Java How to Program Fourth Edition - DCC

678 Graphical User Interface Components: Part 1 Chapter 12
71 // execute application
72 public static void main( String args[] )
73 {
74 MultipleSelection application = new MultipleSelection();
75
76 application.setDefaultCloseOperation(
77 JFrame.EXIT_ON_CLOSE );
78 }
79
80 } // end class MultipleSelection
Fig. Fig. 12.15 12.15 Using a multiple-selection JList (part 3 of 3).
Line 29 creates JList colorList and initializes it with the Strings in the array
colorNames. Line 30 sets the number of visible rows in colorList to 5. Line 31 uses
JList method setFixedCellHeight to specify the height in pixels of each item in
the JList. We do this to ensure that the rows in both JLists in the example have the
same height. Lines 32–33 specify that colorList is a MULTIPLE_INTER-
VAL_SELECTION list. Line 34 adds a new JScrollPane containing colorList to
the content pane. Lines 59–65 perform similar tasks for JList copyList, which is
defined as a SINGLE_INTERVAL_SELECTION list. Line 61 uses JList method set-
FixedCellWidth to set copyList’s width to 100 pixels.
A multiple-selection list does not have a specific event associated with making multiple
selections. 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 JButton copyButton to trigger the event that copies the
selected items in colorList to copyList.
When the user clicks copyButton, method actionPerformed (line 45–50) is
called. Lines 48–49 use JList method setListData to set the items displayed in
copyList. Line 49 calls colorList’s method getSelectedValues, which returns
an array of Objects representing the selected items in colorList. In this example, the
returned array is passed as the argument to copyList’s setListData method.
Many students ask how reference copyList can be used in line 48, when the program
does not create the object to which it refers until Line 59. Remember that method action-
Performed at lines 45–50 does not execute until the user presses the copyButton, which
cannot occur until after the call to the constructor completes. At that point in the program’s
execution, line 59 already has initialized copyList with a new JList object.
12.11 Mouse Event Handling
This section presents the MouseListener and MouseMotionListener event-listener
interfaces for handling mouse events. Mouse events can be trapped for any GUI com-
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