GUI Components: Part 1 - Java: How to Program Early Objects - page 513

Fig. 12.26 | Testing MultipleSelectionFrame . (Part 2 of 2.)
Line 27 of Fig. 12.25 creates JList colorJList and initializes it with the String s in
the array colorNames . Line 28 sets the number of visible rows in colorJList to 5 . Lines
29-30 specify that colorJList is a MULTIPLE_INTERVAL_SELECTION list. Line 31 adds a
new JScrollPane containing colorJList to the JFrame . Lines 51-57 perform similar
tasks for copyJList , which is declared as a SINGLE_INTERVAL_SELECTION list. If a JList
does not contain items, it will not diplay in a FlowLayout . For this reason, lines 53-54 use
JList methods setFixedCellWidth and setFixedCellHeight to set copyJList 's width
to 100 pixels and the height of each item in the JList to 15 pixels, respectively.
Normally, an event generated by another GUI component (known as an external
event ) specifies when the multiple selections in a JList should be processed. In this
example, the user clicks the JButton called copyJButton to trigger the event that copies
the selected items in colorJList to copyJList .
Lines 34-47 declare, create and register an ActionListener for the copyJButton .
When the user clicks copyJButton , method actionPerformed (lines 38-45) uses JList
method setListData to set the items displayed in copyJList . Lines 43-44 call color-
JList 's method getSelectedValuesList , which returns a List<String> (because the
JList was created as a JList<String> ) representing the selected items in colorJList . We
call the List<String> 's toArray method to convert this into an array of String s that can
be passed as the argument to copyJList 's setListData method. List method toArray
receives as its argument an array representing the type of array that the method will return.
You'll learn more about List and toArray in Chapter 16.
You might be wondering why copyJList can be used in line 42 even though the
application does not create the object to which it refers until line 49. Remember that
method actionPerformed (lines 38-45) does not execute until the user presses the copy-
JButton , which cannot occur until after the constructor completes execution and the
application displays the GUI. At that point in the application's execution, copyJList is
already initialized with a new JList object.
12.14 Mouse Event Handling
This section presents the MouseListener and MouseMotionListener event-listener inter-
faces for handling mouse events . Mouse events can be processed for any GUI component
that derives from java.awt.Component . The methods of interfaces MouseListener and
MouseMotionListener are summarized in Figure 12.27. Package javax.swing.event
contains interface MouseInputListener , which extends interfaces MouseListener and
MouseMotionListener to create a single interface containing all the MouseListener and
MouseMotionListener methods. The MouseListener and MouseMotionListener meth-