Java Reference
In-Depth Information
7
// Create a MovableMessagePanel instance for moving a message
MovableMessagePanel p =
new
MovableMessagePanel
8
9
10
11
// Place the message panel in the frame
12 add(p);
13 }
14
15
/** Main method */
16
public static void
main(String[] args) {
17 MoveMessageDemo frame =
new
MoveMessageDemo();
18 frame.setTitle(
"MoveMessageDemo"
);
19 frame.setSize(
200
,
100
);
20 frame.setLocationRelativeTo(
null
);
// Center the frame
21 frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
22 frame.setVisible(
true
);
23 }
24
25
create a panel
(
"Welcome to Java"
);
// Inner class: MovableMessagePanel draws a message
static class
MovableMessagePanel
extends
JPanel {
26
27
inner class
private
String message =
"Welcome to Java"
;
28
private int
x =
20
;
29
private int
y =
20
;
30
31
/** Construct a panel to draw string s */
32
public
MovableMessagePanel(String s) {
33 message = s;
34
35 @Override
/** Handle mouse-dragged event */
36
37
// Get the new location and repaint the screen
38 x = e.getX();
39 y = e.getY();
40 repaint();
41 }
42
43 @Override
/** Handle mouse-moved event */
44
45 }
46 );
47 }
48
49 @Override
50
protected void
paintComponent(Graphics g) {
51
super
.paintComponent(g);
52 g.drawString(message, x, y);
53 }
54 }
55 }
set a new message
anonymous listener
addMouseMotionListener(
new
MouseMotionListener() {
public void
mouseDragged(MouseEvent e) {
override handler
new location
repaint
public void
mouseMoved(MouseEvent e) {
}
paint message
The
MovableMessagePanel
class extends
JPanel
to draw a message (line 26). Addition-
ally, it handles redisplaying the message when the mouse is dragged. This class is defined as
an inner class inside the main class because it is used only in this class. Furthermore, the
inner class is defined as static because it does not reference any instance members of the
main class.
The
MouseMotionListener
interface contains two handlers,
mouseMoved
and
mouseDragged
, for handling mouse-motion events. When you move the mouse with a button
pressed, the
mouseDragged
method is invoked to repaint the viewing area and display the
