Explanation of AWT Components and Window Fundamentals in Java

AWT Abstract Window Toolkit  

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 application window.

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. 
• A Component object is responsible for remembering the current foreground and background colors and the currently selected text font.

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

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.

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( ), setPreferredSize( ), or setBounds( ) methods defined by Component.

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. 
• When a Frame window is created by a stand-alone application 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.