AWT-WINDOW IN JAVA

Window Fundamentals
The AWT defines windows according to a class hierarchy that adds functionality and
specificity with each level. The two most common windows are those derived from
Panel, which is used by applets, and those derived from Frame, which creates a
standard window. Much of the functionality of these windows is derived from their
parent classes. Thus, a description of the class hierarchies relating to these two classes
is fundamental to their understanding. Figure 21-1 shows the class hierarchy for Panel
and Frame. Let’s look at each of these classes now.
Component
At the top of the AWT hierarchy is the Component class. Component is an abstract
class that encapsulates all of the attributes of a visual component. All user interface
elements that are displayed on the screen and that interact with the user are subclasses
of Component. It defines over a hundred public methods that are responsible for
managing events, such as mouse and keyboard input, positioning and sizing the
window, and repainting. (You already used many of these methods when you created
applets in Chapters 19 and 20.) A Component object is responsible for remembering
the current foreground and background colors and the currently selected text font.
Container

The Container class is a subclass of Component. It has additional methods that allow
other Component objects to be nested within it. Other Container objects can be stored
inside of a Container (since they are themselves instances of Component). This makes
for a multileveled containment system. A container is responsible for laying out (that
is, positioning) any components that it contains. It does this through the use of various
layout managers.
Panel
The Panel class is a concrete subclass of Container. It doesn’t add any new methods; it
simply implements Container. A Panel may be thought of as a recursively nestable,
concrete screen component. Panel is the superclass for Applet. When screen output is
directed to an applet, it is drawn on the surface of a Panel object. In essence, a Panel is
a window that does not contain a title bar, menu bar, or border. This is why you don’t
see these items when an applet is run inside a browser. When you run an applet using
an applet viewer, the applet viewer provides the title and border.
Other components can be added to a Panel object by its add( ) method (inherited
from Container). Once these components have been added, you can position and resize
them manually using the setLocation( ), setSize( ), or setBounds( ) methods defined by
Component.

Window
The Window class creates a top-level window. A top-level window is not contained
within any other object; it sits directly on the desktop. Generally, you won’t create
Window objects directly. Instead, you will use a subclass of Window called Frame,
described next.
Frame
Frame encapsulates what is commonly thought of as a “window.” It is a subclass of
Window and has a title bar, menu bar, borders, and resizing corners. If you create a
Frame object from within an applet, it will contain a warning message, such as “Java
Applet Window,” to the user that an applet window has been created. This message
warns users that the window they see was started by an applet and not by software
running on their computer. (An applet that could masquerade as a host-based
application could be used to obtain passwords and other sensitive information without
the user’s knowledge.) When a Frame window is created by a program rather than an
applet, a normal window is created.
Canvas
Although it is not part of the hierarchy for applet or frame windows, there is one other
type of window that you will find valuable: Canvas. Canvas encapsulates a blank
window upon which you can draw.