Java Reference
In-Depth Information
16.16
Why does the
ActionListener
interface have no listener interface adapter?
✓
✓
Check
16.17
What is the advantage of using a listener interface adapter rather than a listener
interface?
Point
A key event is fired whenever a key is pressed, released, or typed on a component.
Key
Point
Key events
enable the use of the keys to control and perform actions or get input from the key-
board. The
KeyEvent
object describes the nature of the event (namely, that a key has been
pressed, released, or typed) and the value of the key, as shown in Figure 16.15. Java provides
the
KeyListener
interface to handle key events, as shown in Figure 16.16.
java.awt.event.InputEvent
java.awt.event.KeyEvent
+getKeyChar(): char
Returns the character associated with the key in this event.
+getKeyCode(): int
Returns the integer key code associated with the key in this event.
F
IGURE
16.15
The
KeyEvent
class encapsulates information about key events.
«interface»
java.awt.event.KeyListener
+
keyPressed(e: KeyEvent): void
Invoked after a key is pressed on the source component.
+
keyReleased(e: KeyEvent): void
Invoked after a key is released on the source component.
Invoked after a key is pressed and then released on the source component.
+
keyTyped(e: KeyEvent): void
F
IGURE
16.16
The
KeyListener
interface handles key pressed, released, and typed events.
The
keyPressed
handler is invoked when a key is pressed, the
keyReleased
handler is
invoked when a key is released, and the
keyTyped
handler is invoked when a Unicode char-
acter is entered. If a key does not have a Unicode (e.g., function keys, modifier keys, action
keys, and control keys), the
keyTyped
handler will not be invoked.
Every key event has an associated key character or key code that is returned by the
getKeyChar()
or
getKeyCode()
method in
KeyEvent
. The key codes are constants
defined in the
KeyEvent
class. Table 16.3 lists some constants. See the Java API for a
complete list of the constants. For a key of the Unicode character, the key code is the same
as the Unicode value. For the key-pressed and key-released events,
getKeyCode()
returns
the value as defined in the table. For the key-typed event,
getKeyCode()
returns
VK_UNDEFINED
(
0
), and
getKeyChar()
returns the character entered.
The program in Listing 16.9 displays a user-input character. The user can move the
character up, down, left, and right, using the arrow keys
VK_UP
,
VK_DOWN
,
VK_LEFT
, and
VK_RIGHT
. Figure 16.17 contains a sample run of the program.
