Creating Windows - Beginning Java - page 813

graphics that need to be updated relatively frequently — such as when you create an animation. When part
of what is displayed is to be animated, a static background can be displayed independently via the con-
tentPane . Because this doesn't need to be reprocessed each time the animated objects need to be redrawn,
the whole process can be much more efficient.
The JFrame class defines methods to provide you with a reference to any of the panes (shown in Table
17-3 ) :
TABLE 17-3 : JFrame Class Methods
METHOD DESCRIPTION
getRootPane() Returns the root pane as type JRootPane .
getLayeredPane() Returns the layered pane as type JLayeredPane .
getContentPane() Returns the content pane as type Container . This is the method you use most frequently because
you normally add components to the content pane.
getGlassPane() Returns the glass pane as type Component.
All the classes here that represent panes are themselves Swing components, defined in the javax.swing
package. A JApplet object has the same arrangement of panes as a JFrame object, so adding components to
an applet, or drawing on it, works in exactly the same way. An applet defined as a JApplet object can also
have a menu bar just like an application window.
All the panes, as well as the menu bar, are components, so before I start delving into how to add a menu
bar or other components to a window, let's unearth a little more about the makeup of components in general.
BASICS OF COMPONENTS
You need to understand several basic concepts common to all components before you can apply them prop-
erly. They also have uses in many different contexts. Although this section may seem like something of a
catalog of classes and methods without much in the way of practical application, please stay with it. You use
most of these capabilities later. To understand the fundamental things you can do with Swing components,
you need to explore the functionality the Swing components inherit from the Component and Container
classes.
When a component is contained within another component, the outer object is referred to as the parent .
You can obtain a reference to the parent object of any given component by calling its getParent() method.
This method is inherited from the Component class, and it returns the parent as type Container because
only a subclass of Container can hold other components. If there is no parent, as is the case with a JFrame
component, this method returns null .
Component Attributes
The Component class defines attributes, which record the following information about an object:
• The position is stored as ( x , y ) coordinates. This fixes where the object is in relation to its contain-
er in the coordinate system of the container object.
• The name of the component is stored as a String object.
• The size is recorded as values for the width and the height of the object.
• The foreground color and background color that apply to the object are used when the object is
displayed.