Java How to Program Fourth Edition - DCC

Chapter 9 Object-Oriented Programming 511
Fig. Fig. 9.34 9.34 Demonstrating anonymous inner classes (part 4 of 4).
Each of the three JTextFields that generate events in this program has a similar anonymous
inner class to handle its events, so we discuss only the anonymous inner class for
hourField here. Lines 33–48 are a call to hourField’s addActionListener
method. The argument to this method must be an object that is an ActionListener (i.e.,
any object of a class that implements ActionListener). Lines 36–46 use special Java
syntax to define an anonymous inner class and create one object of that class that is passed as
the argument to addActionListener. Line 36 uses operator new to create an object. The
syntax ActionListener() begins the definition of an anonymous inner class that implements
interface ActionListener. This is similar to beginning a class definition with
public class MyHandler implements ActionListener {
The parentheses after ActionListener indicate a call to the default constructor of the
anonymous inner class.
The opening left brace ({) at the end of line 36 and the closing right brace (}) at line
46 define the body of the class. Lines 38–44 define the actionPerformed method that
is required in any class that implements ActionListener. Method actionPerformed
is called when the user presses Enter while typing in hourField.
Software Engineering Observation 9.34
When an anonymous inner class implements an interface, the class must define every method
in the interface. 9.34
Method main creates one instance of class TimeTestWindow (line 115), sizes the
window (line 133) and displays the window (line 134).
Windows generate a variety of events that are discussed in Chapter 13. For this
example we discuss the one event generated when the user clicks the window’s close box—
a window closing event. Lines 118–131 enable the user to terminate the application by
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