You cannot just redefine what the "cellHasFocus" argument means.

Here is my attempt at a hover implementation:

import javax.swing.*;
import java.awt.event.*;
import javax.swing.event.*;
import java.awt.*;
import java.beans.PropertyChangeListener;
import java.beans.PropertyChangeEvent;

public class Hover
{
   private static class Listener
    extends MouseInputAdapter
    implements ListDataListener, PropertyChangeListener, ComponentListener, HierarchyListener, HierarchyBoundsListener, Runnable
   {
    private JList list;

    private int hoverIndex;

    private boolean enabled, running;

    private Point lastLocation;

    public Listener(JList l)
    {
       list = l;

       hoverIndex = -1;
    }

    public void setEnabled(boolean value)
    {
       if (enabled == value)
        return;

       enabled = value;

       if (enabled)
           {
        list.addMouseListener(this);
        list.addMouseMotionListener(this);
        list.addPropertyChangeListener(this);
        list.getModel().addListDataListener(this);

        list.addComponentListener(this);
        list.addHierarchyListener(this);
        list.addHierarchyBoundsListener(this);

        list.putClientProperty(HOVER, this);
       }
       else
           {
        repaint(hoverIndex());

        list.removeMouseListener(this);
        list.removeMouseMotionListener(this);
        list.removePropertyChangeListener(this);
        list.getModel().removeListDataListener(this);

        list.removeHierarchyBoundsListener(this);
        list.removeHierarchyListener(this);
        list.removeComponentListener(this);

        list.putClientProperty(HOVER, null);
       }
    }

    public int hoverIndex()
    {
       return hoverIndex;
    }

    private void setHoverIndex(int value)
    {
       hoverIndex = value;
    }

    private void repaint(int index)
    {
       if (index != -1)
        list.repaint(list.getCellBounds(index, index));
    }

    private Point toScreen(Point p)
    {
       p = new Point(p);

       SwingUtilities.convertPointToScreen(p, list);

       return p;
    }

    private Point toList(Point p)
    {
       if (!list.isShowing())
        return null;

       Point s = list.getLocationOnScreen();
       
       s.x = p.x - s.x;
       s.y = p.y - s.y;

       return s;
    }

    // p is in screen coordinate system (or zero, denoting not known or outside)
    private void updateHover(Point p)
    {
       int h = hoverIndex();

       Point q = p == null ? null : toList(p);

       int newHoverIndex = p == null ? -1 : list.locationToIndex(q);

       if (h != newHoverIndex)
       {
        repaint(h);
        repaint(newHoverIndex);

        setHoverIndex(newHoverIndex);
       }

       lastLocation = p;
    }

    // updateLater is used to make sure that the cell bounds are already updated.
    // this may have problems if called initially not from the event-dispatch thread
    private void updateLater()
    {
       if (!running)
       {
        running = true;
        EventQueue.invokeLater(this);
       }
    }

    public void run()
    {
       running = false;

       updateHover(lastLocation);
    }

    public void propertyChange(PropertyChangeEvent e)
    {
       String name = e.getPropertyName();

       if (name.equals("model"))
       {
        ((ListModel)e.getOldValue()).removeListDataListener(this);
        ((ListModel)e.getNewValue()).addListDataListener(this);

        setHoverIndex(-1);
        updateLater();
       }
       else if (name.equals("font") || name.equals("cellRenderer") || name.equals("fixedRowWidth") || name.equals("fixedRowHeight") || name.equals("prototypeCellValue") || name.equals("layoutOrientation"))
       {
        // very unspecific
        list.repaint();

        updateLater();
       }
    }

    public void hierarchyChanged(HierarchyEvent e)
    {
       if ((e.getChangeFlags() & HierarchyEvent.SHOWING_CHANGED) != 0)
       {
        if (!list.isShowing())
        {
           lastLocation = null;
           setHoverIndex(-1);
        }
        // else: nothing to be done as mouse location is not known (only 1.5)
       }
    }

    public void componentShown(ComponentEvent e)
    {
    }

    public void componentHidden(ComponentEvent e)
    {
       // handled by hierarchyChanged
    }

    public void componentMoved(ComponentEvent e)
    {
       updateHover(lastLocation);
    }

    public void ancestorMoved(HierarchyEvent e)
    {
       updateHover(lastLocation);
    }

    public void componentResized(ComponentEvent e)
    {
       updateLater();
    }

    public void ancestorResized(HierarchyEvent e)
    {
       updateLater();
    }

    public void mouseMoved(MouseEvent e)
    {
       updateHover(toScreen(e.getPoint()));
    }

    public void mouseEntered(MouseEvent e)
    {
       updateHover(toScreen(e.getPoint()));
    }

    public void mouseExited(MouseEvent e)
    {
       updateHover(null);
    }

    // These implementations are quite unspecific.

    public void intervalAdded(ListDataEvent e)
    {
       if (hoverIndex() >= e.getIndex0())
        setHoverIndex(-1);

       updateLater();
    }
   
    public void intervalRemoved(ListDataEvent e)
    {
       if (hoverIndex() >= e.getIndex0())
        setHoverIndex(-1);

       updateLater();
    }

    public void contentsChanged(ListDataEvent e)
    {
       updateLater();
    }
   }

   private static Object HOVER = "xxx.Hover";

   public static int index(JList l)
   {
    Listener h = (Listener)l.getClientProperty(HOVER);

    return h == null
       ? -1 : h.hoverIndex();
   }

   public static void install(JList l)
   {
    if (l.getClientProperty(HOVER) == null)
    {
       Listener h = new Listener(l);

       h.setEnabled(true);
    }
   }

   public static void uninstall(JList l)
   {
    Listener h = (Listener)l.getClientProperty(HOVER);

    if (l != null)
       h.setEnabled(false);
   }

   public static void main(String[] args)
   {
    final DefaultListModel data = new DefaultListModel();

    Object[] o = new java.io.File(System.getProperty("user.home")).list();

    for (int i = 0; i < o.length; i++)
       data.addElement(o[i]);

    final JList l = new JList(data);

    Hover.install(l);

    l.setCellRenderer(new DefaultListCellRenderer()
    {
       private Color hoverBackground = new Color(150, 255, 200);

       public Component getListCellRendererComponent
        (JList l, Object value, int index, boolean selected, boolean focused)
       {
        super.getListCellRendererComponent(l, value, index, selected, focused);

        if (Hover.index(l) == index)
           setBackground(hoverBackground);

        return this;
       }
    });

    l.setBorder(BorderFactory.createEmptyBorder(5, 5, 5, 5));

    final JFrame f = new JFrame();

    f.getContentPane().add(new JScrollPane(l));

    f.pack(); f.show();

    Timer t = new Timer(5000, new ActionListener()
    {
       public void actionPerformed(ActionEvent e)
       {
        //data.insertElementAt(e.toString().substring(0, 10), 0);
        f.setLocation(f.getX(), f.getY() + 30);
       }
    });

    t.start();
   }
}
   

Christian