GUI Components: Part 1 - Java: How to Program Early Objects - page 504

Fig. 12.20 | Testing RadioButtonFrame . (Part 2 of 2.)
Lines 35-42 (Fig. 12.19) in the constructor create four JRadioButton objects and add
them to the JFrame . Each JRadioButton is created with a constructor call like that in line
35. This constructor specifies the label that appears to the right of the JRadioButton by
default and the initial state of the JRadioButton . A true second argument indicates that
the JRadioButton should appear selected when it's displayed.
Line 45 instantiates ButtonGroup object radioGroup . This object is the “glue” that
forms the logical relationship between the four JRadioButton objects and allows only one
of the four to be selected at a time. It's possible that no JRadioButton s in a ButtonGroup
are selected, but this can occur only if no preselected JRadioButton s are added to the But-
tonGroup and the user has not selected a JRadioButton yet. Lines 46-49 use ButtonGroup
method add to associate each of the JRadioButton s with radioGroup . If more than one
selected JRadioButton object is added to the group, the selected one that was added first
will be selected when the GUI is displayed.
JRadioButton s, like JCheckBox es, generate ItemEvent s when they're clicked . Lines
59-66 create four instances of inner class RadioButtonHandler (declared at lines 70-85).
In this example, each event-listener object is registered to handle the ItemEvent generated
when the user clicks a particular JRadioButton . Notice that each RadioButtonHandler
object is initialized with a particular Font object (created in lines 52-55).
Class RadioButtonHandler (line 70-85) implements interface ItemListener so it can
handle ItemEvent s generated by the JRadioButton s. The constructor stores the Font
object it receives as an argument in the event-listener object's instance variable font
(declared at line 72). When the user clicks a JRadioButton , radioGroup turns off the pre-
viously selected JRadioButton , and method itemStateChanged (lines 80-84) sets the font
in the JTextField to the Font stored in the JRadioButton 's corresponding event-listener
object. Notice that line 83 of inner class RadioButtonHandler uses the top-level class's
textField instance variable to set the font.
12.11 JComboBox ; Using an Anonymous Inner Class for
Event Handling
A combo box (sometimes called a drop-down list ) enables the user to select one item from
a list (Fig. 12.22). Combo boxes are implemented with class JComboBox , which extends