Restrict drops to JPanel where it's drags are from.

[Matt Kurz]

The attached screenshot consists of 2 JPanels. Each has 2 JLists. I want to allow drag and drop only inside one JPanel. For instance drops from TopPanel to ButtomPanel or complete outside to desktop or something else should NOT be allowed. My solution already works panel specific, but drops outside are still possible. How can I deactivate that? Here is the standalone example code:

import java.awt.GridLayout;

import javax.swing.DefaultListModel;
import javax.swing.JFrame;
import javax.swing.JList;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JSplitPane;
import javax.swing.ListSelectionModel;

public class simpleList2List {
    simpleTransferHandler arrayListHandler = new simpleTransferHandler();    

    private JPanel getContent(String panelName) {
        JPanel panel = new JPanel(new GridLayout(1, 0));
        panel.add(getListComponent("left_" + panelName));
        panel.add(getListComponent2("right" + panelName));
        return panel;
    }

    private JScrollPane getListComponent(String s) {
        DefaultListModel model = new DefaultListModel();
        for (int j = 0; j < 3; j++)
            model.addElement(s + (j + 1));
        JList list = new JList(model);
        list.setName(s);
        list.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
        list.setTransferHandler(arrayListHandler);
        list.setDragEnabled(true);
        return new JScrollPane(list);
    }

    private JScrollPane getListComponent2(String s) {
        DefaultListModel model = new DefaultListModel();
        for (int j = 0; j < 7; j++)
            model.addElement(s + (j + 1));
        JList list = new JList(model);
        list.setName(s);
        list.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
        list.setTransferHandler(arrayListHandler);
        list.setDragEnabled(true);
        return new JScrollPane(list);
    }

    public static void main(String[] args) {      
        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.getContentPane().add(new JSplitPane(JSplitPane.VERTICAL_SPLIT, new simpleList2List().getContent("TopPanel"), new simpleList2List().getContent("ButtomPanel")));
        f.setSize(400, 400);
        f.setLocationRelativeTo(null);
        f.setVisible(true);
    }
}

import java.awt.datatransfer.DataFlavor;
import java.awt.datatransfer.StringSelection;
import java.awt.datatransfer.Transferable;
import javax.swing.DefaultListModel;
import javax.swing.JComponent;
import javax.swing.JList;
import javax.swing.TransferHandler;

public class simpleTransferHandler extends TransferHandler {
    private int drag_index;
    private String drag_list_name = null;
    private String dragPanelName = null;
    
    public boolean canImport(TransferHandler.TransferSupport info) {
        // allow only strings. A Dataflavor is clipboard stuff.
        if (!info.isDataFlavorSupported(DataFlavor.stringFlavor)) {
            return false;
        }
        // only support drops (not clipboard paste)
        if (!info.isDrop()) {
            return false;
        }
        // TODO do not allow drops outside this JPanel! I can still drop outside
        // if I return always false!

        String dropPanel = (((JList) info.getComponent()).getName().toString()).substring(5);
        System.out.println("xxxxx: " + dropPanel + ", " + this.dragPanelName);
        if (dropPanel.equals(this.dragPanelName)) {
            System.out.println("drop location: " + dropPanel);
            return true;
        }
        return false;
    }

    public boolean importData(TransferHandler.TransferSupport info) {
        if (!info.isDrop()) {
            return false;
        }

        JList drop_list = (JList) info.getComponent();
        DefaultListModel listModel = (DefaultListModel) drop_list.getModel();
        JList.DropLocation dl = (JList.DropLocation) info.getDropLocation();
        int drop_index = dl.getIndex();

        // Get the string that is being dropped.
        Transferable t = info.getTransferable();
        String data;
        try {
            data = (String) t.getTransferData(DataFlavor.stringFlavor);
        } catch (Exception e) {
            return false;
        }

        int max = listModel.getSize();
        // Consider special case
        // System.out.println("import data and it's index: " + data + ", " +
        // drop_index + ", " + drop_list.getName());
        if (drop_list.getName().substring(0, 5).equals(drag_list_name)) {
            if (drag_index > drop_index) {
                // System.out.println("spec case");
                drag_index++;
            }
        } else { // we have to consider the case that drop_index = -1 ( in
                 // special when the other list is empty)
            if (drop_index == -1) {
                // System.out.println("spec case 2");
                drop_index = max;
            }
        }
        listModel.add(drop_index, data);
        this.dragPanelName = null;
        return true;
    }

    protected void exportDone(JComponent c, Transferable data, int action) {
        if ((action == MOVE)) { // necessary to avoid drag object loss
            JList drag_list = (JList) c;
            DefaultListModel model = (DefaultListModel) drag_list.getModel();
            model.remove(drag_index);
        }
        this.dragPanelName = null;
    }

    protected Transferable createTransferable(JComponent c) {
        if (c instanceof JList) {
            JList drag_list = (JList) c;
             this.dragPanelName = drag_list.getName().substring(5);
             this.drag_list_name = drag_list.getName().substring(0, 5);
             System.out.println("dragPanel: "+ dragPanelName + ", " + drag_list_name);
            // System.out.println("x: "+ source.getName());
            drag_index = drag_list.getSelectedIndex();
            String value = drag_list.getSelectedValue().toString();
            if (value == null || value.toString().equals("")) {
                return null;
            }
            // System.out.println("export data and it's index: " + value + ", "
            // + drag_index + ", " + drag_list.getName());
            return new StringSelection(value);
        }
        return null;
    }

    public int getSourceActions(JComponent c) {
        return MOVE;
    }

}