Swing I - Absolute Java - page 977

class Container, so any descendent class of the class JFrame can serve as a container
to hold labels, buttons, panels, or other components.
Similarly, the class JPanel is a descendent of the class Container, and any object
of the class JPanel can serve as a container to hold labels, buttons, other panels, or
other components. Display 17.12 shows a portion of the hierarchy of Swing and AWT
classes. Note that the Container class is in the AWT library and not in the Swing
library. This is not a major issue, but it does mean that the import statement for the
Container class is

import java.awt.Container;
A container class is any descendent class of the class Container. The class
JComponent serves a similar roll for components. Any descendent class of the class
JComponent is called a JComponent or sometimes simply a component . You can add
any JComponent object to any container class object.
The class JComponent is derived from the class Container , so you can add a
JComponent to another JComponent . Often, this will turn out to be a viable option;
occasionally it is something to avoid. 6
The classes Component , Frame , and Window shown in Display 17.12 are AWT
classes that some readers may have heard of. We include them for reference value, but
we will have no need for these classes. We will eventually discuss all the other classes
shown in Display 17.12 .
When you are dealing with a Swing container class, you have three kinds of objects
to deal with:

container
class

component

1. The container itself, probably some sort of panel or window-like object
2. The components you add to the container, such as labels, buttons, and panels
3. A layout manager, which positions the components inside the container

You have seen examples of these three kinds of objects in almost every JFrame class we
have defined. Almost every complete GUI you build, and many subparts of the GUIs
you build, will be made up of these three kinds of objects.

Self-Test Exercises

19. What standard Java package contains the layout manager classes discussed in
this chapter?
20. Is an object of the class JPanel a container class? Is it a component class?
21. With a GridLayout manager, you cannot leave any grid element empty, but
you can do something that will make a grid element look empty to the user.
What can you do?

(continued)

6 In particular, it is legitimate and sometimes useful to add JComponents to a JButton . We do not
have space in this topic to develop techniques for doing this effectively, but you may want to give it a
try. You have covered enough material to do it.