How to Write an Item Listener - Java Tutorial 5.0 英文版

The Java^TM Tutorial

Start of Tutorial > Start of Trail > Start of Lesson

Search
Feedback Form

Trail: Creating a GUI with JFC/Swing
Lesson: 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);
        ...
    } else {
        label.setVisible(false);
    }
}

The Item Listener API

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

Method Purpose

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

The ItemEvent Class

Method Purpose

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.

Method	Purpose
`itemStateChanged(ItemEvent)`	Called just after a state change in the listened-to component.

Method	Purpose
`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.

Example 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 How to Use Menus Listens for item events on a check box menu item.

MenuDemo How to Use Scroll Panes Listens for item events on a toggle button.

Example	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`	How to Use Menus	Listens for item events on a check box menu item.
`MenuDemo`	How to Use Scroll Panes	Listens for item events on a toggle button.

Start of Tutorial > Start of Trail > Start of Lesson

Search
Feedback Form