Java Swing Drag and Drop Troubleshooting

Dragging domain objects between JTables

Drag and drop Troubleshooting

Drag and drop is an ubiquitous UI metaphor in common desktop/web applications.  I am in the process of finally learning how to accomplish drag and drop in Java through Swing; it hasn’t been an easy process.  I have been keeping track of some of the problems I have run into along the way, and will be updating this post as I find more.  Make sure you read through the whole set of Drag and Drop tutorials before you attempt to implement Drag and Drop; a lot of my mistakes came from jumping the gun and not reading through the whole document.  I will be making a post later with more complete source code illustrating how to drag and drop domain objects between components (the tutorials online usually show text being dragged and dropped rather than class instances).

Problem: When I attempt to drag an item out of the table, nothing happens
Solution:

• Did you remember to call “component.setDragEnabled(true)” first?

• Make sure you are not using a DataFlavor constructed with default constructor; if it is the drag action will not occur

Problem: When I complete a drag and drop, I get a class cast exception
Solution: Make sure you have created the DataFlavor correctly; you should define the class of the serialized object within the constructor to the DataFlavor.  E.g. if you are sending an instance of a class named Foo.java, you could create a DataFlavor as follows:

public class Foo implements Transferable, Serializable {

    public static final DataFlavor DATA_FLAVOR = new DataFlavor(Foo.class, "Foo");

    public DataFlavor[] getTransferDataFlavors() {
         return new DataFlavor[]{DATA_FLAVOR};
    }

    public boolean isDataFlavorSupported(DataFlavor flavor) {
        return flavor == FLAVOR;
    }

    public Object getTransferData(DataFlavor flavor) throws UnsupportedFlavorException, IOException {
        return this;
    }
}

Problem: When I attempt to drop an item onto a table, nothing happens
Solution:
If the table is empty, make sure you called “JTable.setFillsViewportHeight(true)” first?  Explanation:

JTable.setFillsViewportHeight(true)is invoked to set the fillsViewportHeight property. When this property is true the table uses the entire height of the container, even if the table doesn’t have enough rows to use the whole vertical space. This makes it easier to use the table as a drag-and-drop target. (http://java.sun.com/docs/books/tutorial/uiswing/components/table.html)

See more info about and drag and drop with respect to empty tables.

Problem: When I move an item from one table to another, the item isn’t deleted from the first table
Solution: Make sure you do the modification to the underlying table model in the exportDone method.

For instance:

@Override
public void exportDone(JComponent c, Transferable t, int action) {
    if (action == TransferHandler.MOVE) {
        JTable table = (JTable) c;
        list.remove(table.getSelectedRow());
    }
}