Trail: Creating a GUI With JFC/Swing
Lesson: Writing Event Listeners
Section: Implementing Listeners for Commonly Handled Events

如何编写项目监听器

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 buttons like check boxes, check menu items, toggle buttons etc...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);
        ...
    } else {
        label.setVisible(false);
    }
}

The Item Listener API

The ItemListener Interface

Because ItemListener has only one method, it has no corresponding adapter class.

方法	目的
itemStateChanged(ItemEvent)	Called just after a state change in the listened-to component.

The ItemEvent Class

方法	目的
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 examples that use item listeners.

例子	Where Described	Notes
`ComponentEventDemo`	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`	How to Use Check Boxes	Four check boxes share one item listener, which uses `getItemSelected` to determine which check box fired the event.
`MenuDemo`	如何使用菜单	Listens for item events on a check box menu item.
`MenuDemo`	如何使用滚动窗格	Listens for item events on a toggle button.