GUI Components: Part 1 - Java: How to Program Early Objects - page 489

members, inner classes can be declared public , protected or private . Since event han-
dlers tend to be specific to the application in which they're defined, they're often imple-
mented as private inner classes or as anonymous inner classes (Section 12.11).
GUI components can generate many events in response to user interactions. Each
event is represented by a class and can be processed only by the appropriate type of event
handler. Normally, a component's supported events are described in the Java API docu-
mentation for that component's class and its superclasses. When the user presses Enter in
a JTextField or JPasswordField , an ActionEvent (package java.awt.event ) occurs.
Such an event is processed by an object that implements the interface ActionListener
(package java.awt.event ). The information discussed here is available in the Java API
documentation for classes JTextField and ActionEvent . Since JPasswordField is a sub-
class of JTextField , JPasswordField supports the same events.
To prepare to handle the events in this example, inner class TextFieldHandler
implements interface ActionListener and declares the only method in that interface—
actionPerformed (lines 53-80). This method specifies the tasks to perform when an
ActionEvent occurs. So, inner class TextFieldHandler satisfies Step 1 listed earlier in this
section. We'll discuss the details of method actionPerformed shortly.
Registering the Event Handler for Each Text Field
In the TextFieldFrame constructor, line 42 creates a TextFieldHandler object and as-
signs it to variable handler . This object's actionPerformed method will be called auto-
matically when the user presses Enter in any of the GUI's text fields. However, before this
can occur, the program must register this object as the event handler for each text field.
Lines 43-46 are the event-registration statements that specify handler as the event handler
for the three JTextField s and the JPasswordField . The application calls JTextField
method addActionListener to register the event handler for each component. This meth-
od receives as its argument an ActionListener object, which can be an object of any class
that implements ActionListener . The object handler is an ActionListener , because
class TextFieldHandler implements ActionListener . After lines 43-46 execute, the ob-
ject handler listens for events . Now, when the user presses Enter in any of these four text
fields, method actionPerformed (line 53-80) in class TextFieldHandler is called to han-
dle the event. If an event handler is not registered for a particular text field, the event that
occurs when the user presses Enter in that text field is consumed —i.e., it's simply ignored
by the application.
Software Engineering Observation 12.1
The event listener for an event must implement the appropriate event-listener interface.
Common Programming Error 12.2
If you forget to register an event-handler object for a particular GUI component's event
type, events of that type will be ignored.
Details of Class TextFieldHandler 's actionPerformed Method
In this example, we use one event-handling object's actionPerformed method (lines 53-80)
to handle the events generated by four text fields. Since we'd like to output the name of each
text field's instance variable for demonstration purposes, we must determine which text field