GRAPHICS - Introduction to Java Programming - page 488

The repaint method (lines 75, 86) is defined in the Component class. Invoking repaint
causes the paintComponent method to be called. The repaint method is invoked to
refresh the viewing area. Typically, you call it if you have new things to display.

Caution
The paintComponent method should never be invoked directly. It is invoked either by
the JVM whenever the viewing area changes or by the repaint method. You should
override the paintComponent method to tell the system how to paint the viewing
area, but never override the repaint method.

don't invoke
paintComponent

Note
The repaint method lodges a request to update the viewing area and returns immedi-
ately. Its effect is asynchronous, meaning that it is up to the JVM to execute the
paintComponent method on a separate thread.

request repaint using
repaint()

The getPreferredSize() method (lines 95-97), defined in Component , is overridden
in FigurePanel to specify the preferred size for the layout manager to consider when laying
out a FigurePanel object. This property may or may not be considered by the layout man-
ager, depending on its rules. For example, a component uses its preferred size in a container
with a FlowLayout manager, but its preferred size is ignored if it is placed in a container
with a GridLayout manager. It is a good practice to override getPreferredSize() in a
subclass of JPanel to specify a preferred size, because the default width and height for a
JPanel is 0 . You will see nothing if a JPanel component with a default 0 width and height
is placed in a FlowLayout container.

why override
getPreferredSize() ?

13.12 Why should you override the preferredSize method in a subclass of JPanel ?
13.13 How do you get and set colors and fonts in a Graphics object?

✓ ✓

Check

Point

13.5 Drawing Arcs
An arc is conceived as part of an oval bounded by a rectangle.

Key
Point

The methods to draw or fill an arc are as follows:

drawArc( int x, int y, int w, int h, int startAngle, int arcAngle)
fillArc( int x, int y, int w, int h, int startAngle, int arcAngle)

Parameters x , y , w , and h are the same as in the drawOval method; parameter startAngle
is the starting angle; and arcAngle is the spanning angle (i.e., the angle covered by the arc).
Angles are measured in degrees and follow the usual mathematical conventions (i.e., 0
degrees is in the easterly direction, and positive angles indicate counterclockwise rotation
from the easterly direction); see Figure 13.10.

(x, y)

w

arcAngle

startAngle

h

F IGURE 13.10

The drawArc method draws an arc based on an oval with specified angles.