 | Chapter 16 Drawing with AWT |  |
| Chapter 16 Drawing with AWT | |
The TestPattern applet fills its shapes with a
number of colors, using the setColor() method of the
Graphics object. setColor() sets
the current color in the graphics context, so we set it to a different
color before each drawing operation. But where do these color values
come from?
The java.awt.Color class handles color in Java. A
Color object describes a single color. You can
create an arbitrary Color by specifying the red,
green, and blue values, either as integers between 0 and 255 or as
floating-point values between 0.0 and 1.0. You can also use
getColor() to look up a named color in the system
properties table, as described in Chapter 9.
getColor() takes a
String color property name, retrieves the integer
value from the Properties list, and returns the
Color object that corresponds to that color.
The Color class also defines a number of
static final color values; these are what we used
in the TestPattern example. These constants, such
as Color.black and Color.red,
provide a convenient set of basic colors for your drawings.
The Color class I just described makes it
easy to construct a particular color; however, that's not always what
you want to do. Sometimes you want to match a preexisting color
scheme. This is particularly important when you are designing a user
interface; you might want your components to have the same colors as
other components on that platform, and to change automatically if the
user redefines his or her color scheme.
That's what the SystemColor class is for. A system
color represents the color used by the local windowing system in a
certain context. The SystemColor class holds lots of predefined
system colors, just like the Color class holds some predefined basic
colors. For example, the field activeCaption
represents the color used for the background to the title of an active
window; activeCaptionText represents the color used
for the title itself. menu represents the
background color of menu selection; menuText
represents the color of a menu item's text when it is not selected;
textHighlightText is the color used when the item
is selected; and so on. You could use the window
value to set the color of a Window to match the
other Windows on the user's screen--whether or not they're generated
by Java programs.
myWindow.setBackground( SystemColor.window );
Because the SystemColor class is a
subclass of Color, you can use it
wherever you would use a Color.
However, the SystemColor constants are
tricky. They are constants as far as you, the programmer, are
concerned; your code is not allowed to modify them. However, they can
be modified at run-time by the toolkit. If the user changes his color
scheme, the system colors are automatically updated to follow suit; as
a result, anything displayed with system colors will also change color
the next time it is redrawn. For example, the window
myWindow would automatically change its
background color to the new background color.
The SystemColor class has one noticeable
shortcoming. You can't compare a system color to a
Color directly;
the Color.equals() method doesn't return
reliable results. For example, if you want to find out whether the
window background color is red, you can't call:
Color.red.equals(SystemColor.window);Instead, you should use
getRGB() to find
the color components of both objects and compare them, rather than
comparing the objects themselves. |  | |
| 16.1 Basic Drawing |  | 16.3 Fonts |
