Layouts allow you to format components on the screen in a platform-independent way. Without layouts, you would be forced to place components at explicit locations on the screen, creating obvious problems for programs that need to run on multiple platforms. There's no guarantee that a TextArea or a Scrollbar or any other component will be the same size on each platform; in fact, you can bet they won't be. In an effort to make your Java creations portable across multiple platforms, Sun created a LayoutManager interface that defines methods to reformat the screen based on the current layout and component sizes. Layout managers try to give programs a consistent and reasonable appearance, regardless of the platform, the screen size, or actions the user might take.
The standard JDK provides five classes that implement the LayoutManager interface. They are FlowLayout, GridLayout, BorderLayout, CardLayout, and GridBagLayout. All of these layouts are covered in much greater detail in Chapter 7, Layouts. This chapter also discusses how to create complex layouts by combining layout managers and how to write your own LayoutManager. The Java 1.1 JDK includes the LayoutManager2 interface. This interface extends the LayoutManager interface for managers that provide constraint-based layouts.
The FlowLayout is the default layout for the Panel class, which includes its most famous subclass, Applet. When you add components to the screen, they flow left to right (centered within the applet) based upon the order added and the width of the applet. When there are too many components to fit, they "wrap" to a new row, similar to a word processor with word wrap enabled. If you resize an applet, the components' flow will change based upon the new width and height. Figure 1.11 shows an example both before and after resizing. FlowLayout contains all the FlowLayout details.
The GridLayout is widely used for arranging components in rows and columns. As with FlowLayout, the order in which you add components is relevant. You start at row one, column one, move across the row until it's full, then continue on to the next row. However, unlike FlowLayout, the underlying components are resized to fill the row-column area, if possible. GridLayout can reposition or resize objects after adding or removing components. Whenever the area is resized, the components within it are resized. Figure 1.12 shows an example before and after resizing. GridLayout contains all the details about GridLayout.
BorderLayout is one of the more unusual layouts provided. It is the default layout for Window, along with its children, Frame and Dialog. BorderLayout provides five areas to hold components. These areas are named after the four different borders of the screen, North, South, East, and West, with any remaining space going into the Center area. When you add a component to the layout, you must specify which area to place it in. The order in which components are added to the screen is not important, although you can have only one component in each area. Figure 1.13 shows a BorderLayout that has one button in each area, before and after resizing. BorderLayout covers the details of the BorderLayout.
The CardLayout is a bit on the strange side. A CardLayout usually manages several components, displaying one of them at a time and hiding the rest. All the components are given the same size. Usually, the CardLayout manages a group of Panels (or some other container), and each Panel contains several components of its own. With a little work, you can use the CardLayout to create tabbed dialog boxes or property sheets, which are not currently part of AWT. CardLayout lets you assign names to the components it is managing and lets you jump to a component by name. You can also cycle through components in order. Figure 1.11, Figure 1.12, and Figure 1.13 show multiple cards controlled by a single CardLayout. Selecting the Choice button displays a different card. CardLayout discusses the details of CardLayout.
GridBagLayout is the most sophisticated and complex of the layouts provided in the development kit. With the GridBagLayout, you can organize components in multiple rows and columns, stretch specific rows or columns when space is available, and anchor objects in different corners. You provide all the details of each component through instances of the GridBagConstraints class. Figure 1.14 shows an example of a GridBagLayout. GridBagLayout and GridBagConstraints are discussed in GridBagLayout and GridBagConstraints.