14.2 Menu Bar Selection Models

In all GUI environments, menu components allow only one selection to be made at a time. Swing is no exception. Swing provides a data model that menu bars and menus can use to emulate this behavior: the SingleSelectionModel.

14.2.1 The SingleSelectionModel Interface

Objects implementing the SingleSelectionModel interface do exactly what its name suggests: they maintain an array of possible selections and allow one element in the array to be chosen at a time. The model holds the index of the selected element. If a new element is chosen, the model resets the index representing the chosen element and fires a ChangeEvent to each of the registered listeners.

14.2.1.1 Properties

Objects implementing the SingleSelectionModel interface contain the properties shown in Table 14-1. The selected property is a boolean that tells if there is a selection. The selectedIndex property is an integer index that represents the currently selected item.

Table 14-1. SingleSelectionModel properties

Property	Data type	get	is	set	Default value
selected	boolean		·
selectedIndex	int	·		·

14.2.1.2 Events

Objects implementing the SingleSelectionModel interface must fire a ChangeEvent (not a PropertyChangeEvent) when the object modifies its selectedIndex property, i.e., when the selection has changed. The interface contains the standard addChangeListener( ) and removeChangeListener( ) methods for maintaining a list of ChangeEvent listeners.

void addChangeListener(ChangeListener listener)

void removeChangeListener(ChangeListener listener)

Add or remove the specified ChangeListener from the list of listeners receiving this model's change events.

14.2.1.3 Method

The SingleSelectionModel interface contains one other method:

public void clearSelection( )

Clear the selection value, forcing the selected property to return false.

14.2.2 The DefaultSingleSelectionModel Class

Swing provides a simple default implementation of the SingleSelectionModel interface in the DefaultSingleSelectionModel class.

14.2.2.1 Properties

DefaultSingleSelectionModel contains just the properties required by the SingleSelectionModel interface, as shown in Table 14-2. The selectedIndex property is an integer index that represents the currently selected item. The default value of -1 indicates that there is no selection. The selected property is a boolean that returns true if the selectedIndex is anything other than -1, and false otherwise.

Table 14-2. DefaultSingleSelectionModel properties

Property	Data type	get	is	set	Default value
selected	boolean		·		false
selectedIndex	int	·		·	-1

14.2.2.2 Events and methods

The DefaultSingleSelectionModel object provides all the events and methods specified by the SingleSelectionModel interface discussed earlier.