Polymorphism - Java Software Solutions: Foundations of Program Design - page 522

The answer is polymorphism. If you examine the source
code for the addActionListener method, you'll discover that
it accepts a parameter of type ActionListener , the interface.
Therefore, instead of accepting a parameter of only one object
type, the addActionListener method can accept any object of
any class that implements the ActionListener interface. All
other add listener methods work in similar ways.
The JButton object doesn't know anything particular about the object that
is passed to the addActionListener method, except for the fact that it imple-
ments the ActionListener interface (otherwise the code wouldn't compile). The
JButton object simply stores the listener object and invokes its actionPerformed
method when the event occurs.
In Chapter 9 we discussed that we can also create a listener by extending an
adapter class. Well, it turns out that using an adapter class isn't really a new way
to create a listener after all. Each adapter class is written to implement the appro-
priate listener interface, providing empty methods for all event handlers. So by
extending an adapter class, the new listener class automatically implements the
corresponding listener interface. And that is what really makes it a listener such
that it can be passed to an appropriate add listener method.
Thus, no matter how a listener object is created, we are using polymorphism
via interfaces to set up the relationship between a listener and the component it
listens to. GUI events are a wonderful example of the power and versatility pro-
vided by polymorphism.

KEY CONCEPT
Establishing the relationship between
a listener and the component it
listens to is accomplished using
polymorphism.

10.8 File Choosers

Dialog boxes were introduced in Chapter 6. We used the
JOptionPane class to create several dialog boxes to present
information, accept input, and confirm actions.
The JFileChooser class represents another specialized
dialog box, a file chooser , which allows the user to select
a file from a hard disk or other storage medium. You have
probably run many programs that allow you to open a file using a similar
dialog box.
The program shown in Listing 10.13 uses a JFileChooser dialog box to select
a file. This program also demonstrates the use of another GUI component, a text
area , which is similar to a text field but can display multiple lines of text at one
time. After the user selects a file using the file chooser dialog box, the text con-
tained in that file is displayed in a text area.

KEY CONCEPT
A file chooser allows the user to
browse a disk and select a file to be
processed.