Writing Event Listeners

How to Write an Item Listener

Item events are fired by components that implement the ItemSelectable interface. Generally, ItemSelectable components maintain on/off state for one or more items. The Swing components that fire item events include check boxes, check box menu items, and combo boxes.

Here is some item-event handling code taken from ComponentEventDemo.java:

...
//where initialization occurs
checkbox.addItemListener(this);
...
public void itemStateChanged(ItemEvent e) {
    if (e.getStateChange() == ItemEvent.SELECTED) {
        label.setVisible(true);

        label.revalidate();
        label.repaint();
    } else {
        label.setVisible(false);
    }
}

The Item Event API

The ItemListener interface has just one method, so it has no corresponding adapter class. Here's the method:

void itemStateChanged(ItemEvent)

Called just after a state change in the listened-to component.

The itemStateChanged method hod has a single parameter: an ItemEvent object. The ItemEvent class defines the following handy methods:

Object getItem()

Returns the component-specific object associated with the item whose state changed. Often this is a String containing the text on the selected item.

ItemSelectable getItemSelectable()

Returns the component that fired the item event. You can use this instead of the getSource method.

int getStateChange()

Returns the new state of the item. The ItemEvent class defines two states: SELECTED and DESELECTED.

Examples that Use Item Listeners

The following table lists some of examples that use item listeners.

Example	Where Described	Notes
ComponentEventDemo.java	This section and How to Write a Component Listener	Listens for item events on a check box, which determines whether a lable is visible.
CheckBoxDemo.java	How to Use Check Boxes	Four check boxes share one item listener, which uses getItemSelected to determine which check box fired the event.
MenuDemo.java	How to Use Menus	Listens for item events on a check box menu item.

Writing Event Listeners