Notice that all component classes begin with the letter J. For example, the class for a label
is JLabel; the class for a push button is JButton; and the class for a scroll bar is JScrollBar.
Containers
Swing defines two types of containers. The first are top-level containers: JFrame, JApplet,
JWindow, and JDialog. These containers do not inherit JComponent. They do, however,
inherit the AWT classes Component and Container. Unlike Swing's other components,
which are lightweight, the top-level containers are heavyweight. This makes the top-level
containers a special case in the Swing component library.
As the name implies, a top-level container must be at the top of a containment hierarchy.
A top-level container is not contained within any other container. Furthermore, every
containment hierarchy must begin with a top-level container. The one most commonly used
for applications is JFrame. The one used for applets is JApplet.
The second type of containers supported by Swing are lightweight containers. Lightweight
containers do inherit JComponent. An example of a lightweight container is JPanel, which
is a general-purpose container. Lightweight containers are often used to organize and
manage groups of related components because a lightweight container can be contained
within another container. Thus, you can use lightweight containers such as JPanel to create
subgroups of related controls that are contained within an outer container.
The Top-Level Container Panes
Each top-level container defines a set of panes. At the top of the hierarchy is an instance of
JRootPane. JRootPane is a lightweight container whose purpose is to manage the other
panes. It also helps manage the optional menu bar. The panes that comprise the root pane
are called the glass pane, the content pane, and the layered pane.
The glass pane is the top-level pane. It sits above and completely covers all other panes.
By default, it is a transparent instance of JPanel. The glass pane enables you to manage
mouse events that affect the entire container (rather than an individual control) or to paint
over any other component, for example. In most cases, you won't need to use the glass pane
directly, but it is there if you need it.
The layered pane is an instance of JLayeredPane. The layered pane allows components
to be given a depth value. This value determines which component overlays another. (Thus,
the layered pane lets you specify a Z-order for a component, although this is not something
that you will usually need to do.) The layered pane holds the content pane and the (optional)
menu bar.
Although the glass pane and the layered panes are integral to the operation of a top-level
container and serve important purposes, much of what they provide occurs behind the
scene. The pane with which your application will interact the most is the content pane,
because this is the pane to which you will add visual components. In other words, when
you add a component, such as a button, to a top-level container, you will add it to the
content pane. By default, the content pane is an opaque instance of JPanel.
The Swing Packages
Swing is a very large subsystem and makes use of many packages. These are the packages
used by Swing that are defined by Java SE 6.
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