How to Write a Key Listener

Notifications are sent about two basic kinds of key events:

• The typing of a Unicode character
• The pressing or releasing of a key on the keyboard

In general, you react to only key-typed events unless you need to know when the user presses keys that do not correspond to characters. For example, to know when the user types a Unicode character — whether by pressing one key such as 'a' or by pressing several keys in sequence — you handle key-typed events. On the other hand, to know when the user presses the F1 key, or whether the user pressed the '3' key on the number pad, you handle key-pressed events.

To make a component get the keyboard focus, follow these steps:

1. Make sure the component's isFocusable method returns true. This state allows the component to receive the focus. For example, you can enable keyboard focus for a JLabel component by calling the setFocusable(true) method on the label.
2. Make sure the component requests the focus when appropriate. For custom components, implement a mouse listener that calls the requestFocusInWindow method when the component is clicked.

component.setFocusTraversalKeysEnabled(false)

For key-typed events you can obtain the key character value as well as any modifiers used.

The following example demonstrates key events. It consists of a text field that you can type into, followed by a text area that displays a message every time the text field fires a key event. A button at the bottom of the window lets you clear both the text field and text area.

You can find the example's code in KeyEventDemo.java. Here is the demo's key event handling code:

public class KeyEventDemo ...  implements KeyListener ... {
    ...//where initialization occurs:
	typingArea = new JTextField(20);
	typingArea.addKeyListener(this);

        //Uncomment this if you wish to turn off focus
        //traversal.  The focus subsystem consumes
        //focus traversal keys, such as Tab and Shift Tab.
        //If you uncomment the following line of code, this
        //disables focus traversal and the Tab events 
        //become available to the key event listener.
        //typingArea.setFocusTraversalKeysEnabled(false);
    ...
    /** Handle the key typed event from the text field. */
    public void keyTyped(KeyEvent e) {
	displayInfo(e, "KEY TYPED: ");
    }

    /** Handle the key-pressed event from the text field. */
    public void keyPressed(KeyEvent e) {
	displayInfo(e, "KEY PRESSED: ");
    }

    /** Handle the key-released event from the text field. */
    public void keyReleased(KeyEvent e) {
	displayInfo(e, "KEY RELEASED: ");
    }
    ...
    private void displayInfo(KeyEvent e, String keyStatus){
        
        //You should only rely on the key char if the event
        //is a key typed event.
        int id = e.getID();
        String keyString;
        if (id == KeyEvent.KEY_TYPED) {
            char c = e.getKeyChar();
            keyString = "key character = '" + c + "'";
        } else {
            int keyCode = e.getKeyCode();
            keyString = "key code = " + keyCode
                    + " ("
                    + KeyEvent.getKeyText(keyCode)
                    + ")";
        }
        
        int modifiersEx = e.getModifiersEx();
        String modString = "extended modifiers = " + modifiersEx;
        String tmpString = KeyEvent.getModifiersExText(modifiersEx);
        if (tmpString.length() > 0) {
            modString += " (" + tmpString + ")";
        } else {
            modString += " (no extended modifiers)";
        }
        
        String actionString = "action key? ";
        if (e.isActionKey()) {
            actionString += "YES";
        } else {
            actionString += "NO";
        }
        
        String locationString = "key location: ";
        int location = e.getKeyLocation();
        if (location == KeyEvent.KEY_LOCATION_STANDARD) {
            locationString += "standard";
        } else if (location == KeyEvent.KEY_LOCATION_LEFT) {
            locationString += "left";
        } else if (location == KeyEvent.KEY_LOCATION_RIGHT) {
            locationString += "right";
        } else if (location == KeyEvent.KEY_LOCATION_NUMPAD) {
            locationString += "numpad";
        } else { // (location == KeyEvent.KEY_LOCATION_UNKNOWN)
            locationString += "unknown";
        }
        
        ...//Display information about the KeyEvent...
    }
}

The Key Listener API

The KeyListener Interface

The corresponding adapter class is KeyAdapter.

Method	Purpose
keyTyped(KeyEvent)	Called just after the user types a Unicode character into the listened-to component.
keyPressed(KeyEvent)	Called just after the user presses a key while the listened-to component has the focus.
keyReleased(KeyEvent)	Called just after the user releases a key while the listened-to component has the focus.

The KeyEvent Class

The KeyEvent class inherits many useful methods from the InputEvent class, such as getModifiersEx, and a couple of useful methods from the ComponentEvent and AWTEvent classes. See the InputEvent Class table in the mouse listener page for a complete list.

Method	Purpose
int getKeyChar()	Obtains the Unicode character associated with this event. Only rely on this value for key-typed events.
int getKeyCode()	Obtains the key code associated with this event. The key code identifies the particular key on the keyboard that the user pressed or released. The KeyEvent class defines many key code constants for commonly seen keys. For example, VK_A specifies the key labeled A, and VK_ESCAPE specifies the Escape key.
String getKeyText(int) String getKeyModifiersText(int)	Return text descriptions of the event's key code and modifier keys, respectively.
int getModifiersEx() String getModifiersExText(int modifiers)	Return the extended modifiers mask for this event. There are methods inherited from the InputEvent class. Extended modifiers represent the state of all modal keys. The getModifiersExText method returns a string describing the extended modifier keys and mouse buttons. Since the getModifiersEx and getModifiersExText methods provide more information about key events, they are preferred over the getKeyText or getKeyModifiersText methods.
boolean isActionKey()	Returns true if the key firing the event is an action key. Examples of action keys include Cut, Copy, Paste, Page Up, Caps Lock, the arrow and function keys. This information is valid only for key-pressed and key-released events.
int getKeyLocation()	Returns the location of the key that fired this event. This provides a way to distinguish keys that occur more than once on a keyboard, such as the two shift keys, for example. The possible values are KEY_LOCATION_STANDARD, KEY_LOCATION_LEFT, KEY_LOCATION_RIGHT, KEY_LOCATION_NUMPAD, or KEY_LOCATION_UNKNOWN. This method always returns KEY_LOCATION_UNKNOWN for key-typed events. Introduced in JDK release 1.4.

Examples that Use Key Listeners

Example	Where Described	Notes
KeyEventDemo	This section	Reports all key events that occur on a text field to demonstrate the circumstances under which key events are fired.