Java Reference
In-Depth Information
68
69
/** Enlarge the circle */
70
public void
enlarge() {
71 radius++;
72 repaint();
73 }
74
75
/** Shrink the circle */
76
public void
shrink() {
77
if
(radius >=
1
) radius—-;
78 repaint();
79 }
80
81 @Override
82
protected void
paintComponent(Graphics g) {
83
super
.paintComponent(g);
84 g.drawOval(getWidth() /
2
- radius, getHeight() /
2
- radius,
85
2
* radius,
2
* radius);
86 }
87 }
88 }
A listener for
MouseEvent
is created to handle mouse-clicked events in lines 34-42. If the
left mouse button is clicked, the circle is enlarged (lines 37-38); if the right mouse button is
clicked, the circle is shrunk (lines 39-40).
A listener for
KeyEvent
is created to handle key-pressed events in lines 45-53. If the UP
arrow key is pressed, the circle is enlarged (lines 48-49); if the DOWN arrow key is pressed,
the circle is shrunk (lines 50-51).
Invoking
setFocusable
on
canvas
makes
canvas
focusable. However, once a button is
clicked, the canvas is no longer focused. Invoking
canvas.requestFocusInWindow()
(lines 22, 30) resets the focus on
canvas
so that
canvas
can listen for key events.
MouseEvent
KeyEvent
setFocusable
requestFocusInWindow()
16.18
What method do you use to get the timestamp for an action event, a mouse event, or
a key event?
✓
✓
Check
Point
16.19
What method do you use to get the key character for a key event?
16.20
How do you set focus on a component so it can listen for key events?
16.21
Does every key in the keyboard have a Unicode? Is a key code in the
KeyEvent
class
equivalent to a Unicode?
16.22
Is the
keyPressed
handler invoked after a key is pressed? Is the
keyReleased
handler invoked after a key is released? Is the
keyTyped
handler invoked after
any
key is typed?
A
Timer
is a source object that fires
ActionEvent
at a fixed rate.
Key
Point
Not all source objects are GUI components. The
javax.swing.Timer
class is a source
component that fires an
ActionEvent
at a predefined rate. Figure 16.18 lists some of the
methods in the class.
A
Timer
object serves as the source of an
ActionEvent
. The listeners must be instances
of
ActionListener
and registered with a
Timer
object. You create a
Timer
object using its
sole constructor with a delay and a listener, where
delay
specifies the number of milliseconds
between two action events. You can add additional listeners using the
addActionListener
method and adjust the
delay
using the
setDelay
method. To start the timer, invoke the
start()
method; to stop the timer, invoke the
stop()
method.
