GUI Components: Part 1 - Java: How to Program Early Objects - page 501

tialized with "Serif" (a generic font name that represents a font such as Times and is sup-
ported on all Java platforms), Font.PLAIN style and 14 -point size. Next, lines 28-29 create
two JCheckBox objects. The String passed to the JCheckBox constructor is the checkbox
label that appears to the right of the JCheckBox by default.
When the user clicks a JCheckBox , an ItemEvent occurs. This event can be handled
by an ItemListener object, which must implement method itemStateChanged . In this
example, the event handling is performed by an instance of private inner class CheckBox-
Handler (lines 40-60). Lines 34-36 create an instance of class CheckBoxHandler and reg-
ister it with method addItemListener as the listener for both the JCheckBox objects.
CheckBoxHandler method itemStateChanged (lines 43-59) is called when the user
clicks the either boldJCheckBox or italicJCheckBox . In this example, we do not deter-
mine which JCheckBox was clicked—we use both of their states to determine the font to
display. Line 49 uses JCheckBox method isSelected to determine if both JCheckBox es are
selected. If so, line 50 creates a bold italic font by adding the Font constants Font.BOLD
and Font.ITALIC for the font-style argument of the Font constructor. Line 51 determines
whether the boldJCheckBox is selected, and if so line 52 creates a bold font. Line 53 deter-
mines whether the italicJCheckBox is selected, and if so line 54 creates an italic font. If
none of the preceding conditions are true, line 56 creates a plain font using the Font con-
stant Font.PLAIN . Finally, line 58 sets textField 's new font, which changes the font in
the JTextField on the screen.
Relationship Between an Inner Class and Its Top-Level Class
Class CheckBoxHandler used variables boldJCheckBox (lines 49 and 51), italicJCheck-
Box (lines 49 and 53) and textField (line 58) even though they are not declared in the
inner class. Recall that an inner class has a special relationship with its top-level class —it's
allowed to access all the variables and methods of the top-level class. CheckBoxHandler
method itemStateChanged (line 43-59) uses this relationship to determine which
JCheckBox es are checked and to set the font on the JTextField . Notice that none of the
code in inner class CheckBoxHandler requires an explicit reference to the top-level class
object.
12.10.2 JRadioButton
Radio buttons (declared with class JRadioButton ) are similar to checkboxes in that they
have two states— selected and not selected (also called deselected ). However, radio buttons
normally appear as a group in which only one button can be selected at a time (see the out-
put of Fig. 12.20). Radio buttons are used to represent mutually exclusive options (i.e.,
multiple options in the group cannot be selected at the same time). The logical relationship
between radio buttons is maintained by a ButtonGroup object (package javax.swing ),
which itself is not a GUI component. A ButtonGroup object organizes a group of buttons
and is not itself displayed in a user interface. Rather, the individual JRadioButton objects
from the group are displayed in the GUI.
The application of Figs. 12.19-12.20 is similar to that of Figs. 12.17-12.18. The user
can alter the font style of a JTextField 's text. The application uses radio buttons that
permit only a single font style in the group to be selected at a time. Class RadioButtonTest
(Fig. 12.20) contains the main method that executes this application.