A Container is a type of component that provides a rectangular area within which other components can be organized by a LayoutManager. Because Container is a subclass of Component, a Container can go inside another Container, which can go inside another Container, and so on, like Russian nesting dolls. Subclassing Container allows you to encapsulate code for the components within it. This allows you to create reusable higher-level objects easily. Figure 1.15 shows the components in a layout built from several nested containers.
A Panel is the basic building block of an applet. It provides a container with no special features. The default layout for a Panel is FlowLayout. The details of Panel are discussed in Panel. Figure 1.16 shows an applet that contains panels within panels within panels.
A Window provides a top-level window on the screen, with no borders or menu bar. It provides a way to implement pop-up messages, among other things. The default layout for a Window is BorderLayout. Window explores the Window class in greater detail. Figure 1.17 shows a pop-up message using a Window in Microsoft Windows and Motif.
A Frame is a Window with all the window manager's adornments (window title, borders, window minimize/maximize/close functionality) added. It may also include a menu bar. Since Frame subclasses Window, its default layout is BorderLayout. Frame provides the basic building block for screen-oriented applications. Frame allows you to change the mouse cursor, set an icon image, and have menus. All the details of Frame are discussed in Frames. Figure 1.18 shows an example Frame.
Dialog and FileDialog
A Dialog is a Window that accepts input from the user. BorderLayout is the default layout of Dialog because it subclasses Window. A Dialog is a pop-up used for user interaction; it can be modal to prevent the user from doing anything with the application before responding. A FileDialog provides a prebuilt Dialog box that interacts with the filesystem. It implements the Open/Save dialog provided by the native windowing system. You will primarily use FileDialog with applications since there is no guarantee that an applet can interact with the local filesystem. (Netscape Navigator will throw an exception if you try to use it.) The details of Dialog are revealed in Dialogs, while FileDialog is discussed in FileDialog. Figure 1.19 shows sample Dialog and FileDialog boxes.
Java 1.1 introduces the ScrollPane container. In version 1.0, if you want to have a scrolling area (for example, to display an image that won't fit onto the screen), you create a panel using BorderLayout that contains scrollbars on the right and bottom, and display part of the image in the rest of the screen. When the user scrolls, you capture the event, figure out what part of the image to display, and update the screen accordingly. Although this works, its performance is poor, and it's inconvenient. With version 1.1 of Java, you can tell the ScrollPane what needs to scroll; it creates the scrollbars and handles all the events automatically. ScrollPane covers the ScrollPane; Figure 1.20 shows a ScrollPane. Chapter 11, Scrolling, covers the Adjustable interface that Scrollbar implements and ScrollPane utilizes.