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 7, Basic Utility Classes. 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 pre-defined SystemColors, just like the Color Class holds some pre-defined 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: