This Java Swing tip illustrates a method of implementing functionality of drag & drop of a group of files into a tree. The tip uses getdatatransfer function that returns an object that represents the data to be transferred. The class of the object returned is defined by the representation class of the data flavor.

 import javax.swing.*;
import javax.swing.tree.*;
import java.awt.dnd.*;
import java.awt.datatransfer.*;
import java.util.List;
import java.util.Iterator;
import java.awt.Point;
import java.awt.Rectangle;
import java.awt.Insets;
import java.io.File;
import java.io.IOException;

public class FSTree extends JTree {

  public FSTree() { super(); init(); }
  public FSTree(TreeModel newModel) { super(newModel); init(); }
  public FSTree(TreeNode root) { super(root); init(); }
  public FSTree(TreeNode root, boolean asks) { super(root, asks); init(); }

  private void init() {
    // We don't want to export anything from this tree, only import
    setDragEnabled(false);
    setTransferHandler(new FSTransfer());
  }

  public class FSTransfer extends TransferHandler {
    public boolean importData(JComponent comp, Transferable t) {
      // Make sure we have the right starting points
      if (!(comp instanceof FSTree)) {
        return false;
      }
      if (!t.isDataFlavorSupported(DataFlavor.javaFileListFlavor)) {
        return false;
      }

      // Grab the tree, its model and the root node
      FSTree tree = (FSTree)comp;
      DefaultTreeModel model = (DefaultTreeModel)tree.getModel();
      DefaultMutableTreeNode root = (DefaultMutableTreeNode)model.getRoot();
      try {
        List data = (List)t.getTransferData(DataFlavor.javaFileListFlavor);
        Iterator i = data.iterator();
        while (i.hasNext()) {
          File f = (File)i.next();
          root.add(new DefaultMutableTreeNode(f.getName()));
        }
        model.reload();
        return true;
      }
      catch (UnsupportedFlavorException ufe) {
        System.err.println("Ack! we should not be here.\nBad Flavor.");
      }
      catch (IOException ioe) {
        System.out.println("Something failed during import:\n" + ioe);
      }
      return false;
    }
    
    // We only support file lists on FSTrees...
    public boolean canImport(JComponent comp, DataFlavor[] transferFlavors) {
      if (comp instanceof FSTree) {
        for (int i = 0; i < transferFlavors.length; i++) {
          if (!transferFlavors[i].equals(DataFlavor.javaFileListFlavor)) {
            return false;
          }
        }
        return true;
      }
      return false;
    }
  }
}