Drag and drop custom object from JList to JLabel

Question

I have a JList containing an ArrayList for custom objects, and I'm trying to create drag and drop into fields. I am having trouble understanding how to package and receive an object in Transferable.

This is something like I got:

import java.awt.*; import java.awt.event.*; import javax.swing.*; import javax.swing.event.*; import java.util.*; public class FlightProjectInterface extends JFrame{ //create GUI Objects private JFrame primaryFrame; private JPanel createFlightPanel; private JPanel aircraftLayout; private JList personsJList, personsOnFlightJList; private JTextField pilotLabel, coPilotLabel, backseat1Label, backseat2Label; public FlightProjectInterface(){ //establish frame super("Create Flight"); setLayout( new FlowLayout()); //aircraftPanel aircraftLayout = new JPanel(); aircraftLayout.setLayout(new GridLayout(2,2)); pilotLabel = new JTextField("Drag Pilot Here"); //build person load list DefaultListModel listModel = new DefaultListModel(); for (Person person : Database.persons) listModel.addElement(person); personsJList = new JList(listModel); personsJList.setVisibleRowCount(5); personsJList.setSelectionMode(ListSelectionModel.SINGLE_SELECTION); personsJList.setDragEnabled(true); add( new JScrollPane(personsJList) ); aircraftLayout.add(pilotLabel); add(aircraftLayout); }//end constructor } 

Clarification: I am having problems selecting an object from a JList and creating a portable one from it. With the above code, the toString view of the object is simply inserted into the text field, so I cannot pull the object data from the dropped location. How can I “pack” the object itself and put it in a placeholder that I can associate with the object itself from the GUI?

Ideally, there would be 4 fields, each of which contains an object that can be dropped. A person will be removed from the list if he is removed, but will be returned to the list if he is replaced.

Answer 1

Dragging and dropping can be a difficult animal, and not be facilitated due to conflicting information. Personally, I like to avoid the transfer API, but I'm so old school.

The glue to DnD is really DataFlavor . I prefer to roll on my own, making life easier.

In this example, I used one TransferHandler , but really, you really have one to drag and one to delete, in particular, you must have one for each component that you want to switch to.

The main reason for this is because I put a trap in my canImport method to reject it if you drag it behind a JList , so you can just drop it onto JLabel , this is a bit of a hack and probably not a good idea.

 import java.awt.BorderLayout; import java.awt.Component; import java.awt.EventQueue; import java.awt.GridBagConstraints; import java.awt.GridBagLayout; import java.awt.datatransfer.DataFlavor; import java.awt.datatransfer.Transferable; import java.awt.datatransfer.UnsupportedFlavorException; import java.awt.dnd.DnDConstants; import java.io.IOException; import javax.swing.DefaultListModel; import javax.swing.JComponent; import javax.swing.JFrame; import javax.swing.JLabel; import javax.swing.JList; import javax.swing.JPanel; import javax.swing.JScrollPane; import javax.swing.TransferHandler; import javax.swing.UIManager; import javax.swing.UnsupportedLookAndFeelException; public class DnDTransferableTest { public static void main(String[] args) { new DnDTransferableTest(); } public DnDTransferableTest() { EventQueue.invokeLater(new Runnable() { @Override public void run() { try { UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName()); } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) { } JFrame frame = new JFrame("Testing"); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame.setLayout(new BorderLayout()); frame.add(new TestPane()); frame.pack(); frame.setLocationRelativeTo(null); frame.setVisible(true); } }); } @SuppressWarnings("serial") public class TestPane extends JPanel { private JList<ListItem> list; private JLabel label; public TestPane() { list = new JList<ListItem>(); list.setDragEnabled(true); list.setTransferHandler(new ListTransferHandler()); DefaultListModel<ListItem> model = new DefaultListModel<ListItem>(); for (int index = 0; index < 10; index++) { model.addElement(new ListItem("Item " + index)); } list.setModel(model); setLayout(new GridBagLayout()); GridBagConstraints gbc = new GridBagConstraints(); gbc.gridx = 0; gbc.gridy = 0; gbc.weighty = 1; gbc.weightx = 1; gbc.fill = GridBagConstraints.BOTH; add(new JScrollPane(list), gbc); label = new JLabel("Drag on me..."); gbc.gridx++; gbc.weightx = 1; gbc.fill = GridBagConstraints.NONE; add(label, gbc); label.setTransferHandler(new ListTransferHandler()); } } @SuppressWarnings("serial") public class ListTransferHandler extends TransferHandler { @Override public boolean canImport(TransferSupport support) { return (support.getComponent() instanceof JLabel) && support.isDataFlavorSupported(ListItemTransferable.LIST_ITEM_DATA_FLAVOR); } @Override public boolean importData(TransferSupport support) { boolean accept = false; if (canImport(support)) { try { Transferable t = support.getTransferable(); Object value = t.getTransferData(ListItemTransferable.LIST_ITEM_DATA_FLAVOR); if (value instanceof ListItem) { Component component = support.getComponent(); if (component instanceof JLabel) { ((JLabel)component).setText(((ListItem)value).getText()); accept = true; } } } catch (Exception exp) { exp.printStackTrace(); } } return accept; } @Override public int getSourceActions(JComponent c) { return DnDConstants.ACTION_COPY_OR_MOVE; } @Override protected Transferable createTransferable(JComponent c) { Transferable t = null; if (c instanceof JList) { @SuppressWarnings("unchecked") JList<ListItem> list = (JList<ListItem>) c; Object value = list.getSelectedValue(); if (value instanceof ListItem) { ListItem li = (ListItem) value; t = new ListItemTransferable(li); } } return t; } @Override protected void exportDone(JComponent source, Transferable data, int action) { System.out.println("ExportDone"); // Here you need to decide how to handle the completion of the transfer, // should you remove the item from the list or not... } } public static class ListItemTransferable implements Transferable { public static final DataFlavor LIST_ITEM_DATA_FLAVOR = new DataFlavor(ListItem.class, "java/ListItem"); private ListItem listItem; public ListItemTransferable(ListItem listItem) { this.listItem = listItem; } @Override public DataFlavor[] getTransferDataFlavors() { return new DataFlavor[]{LIST_ITEM_DATA_FLAVOR}; } @Override public boolean isDataFlavorSupported(DataFlavor flavor) { return flavor.equals(LIST_ITEM_DATA_FLAVOR); } @Override public Object getTransferData(DataFlavor flavor) throws UnsupportedFlavorException, IOException { return listItem; } } public static class ListItem { private String text; public ListItem(String text) { this.text = text; } public String getText() { return text; } @Override public String toString() { return getText(); } } }