Java Reference
In-Depth Information
Much of the program will be familiar, but a few parts warrant special attention. First,
notice that the action command associated with the text field is set to "myTF" by the fol-
lowing line:
After this line executes, the action command string will always be "myTF" no matter what
text is currently held in the text field. Therefore, the action command generated by
jtf
will
not accidentally conflict with the action command associated with the Reverse push but-
ton. The
actionPerformed( )
method makes use of this fact to determine what event has
occurred. If the action command string is "Reverse", it can mean only one thing: that the
Reverse push button has been pressed. Otherwise, the action command was generated by
the user pressing
ENTER
while the text field had input focus.
Finally, notice this line from within the
actionPerformed( )
method:
As explained, when the user presses
ENTER
while focus is inside the text field, an
Ac-
tionEvent
is generated and sent to all registered action listeners, through the
actionPer-
formed( )
method. For
TFDemo
, this method simply obtains the text currently held in the
text field by calling
getText( )
on
jtf
. It then displays the text through the label referred to
by
jlabContents
.
Create a JCheckBox
After the push button, perhaps the next most widely used control is the check box. In
Swing, a check box is an object of type
JCheckBox
.
JCheckBox
inherits
AbstractButton
and
JToggleButton
. Thus, a check box is, essentially, a special type of button.
JCheckBox
defines several constructors. The one used here is
JCheckBox(String
str
)
It creates a check box that has the text specified by
str
as a label.
When a check box is selected or deselected (that is, checked or unchecked), an item
event is generated. Item events are represented by the
ItemEvent
class. Item events are
handled by classes that implement the
ItemListener
interface. This interface specifies only
one method:
itemStateChanged( )
, which is shown here:
void itemStateChanged(ItemEvent
ie
)
