or ItemListener you would then access the country code value by doing:

JComboBox comboBox = (JComboBox)e.getSource();
Item item = (Item)comboBox.getSelectedItem();
String code = (String)item.getValue();


Using the Item class as a Wrapper

There may be times when you already have a custom object that you want to display in a combo box but the problem is that the toString() method has already been implemented and the implementation is not appropriate for display in a combo box. In this case you can use the Item class as a wrapper for your custom class.

For this next example we will assume we have a Car class with the properties “make” and “model” and we want the “model” to be displayed in the combo box. The code to create the combo box would be something like:

JComboBox<Item<Car>> comboBox = new JComboBox<Item<Car>>();
Car c1 = new Car("Ford", "Crown Victoria");
Car c2 = new Car("Ford", "Focus");
comboBox.addItem( new Item<Car>(c1, c1.getModel()) );
comboBox.addItem( new Item<Car>(c2, c2.getModel()) );

In your ActionListener or ItemListener you would then access the Car object by doing:

JComboBox comboBox = (JComboBox)e.getSource();
Item item = (Item)comboBox.getSelectedItem();
Car car = (Car)item.getValue();

Hopefully you will find the Item class helpful for including hidden data in a combo box. I couldn’t think of how to use a WebStart demo to show the usage of the Item class, so instead I have included my ItemTest code below which contains examples of the two approaches suggested here.

Note: Although the Item class was created for use with a JComboBox, there is no reason it can’t be used with other components that use renderers where the renderer relies on the toString() method for getting the text to render. For example, the Item class could also be used in a JList or JTable. Although, if used in a JTable it can only be used in a non-editable column.

Using a Custom Renderer

On various forums I have seen suggestions to use a custom render to solve this problem. That is, instead of overriding the toString() method of a custom class, the suggestion is to create a custom renderer to display a specific property from the class. I am not a big fan of this suggestion for the simple reason that the default KeySelectionManager will still use the toString() method of the class for selection purposes which can be very confusing to users. So if you decide to use this approach then you also need to implement a custom KeySelectionManager. This means you now have related logic in two separate classes.

Update:

If you decide to use the renderer approach then check out the Combo Box With Custom Renderer link below for a class that does most of the work for you.

Get The Code

Item.java
ItemDemo.java

See Also

Sorted Combo Box Model
Combo Box With Custom Renderer

Related Reading

Java API: Comparable
Java API: JComboBox.KeySelectionManager