Java Reference
In-Depth Information
There's a special kind of listener for
JList
selection events that implements the
ListSelectionListener
interface. Since we set the
FontDialog
object as the listener for the list
in the call to the
addListSelectionListener()
method, we had better make sure the
FontDialog
class implements the interface:
class FontDialog extends JDialog
implements Constants, ActionListener, // For buttons etc.
ListSelectionListener { // For list box
There's only one method in the
ListSelectionListener
interface, and we can implement it like this:
// List selection listener method
public void valueChanged(ListSelectionEvent e) {
if(!e.getValueIsAdjusting()) {
font = new Font((String)fontList.getSelectedValue(), fontStyle, fontSize);
fontDisplay.setFont(font);
fontDisplay.repaint();
}
}
This method is called when you select an item in the list. We have only one list so we don't need to
check which object was the source of the event. If we needed to, we could call the
getSource()
method for the event object that is passed to
valueChanged()
, and compare it with the references to
the
JList
objects.
The
ListSelectionEvent
object that is passed to the
valueChanged()
method contains records
of the index positions of the list items that changed. You can obtain these as a range by calling
getFirstIndex()
for the event object for the first in the range, and
getLastIndex()
for the last.
We don't need to worry about this because we have disallowed multiple selections and we just want the
newly selected item in the list.
We have to be careful though. Since we start out with an item already selected, selecting another font
name from the list will cause two events - one for deselecting the original font name, and the other for
selecting the new name. We make sure we only deal with the last event by calling the
g
etValueIsAdjusting()
method for the event object in the
if
expression. This returns
false
when all changes due to a selection are complete, and
true
if things are still changing. Once we are
sure nothing further is changing, we retrieve the selected font name from the list by calling its
getSelectedValue()
method. The item is returned as type
Object
so we have to cast it to type
String
before using it. We create a new
Font
object using the selected family name and the current
values for
fontStyle
and
fontSize
. We store the new font in the data member
font
, and also call
the
setFont()
member of a data member
fontDisplay
that we haven't added yet. This will be a
JLabel
object displaying a sample of the current font. After we've set the new font, we call
repaint()
for the label,
fontDisplay
, to get it redrawn.
