5.1 The ButtonModel Interface

The state of any Swing button is maintained by a ButtonModel object. This interface defines methods for reading and writing a model's properties and for adding and removing various types of event listeners.

5.1.1 Properties

The properties for the ButtonModel interface are listed in Table 5-1. The actionCommand property specifies the name of the command to be sent as part of the ActionEvent that is fired when the button is pressed. This can be used by event handlers that are listening to multiple buttons to determine which button is pressed.

Table 5-1. ButtonModel properties

Property	Data type	get	is	set	Default value
actionCommand	String	·		·
armed	boolean		·	·
enabled	boolean		·	·
group	ButtonGroup			·
mnemonic	int	·		·
pressed	boolean		·	·
rollover	boolean		·	·
selected	boolean		·	·
See also java.awt.ItemSelectable.

If no actionCommand is specified, an ActionEvent takes the button's text for its command string, so it is usually not necessary to specify an explicit actionCommand. You may find it useful to do so for buttons that have icons but no text or for multiple buttons with the same text. actionCommand properties can also be handy for internationalization. For example, if you need to change a button's text from "Hot" to "Caliente", you won't have to change any event-handling code if you set the actionCommand to "Hot".

The group property refers to the ButtonGroup that contains the button (if any). mnemonic contains the key that can be pressed in conjunction with a L&F-specific modifier key in order to produce the same effect as clicking the button with the mouse. The modifier key is currently the Alt^[1] key for all Swing L&Fs.

^[1] On the Macintosh, the Option key is used for Alt. Newer Mac keyboards have both labels.

The type of the mnemonic property is int because its value is intended to be one of the VK_ "virtual keycode" constants defined in java.awt.KeyEvent (see Table 27-6). However, a setMnemonic( ) method that takes a char is defined in AbstractButton. It's usually easier to call setMnemonic('a') than it is to call setMnemonic(KeyEvent.VK_A) unless you are working with the model directly. If you use the char version, it doesn't matter if you specify an uppercase or lowercase character.

The other properties are boolean flags that reflect certain aspects of the button's state. The properties are:

armed

Indicates whether releasing the button causes an action to be performed. This becomes false if the cursor is moved away from the button while the mouse button is still being held down.

enabled

Indicates whether the button is currently enabled. A button must be enabled to be pressed.

pressed

Indicates whether the button is currently being pressed (meaning that the button is being held down).

rollover

Indicates whether the mouse cursor is currently over the button. This allows an alternate image to be displayed.

selected

Indicates whether the button is currently selected. This is used only by JToggleButton and its subclasses. This property toggles between true and false each time the button is clicked.

5.1.2 Events

Objects implementing the ButtonModel interface fire action events, change events, and item events, as shown in Table 5-2.

Table 5-2. ButtonModel events

Event	Description
ActionEvent	The button is pressed.
ChangeEvent	A change has occurred in one or more properties of the button model.
ItemEvent	The button is toggled on or off.

The ButtonModel interface contains the following standard methods for maintaining event subscribers:

public void addActionListener(ActionListener l)

public void removeActionListener(ActionListener l)

public void addItemListener(ItemListener l)

public void removeItemListener(ItemListener l)

public void addChangeListener(ChangeListener l)

public void removeChangeListener(ChangeListener l)