ListDemo from Liang - top of Musings page

By: Nicholas Duchon.

• These lectures address some details about the way Java array references work.
  

 Here are a few lectures on the ListDemo class in Liang 7, pp 536-7:

• ListDemoA.swf -
  
• the instance variables, including 3 arrays
• ListDemoB.swf -
  
• main and GUI panels
• ListDemoC.swf -
  
• constructor
• creating the JLabel array,
  
• initializing with null's
• ListDemoD.swf -
  
• constructor as well
• list listener
• anonymous inner class
  

I hope you find these helpful.

Code:

 import java.awt.GridLayout;
 import java.awt.BorderLayout;
 import javax.swing.JFrame;
 import javax.swing.JList;
 import javax.swing.JPanel;
 import javax.swing.JLabel;
 import javax.swing.JScrollPane;
 import javax.swing.ImageIcon;
 import javax.swing.SwingConstants;
 import javax.swing.event.ListSelectionListener;
 import javax.swing.event.ListSelectionEvent;

  public class ListDemoND extends JFrame {
    final static int NUMBER_OF_FLAGS = 9;
    final static JLabel nulljl = new JLabel ();
 
 // Declare an array of Strings for flag titles
    private String[] flagTitles = {"Canada", "China", "Denmark",
       "France", "Germany", "India", "Norway", "United Kingdom",
       "United States of America"
       }; // end immediate initialization of String array
 
 // The list for selecting countries
    private JList jlst = new JList(flagTitles);
 
 // Declare an ImageIcon array for the national flags of 9 countries
    private JLabel[] imageLabels = {
       new JLabel (new ImageIcon("flags/ca.gif"     )),
       new JLabel (new ImageIcon("flags/china.gif"  )),
       new JLabel (new ImageIcon("flags/denmark.gif")),
       new JLabel (new ImageIcon("flags/fr.gif"     )),
       new JLabel (new ImageIcon("flags/germany.gif")),
       new JLabel (new ImageIcon("flags/india.gif"  )),
       new JLabel (new ImageIcon("flags/norway.gif" )),
       new JLabel (new ImageIcon("flags/uk.gif"     )),
       new JLabel (new ImageIcon("flags/us.gif"     ))
       }; // end immediate initialization of ImageIcon array
 
 // Arrays of labels for displaying images
    private JLabel[] jlblImageViewer = new JLabel[NUMBER_OF_FLAGS];
 
    public static void main(String[] args) {   
       ListDemo frame = new ListDemo();
       frame.setSize(650, 500);
       frame.setTitle("ListDemo");
       frame.setLocationRelativeTo(null); // Center the frame
       frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
       frame.setVisible(true);
    } // end main
 
    public ListDemoND () {   
    // Create a panel to hold nine labels
       JPanel p = new JPanel(new GridLayout(3, 3, 5, 5));
   
       for (int i = 0; i < NUMBER_OF_FLAGS; i++) {
          p.add(jlblImageViewer[i] = new JLabel());
          jlblImageViewer[i].setHorizontalAlignment
             (SwingConstants.CENTER);
       } // end instantiate JLabels, empty, centered and added to image panel
   
    // Add p and the list to the frame
       add(p, BorderLayout.CENTER); // image panel
       add(new JScrollPane(jlst), BorderLayout.WEST); // selection panel
   
    // Register listeners
       jlst.addListSelectionListener
          (
             // instatiate anonymous class (ListDemo$1) which implements ListSelectionListener interface
             new ListSelectionListener()
             // Define anonymous inner class (ListDemo$1) immediately
             {
             /** Handle list selection */
                public void valueChanged(ListSelectionEvent e) {
                // Get selected indices
                   int[] indices = jlst.getSelectedIndices();
               
                   int i;
                // Set icons in the labels
                   for (i = 0; i < indices.length; i++) {
                      jlblImageViewer[i] = imageLabels [indices[i]];
                   } // end for each selectected image
               
                // Remove icons from the rest of the labels
                   for (; i < NUMBER_OF_FLAGS; i++) {
                      jlblImageViewer[i] = nulljl;
                   } // end for the rest of the locations
                } // end method valueChanged - interface specified method
               
             } // end anonymous inner class immediate definition
            
          ); // end addListSelectionListener method parameter list
   
    } // end constructor
   
 } // end class List Demo

ND.