The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.
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See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.
The JComponent
class supports key bindings as a way of responding to individual keys typed by a user. Here are some examples of when key bindings are appropriate:
You often don't need to use key bindings directly. They're used behind the scenes by mnemonics (supported by all buttons and by tabbed panes as well as by
JLabel
) and accelerators (supported by menu items). You can find coverage of mnemonics and accelerators in the section
Enabling Keyboard Operation.
An alternative to key bindings is using
key listeners. Key listeners have their place as a low-level interface to keyboard input, but for responding to individual keys key bindings are more appropriate and tend to result in more easily maintained code. Key listeners are also difficult if the key binding is to be active when the component doesn't have focus. Some of the advantages of key bindings are they're somewhat self documenting, take the containment hierarchy into account, encourage reusable chunks of code (Action
objects), and allow actions to be easily removed, customized, or shared. Also, they make it easy to change the key to which an action is bound. Another advantage of Actions is that they have an enabled state which provides an easy way to disable the action without having to track which component it is attached to.
The rest of this section gives you the details you need to use key bindings:
The key binding support provided by JComponent
relies on the
InputMap
and
ActionMap
classes. An input map binds key strokes to action names, and an action map specifies the
action corresponding to each action name. Technically, you don't need to use action names in the maps; you can use any object as the "key" into the maps. By convention, however, you use a string that names an action.
Each InputMap
/ActionMap
has a parent that typically comes from the UI. Any time the look and feel is changed, the parent is reset. In this way, any bindings specified by the developer are never lost on look and feel changes.
Each JComponent
has one action map and three input maps. The input maps correspond to the following focus situations:
JComponent.WHEN_FOCUSED
WHEN_FOCUSED
input map is typically used when the component has no children. For example, buttons bind the Space key using the WHEN_FOCUSED
map.JComponent.WHEN_ANCESTOR_OF_FOCUSED_COMPONENT
JTable
s make all their bindings using WHEN_ANCESTOR_OF_FOCUSED_COMPONENT
so that if the user is editing, the up-arrow key (for example) still changes the selected cell.JComponent.WHEN_IN_FOCUSED_WINDOW
When the user types a key, the JComponent
key event processing code searches through one or more input maps to find a valid binding for the key. When it finds a binding, it looks up the corresponding action in the action map. If the action is enabled, the binding is valid and the action is executed. If it's disabled, the search for a valid binding continues.
If more than one binding exists for the key, only the first valid one found is used. Input maps are checked in this order:
WHEN_FOCUSED
input map.WHEN_ANCESTOR_OF_FOCUSED_COMPONENT
input map.WHEN_ANCESTOR_OF_FOCUSED_COMPONENT
input maps of the focused component's parent, and then its parent's parent, and so on, continuing up the containment hierarchy. Note: Input maps for disabled components are skipped.WHEN_IN_FOCUSED_WINDOW
input maps of all the enabled components in the focused window are searched. Because the order of searching the components is unpredictable, avoid duplicate WHEN_IN_FOCUSED_WINDOW
bindings!Let's consider what happens in two typical key binding cases: a button reacting to the Space key, and a frame with a default button reacting to the Enter key.
In the first case, assume the user presses the Space key while a JButton
has the keyboard focus. First, the button's key listeners are notified of the event. Assuming none of the key listeners consumes the event (by invoking the
consume
method on the KeyEvent
) the button's WHEN_FOCUSED
input map is consulted. A binding is found because JButton
uses that input map to bind Space to an action name. The action name is looked up in the button's action map, and the actionPerformed
method of the action is invoked. The KeyEvent
is consumed, and processing stops.
In the second case, assume the Enter key is pressed while the focus is anywhere inside a frame that has a default button (set using the JRootPane
setDefaultButton
method). Whatever the focused component is, its key listeners are first notified. Assuming none of them consumes the key event the focused component's WHEN_FOCUSED
input map is consulted. If it has no binding for the key or the Action bound to the key is disabled, the focused component's WHEN_ANCESTOR_OF_FOCUSED_COMPONENT
input map is consulted and then (if no binding is found or the Action bound to the key is disabled) the WHEN_ANCESTOR_OF_FOCUSED_COMPONENT
input maps of each of the component's ancestors in the containment hierarchy. Eventually, the root pane's WHEN_ANCESTOR_OF_FOCUSED_COMPONENT
input map is searched. Since that input map has a valid binding for Enter, the action is executed, causing the default button to be clicked.
Here is an example of specifying that a component should react to the F2 key:
component.getInputMap().put(KeyStroke.getKeyStroke("F2"), "doSomething"); component.getActionMap().put("doSomething", anAction); //where anAction is a javax.swing.Action
As the preceding code shows, to get a component's action map you use the getActionMap
method (inherited from JComponent
). To get an input map, you can use the getInputMap(int)
method, where the integer is one of the JComponent.WHEN_*FOCUSED*
constants shown in the preceding list. Or, in the usual case where the constant is JComponent.WHEN_FOCUSED
, you can just use getInputMap
with no arguments.
To add an entry to one of the maps, use the put
method. You specify a key using a KeyStroke
object, which you can get using the
KeyStroke.getKeyStroke(String)
method. You can find examples of creating an Action
(to put in an action map) in
How to Use Actions.
Here's a slightly more complex example that specifies that a component should react to the Space key as if the user clicked the mouse.
component.getInputMap().put(KeyStroke.getKeyStroke("SPACE"), "pressed"); component.getInputMap().put(KeyStroke.getKeyStroke("released SPACE"), "released"); component.getActionMap().put("pressed", pressedAction); component.getActionMap().put("released", releasedAction); //where pressedAction and releasedAction are javax.swing.Action objects
To make a component ignore a key that it normally responds to, you can use the special action name "none". For example, the following code makes a component ignore the F2 key.
component.getInputMap().put(KeyStroke.getKeyStroke("F2"), "none");
The preceding code doesn't prevent the relevant WHEN_ANCESTOR_OF_FOCUSED_COMPONENT
and WHEN_IN_FOCUSED_WINDOW
input maps from being searched for an F2 key binding. To prevent this search, you must use a valid action instead of "none". For example:
Action doNothing = new AbstractAction() { public void actionPerformed(ActionEvent e) { //do nothing } }; component.getInputMap().put(KeyStroke.getKeyStroke("F2"), "doNothing"); component.getActionMap().put("doNothing", doNothing);
The following tables list the commonly used API for key bindings. Also see the API table Creating and Using an Action, in the section How to Use Actions.
Method | Purpose |
---|---|
InputMap getInputMap() InputMap getInputMap(int) (in JComponent ) |
Get one of the input maps for the component. The arguments can be one of these JComponent constants: WHEN_FOCUSED , WHEN_IN_FOCUSED_WINDOW , or WHEN_ANCESTOR_OF_FOCUSED_COMPONENT . The no-argument method gets the WHEN_FOCUSED input map. |
void put(KeyStroke, Object) (in InputMap ) |
Set the action name associated with the specified key stroke. If the second argument is null , this method removes the binding for the key stroke. To make the key stroke be ignored, use "none" as the second argument. |
static KeyStroke getKeyStroke(String) (in KeyStroke ) |
Get the object specifying a particular user keyboard activity. Typical arguments are "alt shift X", "INSERT", and "typed a". See the
KeyStroke API documentation for full details and for other forms of the getKeyStroke method. |
Method | Purpose |
---|---|
ActionMap getActionMap() (in JComponent ) |
Get the object that maps names into actions for the component. |
void put(Object, Action) (in ActionMap ) |
Set the action associated with the specified name. If the second argument is null , this method removes the binding for the name. |
The following table lists examples that use key bindings:
Example | Where Described | Notes |
---|---|---|
TableFTFEditDemo | How to Use Tables | The IntegerEditor class registers a key binding on a formatted text field to validate the input when the user presses the Enter key. |
TextComponentDemo | Text Component Features | Key bindings are registered on a text pane to navigate through the text when the user presses the Control-B, Control-F, Control-P, and Control-N keys. |