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Java AWT

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10.6 CheckboxMenuItem

The CheckboxMenuItem is a subclass of MenuItem that can be toggled. It is similar to a Checkbox but appears on a Menu. The appearance depends upon the platform. There may or may not be a visual indicator next to the choice. However, when the MenuItem is selected (true), a checkmark or some similar graphic will be displayed next to the label.

There is no way to put CheckboxMenuItem components into a CheckboxGroup to form a radio menu group.

An example of a CheckboxMenuItem is the Show Java Console menu item in Netscape Navigator.

CheckboxMenuItem Methods

Constructors

public CheckboxMenuItem (String label)

The first CheckboxMenuItem constructor creates a CheckboxMenuItem with no label displayed next to the check toggle. The initial value of the CheckboxMenuItem is false. To set the label at a later time, use setLabel().

public CheckboxMenuItem (String label)

The next CheckboxMenuItem constructor creates a CheckboxMenuItem with label displayed next to the check toggle. The initial value of the CheckboxMenuItem is false.

public CheckboxMenuItem (String label, boolean state)

The final CheckboxMenuItem constructor creates a CheckboxMenuItem with label displayed next to the check toggle. The initial value of the CheckboxMenuItem is state.

Selection

public boolean getState ()

The getState() method retrieves the current state of the CheckboxMenuItem.

public void setState (boolean condition)

The setState() method changes the current state of the CheckboxMenuItem to condition. When true, the CheckboxMenuItem will have the toggle checked.

public Object[] getSelectedObjects () (New)

The getSelectedItems() method returns the currently selected item as an Object array. This method, which is required by the ItemSelectable interface, allows you to use the same methods to retrieve the selected items of any Checkbox, Choice, or List. The array has at most one element, which contains the label of the selected item; if no item is selected, getSelectedItems() returns null.

Miscellaneous methods

public synchronized void addNotify ()

The addNotify() method creates the CheckboxMenuItem peer.

public String paramString ()

The paramString() method of CheckboxMenuItem should be protected like other paramString() methods. However, it is public, so you have access to it. When you call the toString() method of a CheckboxMenuItem, the default toString() method of MenuComponent is called. This in turn calls paramString() which builds up the string to display. At the CheckboxMenuItem level, the current state of the object is appended to the output. If the constructor for the CheckboxMenuItem was new CheckboxMenuItem(`File`) the results would be:

java.awt.CheckboxMenuItem[label=File,state=false]

CheckboxMenuItem Events

Event handling

A CheckboxMenuItem generates an ACTION_EVENT when it is selected. The argument to action() is the label of the CheckboxMenuItem, like the method provided by MenuItem, not the state of the CheckboxMenuItem as used in Checkbox. The target of the ACTION_EVENT is the Frame containing the menu. You cannot subclass CheckboxMenuItem and handle the Event within the subclass unless you override postEvent(). Listeners and 1.1 event handling

With the Java 1.1 event model, you register listeners, which are told when the event happens.

public void addItemListener(ItemListener listener) (New)

The addItemListener() method registers listener as an object that is interested in being notified when an ItemEvent passes through the EventQueue with this CheckboxMenuItem as its target. When these item events occur, the listener.itemStateChanged() method is called. Multiple listeners can be registered.

public void removeItemListener(ItemListener listener) (New)

The removeItemListener() method removes listener as a interested listener. If listener is not registered, nothing happens.

protected void processEvent(AWTEvent e) (New)

The processEvent() method receives every AWTEvent with this CheckboxMenuItem as its target. processEvent() then passes it along to any listeners for processing. When you subclass CheckboxMenuItem, overriding processEvent() allows you to process all events yourself, before sending them to any listeners. In a way, overriding processEvent() is like overriding postEvent() using the 1.0 event model.

If you override processEvent(), remember to call super.processEvent(e) last to ensure that regular event processing can occur. If you want to process your own events, it's a good idea to call enableEvents() to ensure that events are delivered, even in the absence of registered listeners.

protected void processItemEvent(ItemEvent e) (New)

The processItemEvent() method receives every ItemEvent with this CheckboxMenuItem as its target. processItemEvent() then passes it along to any listeners for processing. When you subclass CheckboxMenuItem, overriding processItemEvent() allows you to process all item events yourself, before sending them to any listeners. In a way, overriding processItemEvent() is like overriding action() using the 1.0 event model.

If you override processItemEvent(), remember to call the method super.processItemEvent(e) last to ensure that regular event processing can occur. If you want to process your own events, it's a good idea to call enableEvents() to ensure that events are delivered even in the absence of registered listeners.


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