Drag and drop with objects other than strings

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Hi,

I have a little Swing app which has an edit field, implemented by a
JList, containing formulas.  The JList has a DefaultListModel which
contains objects of class Formula.  I would like to enable DnD.  I read
through the Swing tutorial on DnD
(http://java.sun.com/docs/books/tutorial/uiswing/dnd/intro.html),
but it
only gives examples using Strings.

Did someone implement drag and drop for JVM objects already?

Below is my go at it, but it doesn’t work in that the cursor shows the
‘forbidden’ icon all the time, i.e. canImport() always returns false.

Help appreciated.

==FormulaTransferHandler.java==
import java.awt.datatransfer.*;
import java.io.IOException;
import javax.activation.DataHandler;
import javax.swing.*;

/**
~ * A transfer handler for formulas. Based on Sun’s Swing DnD tutorial.
~ */
public class FormulaTransferHandler extends TransferHandler {
~  /**
~   * Eclipse generated 16/4/8
~   */
~  private static final long serialVersionUID = 4382734030287436893L;

~  /**
~   * Initialize the formula transfer handler. Sets the supported
~   * flavors. For now these are:
~   * <ul>
~   * <li>new DataFlavor(DataFlavor.javaJVMLocalObjectMimeType +
~   * ";class=de.uni_tuebingen.sfb.lichtenstein.formulas.Formula")</li>
~   * </ul>
~   */
~  public FormulaTransferHandler() {
~    try {
~      this.localFormulaFlavor = new DataFlavor(
~          DataFlavor.javaJVMLocalObjectMimeType + ";class=\""
~              + Formula[].class.getName() + "\"");
~      this.localFormulaFlavor
~          .setHumanPresentableName("Arrays of formulas");
~    } catch (final ClassNotFoundException exc) {
~      exc.printStackTrace();
~    }
~      this.supportedFlavors = new DataFlavor[1];
~      this.supportedFlavors[0] = this.localFormulaFlavor;
~      this.supportedFlavors[1] = new DataFlavor(Formula[].class,
~          "Formulas");
~    }
~  }

~  /**
~   * The data flavor for formulas local to the JVM.
~   */
~  private DataFlavor localFormulaFlavor;
~  /**
~   * The supported flavors of data.
~   */
~  private final DataFlavor[] supportedFlavors;
~  /**
~   * The formulas being transfered.
~   */
~  private Object[] transferedFormulas = null;

~  @[EMAIL PROTECTED]
  protected Transferable createTransferable(final JComponent c) {
~    final JList formulaList = (JList) c;
~    this.transferedFormulas = formulaList.getSelectedValues();
~    return new DataHandler(this.transferedFormulas,
~        this.localFormulaFlavor.getMimeType());
~    // TODO: add support via serialization?
~  }

~  @[EMAIL PROTECTED]
  public boolean canImport(final TransferHandler.TransferSupport info) {
~    // only support drop, I don’t see a use for paste here
~    if (!info.isDrop()) {
~      return false;
~    }
~    if (!info.isDataFlavorSupported(this.localFormulaFlavor)) {
~      // TODO: allow serialization?
~      return false;
~    }
~    final JList.DropLocation dl = (JList.DropLocation) info
~        .getDropLocation();
~    return true;
~  }

~  @[EMAIL PROTECTED]
  public int getSourceActions(final JComponent c) {
~    return TransferHandler.COPY_OR_MOVE;
~  }

~  @[EMAIL PROTECTED]
  public boolean importData(final TransferHandler.TransferSupport info) {
~    if (this.canImport(info)) {
~      try {
~        final Formula[] formulas = (Formula[]) info.getTransferable()
~            .getTransferData(this.localFormulaFlavor);
~        int index = ((JList.DropLocation) info.getDropLocation())
~            .getIndex();
~        final DefaultListModel listModel = (DefaultListModel) ((JList)
info
~            .getComponent()).getModel();
~        for (final Formula element : formulas) {
~          listModel.add(index++, element);
~        }
~        return true;
~      } catch (final UnsupportedFlavorException ufe) {
~        ufe.printStackTrace();
~      } catch (final IOException ioe) {
~        ioe.printStackTrace();
~      }
~    }
~    return false;
~  }

~  @[EMAIL PROTECTED]
  protected void exportDone(final JComponent c,
~      final Transferable data, final int action) {
~    if (action == TransferHandler.MOVE
~        && this.transferedFormulas != null) {
~      final DefaultListModel model = (DefaultListModel) ((JList) c)
~          .getModel();
~      for (int i = this.transferedFormulas.length - 1; i >= 0; i--) {
~        model.removeElement(this.transferedFormulas[i]);
~      }
~    }
~  }
}
== ==

Formula contains a lot more, but to try the example,

public interface Formula {}

will suffice.

I know there are other problems with this, but for now I’d like to get
it ‘basically’ to work.

It is difficult to debug, since the Eclipse’s debugging program requires
me to use the mouse, so stuf gets strange.

All I can find out is that
info.isDataFlavorSupported(this.localFormulaFlavor) returns false, since
localFormulaFlavor is
java.awt.datatransfer.DataFlavor[mimetype=application/x-java-jvm-local-objectref;representationclass=[Lde.uni_tuebingen.sfb.lichtenstein.formulas.Formula;]
whereas the supported flavors of info are
javax.activation.ActivationDataFlavor[mimetype=application/x-java-jvm-local-objectref;representationclass=[Lde.uni_tuebingen.sfb.lichtenstein.formulas.Formula;]

But why this is so is unclear to me.

TIA, H.
- --
Hendrik Maryns
http://tcl.sfs.uni-tuebingen.de/~hendrik/
==================
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