Java Reference
In-Depth Information
tions do nothing when they're called. So, we override method
mouseDragged
at lines 23-
28 to capture the coordinates of a mouse drag event and store them as a
Point
object. Line
26 invokes the
MouseEvent
's
getPoint
method to obtain the
Point
where the event
occurred and stores it in the
ArrayList
. Line 27 calls method
repaint
(inherited indi-
rectly from class
Component
) to indicate that the
PaintPanel
should be refreshed on the
screen as soon as possible with a call to the
PaintPanel
's
paintComponent
method.
Method
paintComponent
(lines 34-42), which receives a
Graphics
parameter, is
called automatically any time the
PaintPanel
needs to be displayed on the screen—such
as when the GUI is first displayed—or refreshed on the screen—such as when method
repaint
is called or when the GUI component has been
hidden
by another window on the
screen and subsequently becomes visible again.
Look-and-Feel Observation 12.13
Calling
repaint
for a Swing GUI component indicates that the component should be re-
freshed on the screen as soon as possible. The component's background is cleared
only if
the component is opaque.
JComponent
method
setOpaque
can be passed a
boolean
argu-
ment indicating whether the component is opaque (
true
) or transparent (
false
).
Line 37 invokes the superclass version of
paintComponent
to clear the
PaintPanel
's
background (
JPanel
s are opaque by default). Lines 40-41 draw an oval at the location
specified by each
Point
in the
ArrayList
.
Graphics
method
fillOval
draws a solid oval.
The method's four parameters represent a rectangular area (called the
bounding box
) in
which the oval is displayed. The first two parameters are the upper-left
x
-coordinate and
the upper-left
y
-coordinate of the rectangular area. The last two coordinates represent the
rectangular area's width and height. Method
fillOval
draws the oval so it touches the
middle of each side of the rectangular area. In line 41, the first two arguments are specified
by using class
Point
's two
public
instance variables—
x
and
y
. You'll learn more
Graphics
features in Chapter 13.
Look-and-Feel Observation 12.14
Drawing on any GUI component is performed with coordinates that are measured from
the upper-left corner (0, 0) of that GUI component,
not
the upper-left corner of the screen.
Using the Custom
JPanel
in an Application
Class
Painter
(Fig. 12.35) contains the
main
method that executes this application. Line
14 creates a
PaintPanel
object on which the user can drag the mouse to draw. Line 15
attaches the
PaintPanel
to the
JFrame
.
1
// Fig. 12.35: Painter.java
2
// Testing PaintPanel.
3
import
java.awt.BorderLayout;
4
import
javax.swing.JFrame;
5
import
javax.swing.JLabel;
6
7
public
class
Painter
8
{
Fig. 12.35
|
Testing
PaintPanel
. (Part 1 of 2.)
