Inheritance - Java Software Solutions: Foundations of Program Design - page 470

ability to put a border on any Swing component (as we saw in Chapter 6). This
ability is defined once in the JComponent class and is inherited by any class that is
derived, directly or indirectly, from it.
Some component classes, such as JPanel and JLabel , are derived directly from
JComponent . Other component classes are nested further down in the inheritance
hierarchy structure. For example, the AbstractButton class is an abstract class
that defines the functionality that applies to all types of GUI buttons. JButton is
derived directly from it. However, note that JCheckBox and JRadioButton are
both derived from a class called JToggleButton , which embodies the common
characteristics for buttons that can be in one of two states. The set of classes that
define GUI buttons shows once again how common characteristics are put at
appropriately high levels of the class hierarchy rather than duplicated in multiple
classes.
The world of text components demonstrates this as well. The JTextField class
that we've used in previous examples is one of many Java GUI components that
support the management of text data. They are organized under a class called
JTextComponent . Keep in mind that there are many GUI component classes that
are not shown in the diagram in Figure 9.7.
In previous chapters we've extended the component class hierarchy further by
defining panel and applet classes of our own using inheritance. By extending the
JPanel or JApplet class, we create our own classes that automatically have all the
characteristics of those components. Sometimes we then overrode the definition
of a method, such as the paintComponent method, to behave in a particular way.
Creating our own panel and applet classes is a classic use of inheritance,
allowing the parent class to shoulder the responsibilities that apply to all of its
descendants. For example, the JApplet class is already designed to handle all of
the details concerning applet creation and execution. An applet interacts with a
browser, can accept parameters through HTML code, and is constrained by cer-
tain security limitations. The JApplet class already takes care of these details in
a generic way that applies to all applets. The applet class that we write (the one
derived from JApplet ) is ready to focus on the purpose of that particular pro-
gram. In other words, the only issues that we address in our applet code are those
that make it different from other applets.
SELF-REVIEW QUESTIONS (see answers in Appendix N)
SR 9.23 True or False, based on the GUI component class hierarchy diagram
displayed in this section.
a. A JCheckBox object is a JToggleButton .
b. A JCheckBox object is an AbstractButton .
c. A JTextField object is a JLabel .