Java Reference
In-Depth Information
115 repaint();
116 }
117
118
/** Move the message up */
119
public void
moveUp() {
120 yCoordinate -= interval;
121 repaint();
122 }
123
124
/** Move the message down */
125
public void
moveDown() {
126 yCoordinate += interval;
127 repaint();
128 }
129
130 @Override
/** Override get method for preferredSize */
131
override
getPreferredSize
public
Dimension getPreferredSize() {
132
return new
Dimension(
200
,
30
);
133 }
134 }
The
paintComponent
method displays the message centered, if the
centered
property
is
true
(line 91).
message
is initialized to
Welcome to Java
in line 8. If it were not
initialized, a
NullPointerException
runtime error would occur when you created
a
MessagePanel
using the no-arg constructor, because
message
would be
null
in
line 103.
Caution
The
MessagePanel
class uses the properties
xCoordinate
and
yCoordinate
to
specify the position of the message displayed on the panel. Do not use the property
names
x
and
y
, because they are already defined in the
Component
class to return the
position of the component in the parent's coordinate system using
getX()
and
getY()
.
Note
The
Component
class has the
setBackground
,
setForeground
, and
setFont
methods. These methods are for setting colors and fonts for the entire component. If
you wanted to draw several messages in a panel with different colors and fonts, you
would have to use the
setColor
and
setFont
methods in the
Graphics
class to
set the color and font for the current drawing.
Note
A key feature of Java programming is the reuse of classes. Throughout this topic,
reusable classes are developed and later reused.
MessagePanel
is an example, as are
Loan
in Listing 10.2 and
FigurePanel
in Listing 13.3.
MessagePanel
can be
reused whenever you need to display a message on a panel. To make your class reusable
in a wide range of applications, you should provide a variety of ways to use it.
MessagePanel
provides many properties and methods that will be used in several
examples in the topic.
design classes for reuse
13.21
If
message
is not initialized in line 8 in Listing 13.8, MessagePanel.java, what will
happen when you create a
MessagePanel
using its no-arg constructor?
13.22
The following program is supposed to display a message on a panel, but nothing is
displayed. There are problems in lines 2 and 15. Correct them.
✓
✓
Check
Point
