Java How to Program Fourth Edition - DCC

Chapter 12 Graphical User Interface Components: Part 1 659
Fig. Fig. 12.7 12.7 Demonstrating JTextFields and JPasswordFields (part 4 of 4).
Lines 12–13 declare three references for JTextFields (textField1,
textField2 and textField3) and a JPasswordField (passwordField).
Each of these is instantiated in the constructor (line 16–50). Line 24 defines JTextField
textField1 with 10 columns of text. The width of the text field will be the width in
pixels of the average character in the text field’s current font multiplied by 10. Line 25 adds
textField1 to the content pane.
Line 28 defines textField2 with the initial text "Enter text here" to display
in the text field. The width of the text field is determined by the text. Line 29 adds
textField2 to the content pane.
Line 33 defines textField3 and call the JTextField constructor with two arguments—the
default text "Uneditable text field" to display in the text field and the
number of columns (20). The width of the text field is determined by the number of columns
specified. Line 34 uses method setEditable (inherited into JTextField from
class JTextComponent) to indicate that the user cannot modify the text in the text field.
Line 35 adds textField3 to the content pane.
Line 38 defines JPasswordField passwordField with the text "Hidden
text" to display in the text field. The width of the text field is determined by the text.
Notice that the text is displayed as a string of asterisks when the program executes. Line 39
adds passwordField to the content pane.
For the event-handling in this example, we defined inner class TextFieldHandler
(lines 62–92), which implements interface ActionListener (class TextFieldHandler
is discussed shortly). Thus, every instance of class TextFieldHandler is an
ActionListener. Line 42 defines an instance of class TextFieldHandler and
assigns it to reference handler. This one instance will be used as the event-listener object
for the JTextFields and the JPasswordField in this example.
Lines 43–46 are the event registration statements that specify the event listener object for
each of the three JTextFields and for the JPasswordField. After these statements
execute, the object to which handler refers is listening for events (i.e., it will be notified
when an event occurs) on these four objects. The program calls JTextField method
addActionListener to register the event for each component. Method addAction-
Listener receives as its argument an ActionListener object. Thus, any object of a
class that implements interface ActionListener (i.e., any object that is an Action-
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