TransferHandler problem

Question

I've got Java 1.4 dragging between tables and lists in my application. Every drag and drop is a move, so when I drop an item onto one list, it is removed from the source.
Everything works great until I try to drag and drop on the same list. Seems like Swing first does the importData, then the exportDone, but because everything is serialized through the JVM, there doesn't seem to be a way to tell the transfer handler that the source and the target are the same object. How exactly do I teach my TransferHandler class to not importData if the source and target are the same.
Right now I'm cheating and saving the source of the drag as a static variable in the application class, but that doesn't seem like it should be the right way to do things. Is the new DnD code just poorly implemented, or am I missing something?
Oh, and I've tried saving the source component in the transfer data, but I get some weird null pointer exception somewhere in the UI classes when I do that.
Here's my TransferHandler class:
import javax.swing.*;
import javax.swing.tree.TreePath;
import java.awt.datatransfer.Transferable;
import java.awt.datatransfer.DataFlavor;
import java.awt.datatransfer.UnsupportedFlavorException;
import java.util.List;
import java.util.Vector;
import java.io.IOException;
* This handles the movement of CommentTextValues around the system.  Combined
* with properly build CommonTextModel and TermModel table and tree models,
* this method makes the dragging and dropping of common text and terms in an
* application completely generic.
public class TermSelection extends TransferHandler implements Transferable {
    // This looks a little weird, but it's because the TransferHandler is also the Transferable
    private static final DataFlavor[] flavors = {
            new DataFlavor(TermSelection.class, DataFlavor.javaJVMLocalObjectMimeType)
    // The data to be moved
    private List moving;
     * All this handler does is move operations; copy may be supported later if
     * the need arises.
    public int getSourceActions(JComponent c) {
        return MOVE;
     * Indicate what we can import (TermSelection objects).
    public boolean canImport(JComponent c, DataFlavor[] flavor) {
        if (c instanceof JTree || c instanceof JTable || c instanceof JList) {
            for (int idx = 0; idx < flavor.length; idx++) {
                if (flavor[idx].equals(flavors[0])) {
                    return true;
        return false;
     * Create the transferable object to pass to the drop listener.
     * @param component     the component that we're dragging from
     * @return  a loaded (set <b>moving</b>) Transferable object
    public Transferable createTransferable(JComponent component) {
        if (component instanceof JTable) {
            JTable table = (JTable) component;
            if (table.getModel() instanceof CommonTextModel) {
                CommonTextModel model = (CommonTextModel) table.getModel();
                // Get the selected items and save them for transfer
                int[] rows = table.getSelectedRows();
                List terms = model.getCommonText();
                moving = new Vector(rows.length);
                for (int idx = 0; idx < rows.length; idx++) {
                    moving.add(terms.get(rows[idx]));
                return this;
            } else if (table.getModel() instanceof TermModel) {
                TermModel model = (TermModel) table.getModel();
                // Get the selected items and save them for transfer
                int[] rows = table.getSelectedRows();
                List terms = model.getTerms();
                moving = new Vector(rows.length);
                for (int idx = 0; idx < rows.length; idx++) {
                    moving.add(terms.get(rows[idx]));
                return this;
        } else if (component instanceof JTree) {
            JTree tree = (JTree) component;
            if (tree.getModel() instanceof TermModel) {
//                TreePath path = tree.get
        } else if (component instanceof JList) {
            JList list = (JList) component;
            if (list.getModel() instanceof TermModel) {
                TermModel model = (TermModel) list.getModel();
                // Get the selected items and save them for transfer
                int[] rows = list.getSelectedIndices();
                List terms = model.getTerms();
                moving = new Vector(rows.length);
                for (int idx = 0; idx < rows.length; idx++) {
                    moving.add(terms.get(rows[idx]));
                return this;
        return null;
     * Import the data into components onto which they were dropped.  Supports
     * JTree and JTable components that have models implementing
     * CommonTextModel and TermModel.
     * @param c     the componet we're dropping on
     * @param t     the data object (with the moved values)
     * @return  true, if the drop was successful
    public boolean importData(JComponent c, Transferable t) {
        List moved = getWrapped(t);
        if (moved != null && t.isDataFlavorSupported(flavors[0])) {
            // Drop it on a tree?
            if (c instanceof JTree) {
                // Drop it on a terms tree?
                if (((JTree) c).getModel() instanceof TermModel) {
                    TermModel model = (TermModel) ((JTree) c).getModel();
                    model.addTerms(moved);
                    return true;
                // Drop it on a common text tree?
                // NOT YET IMPLEMENTED !!!
            // Drop it on the stop table or unreferenced terms table?
            } else if (c instanceof JTable) {
                JTable table = (JTable) c;
                if (table.getModel() instanceof CommonTextModel) {
                    CommonTextModel model = (CommonTextModel) table.getModel();
                    model.addCommonText(moved);
                    return true;
            } else if (c instanceof JList) {
                JList list = (JList) c;
                if (list.getModel() instanceof TermModel) {
                    TermModel model = (TermModel) list.getModel();
                    model.addTerms(moved);
                    return true;
        return false;
     * All drags in Rosetta are "moves" as opposed to "copies."  Remove the
     * objects from their source.
    protected void exportDone(JComponent source, Transferable data, int action) {
        List moved = getWrapped(data);
        if (moved != null) {
            if (source instanceof JTable) {
                JTable table = (JTable) source;
                if (table.getModel() instanceof CommonTextModel) {
                    CommonTextModel model = (CommonTextModel) table.getModel();
                    model.removeText(moved);
            } else if (source instanceof JList) {
                JList list = (JList) source;
                if (list.getModel() instanceof TermModel) {
                    TermModel model = (TermModel) list.getModel();
                    model.removeTerms(moved);
        super.exportDone(source, data, action);
     * Convert the transferable object to a wrapped object.
     * @param data
     * @return  returns null if the transferable object is not a Wrapped
     *          object, or an exception was thrown
    private static List getWrapped(Transferable data) {
        try {
            if (data.getTransferData(flavors[0]) instanceof List) {
                return (List) data.getTransferData(flavors[0]);
        } catch (UnsupportedFlavorException e) {
        } catch (IOException e) {
        return null;
    public DataFlavor[] getTransferDataFlavors() {
        return flavors;
    public boolean isDataFlavorSupported(DataFlavor flavor) {
        return flavor.equals(flavors[0]);
     * @param flavor    ignored; we always return the list
     * @return the list of CommonTextValue objects that we're dragging
     * @throws UnsupportedFlavorException   not thrown
     * @throws IOException  not thrown
    public Object getTransferData(DataFlavor flavor)
            throws UnsupportedFlavorException, IOException {
        return moving;
}

Answer 1

You find an example here:
http://forum.java.sun.com/thread.jspa?threadID=646602