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From: Knute Johnson <nospam@knutejohnson.com>
Newsgroups: comp.lang.java.programmer
Subject: Generic JList and ListCellRenderer?
Date: Fri, 28 Dec 2012 11:01:58 -0800
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I've been trying to clean up some really old code and I've hit some 
snags.  I've got several modified JLists and the ListCellRenderers for 
them and thought it would make sense to have generic classes that could 
be extended for different data types that need to be displayed.  The 
example below displays InetAddresses in the JList.  I've got another 
implementation of JList that does a lot more than what this one does but 
I wanted to keep this example simple and focus on the ListCellRenderer.

MyListCellRenderer extends the getListCellRenderer method of 
DefaultListCellRenderer and adds a new method, textToDisplay().  I added 
a field to the constructor that specifies the class of the data element 
to be displayed.  That class information is the test to make the call to 
the textToDisplay() method.  The InetAddressListCellRenderer class 
extends MyListCellRenderer to get this to display nice neat Strings for 
the InetAddresses.

I think this whole thing is a little kludgie.  I've been playing with it 
for so long now I'm not getting anywhere anymore.  I was hoping somebody 
would say "gee you ought to go this way" or "no that is really brilliant 
code my man!" and I'll leave it as is :-).

Anyway, please take a look and if you have any great ideas, I would 
appreciate the feedback.

Thanks,

import java.awt.*;
import java.io.*;
import java.net.*;
import java.util.*;
import javax.swing.*;
import javax.swing.border.*;

public class test6 extends JPanel {
     public test6() throws UnknownHostException {
         Vector<InetAddress> v = new Vector<>();
         v.addElement(InetAddress.getByName("216.240.58.139"));
         v.addElement(InetAddress.getByName("4.2.2.1"));
         v.addElement(InetAddress.getByName("knutejohnson.com"));
         v.addElement(InetAddress.getByName("192.168.3.6"));

         MyJList<InetAddress> list =
          new MyJList<>(new DefaultListModel<InetAddress>());
         list.setCellRenderer(new InetAddressListCellRenderer());
         list.setListData(v);
         add(list);
     }

     public class MyJList<E> extends JList<E> {
         private final DefaultListModel<E> model;

         public MyJList(DefaultListModel<E> lm) {
             super(lm);
             this.model = lm;
         }

         public void setListData(E[] listData) {
             model.clear();
             for (E e : listData)
                 model.addElement(e);
         }

         public void setListData(Vector<? extends E> v) {
             model.clear();
             Iterator<? extends E> iter = v.iterator();
             while (iter.hasNext())
                 model.addElement(iter.next());
         }
     }

     public class MyListCellRenderer extends DefaultListCellRenderer {
         private final Class clazz;

         public MyListCellRenderer(Class clazz) {
             super();
             setOpaque(true);
             setBorder(new EmptyBorder(1,1,1,1));
             setName("List.cellRenderer");

             this.clazz = clazz;
         }

         public Component getListCellRendererComponent(JList<?> list,
          Object value, int index, boolean isSelected, boolean 
cellHasFocus) {
             setComponentOrientation(list.getComponentOrientation());

             UIDefaults defaults = UIManager.getDefaults();

             Color bg = null;
             Color fg = null;

             JList.DropLocation dropLocation = list.getDropLocation();
             if (dropLocation != null &&
               !dropLocation.isInsert() &&
               dropLocation.getIndex() == index) {
                 bg = (Color)defaults.get("List.dropCellBackground");
                 fg = (Color)defaults.get("List.dropCellForeground");

                 isSelected = true;
             }

             if (isSelected) {
                 setBackground(bg == null ? 
list.getSelectionBackground() : bg);
                 setForeground(fg == null ? 
list.getSelectionForeground() : fg);
             } else {
                 setBackground(list.getBackground());
                 setForeground(list.getForeground());
             }

             if (value instanceof Icon) {
                 setIcon((Icon)value);
                 setText("");
             } else if (clazz.isInstance(value)) {
                 setIcon(null);
//                setText(((InetAddress)value).getHostAddress());
                 setText(textToDisplay(value));
             /*
             } else {
                 setIcon(null);
                 setText((value == null) ? "" : value.toString());
             */
             }

             setEnabled(list.isEnabled());
             setFont(list.getFont());

             Border border = null;
             if (cellHasFocus) {
                 if (isSelected)
                     border = (Border)defaults.get(
                      "List.focusSelectedCellHighlightBorder");
                 if (border == null)
                     border = (Border)defaults.get(
                      "List.focusCellHighlightBorder");
             } else
                 border = new EmptyBorder(1,1,1,1);
             setBorder(border);

             return this;
         }

         public String textToDisplay(Object value) {
             return value == null ? "" : value.toString();
         }
     }

     public class InetAddressListCellRenderer extends MyListCellRenderer {
         public InetAddressListCellRenderer() {
             super(InetAddress.class);
         }

         public String textToDisplay(Object value) {
             return ((InetAddress)value).getHostAddress();
         }
     }

     public static void main(String[] args) {
         EventQueue.invokeLater(new Runnable() {
             public void run() {
                 try {
                     JFrame f = new JFrame();
                     f.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
                     test6 t6 = new test6();
                     f.add(t6);
                     f.pack();
                     f.setVisible(true);
                 } catch (Exception e) {
                     e.printStackTrace();
                 }
             }
         });
     }
}


-- 

Knute Johnson