Swing Hacks

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Chapter 2, Lists and Combos

#17 Reorder a JList with Drag-and-Drop

HACK

H A C K

#17

Reorder a JList with Drag-and-Drop Hack #17

Let users put things where they want.

You may be so used to immutable lists that the idea of reordering a list with

drag-and-drop seems unnatural. The first time I saw it—rearranging the

order of network devices in Mac OS X to establish a priority (e.g., try Ether-

net, then wireless, then modem)—I thought it was kind of odd. In fact,

Apple felt it necessary to put a label on the list to tell users they could drag-

and-drop the list items to rearrange them. Now that I’m used to it, it’s

totally cool, and I’d like to see it done in more places.

To implement this functionality in a

JList, you basically just have to imple-

ment the full set of AWT drag-and-drop interfaces because the list will be

both the source of the drag and the target of the drop. The other thing you

need to do is to use some cell rendering tricks to provide a visual cue as to

where the drop will occur.

The

ReorderableJList, shown in Example 2-12, is a JList that uses a

DefaultListModel, which is mutable for the obvious reason that it will

need to change in response to drag-and-drops. The bulk of it is concerned

with implementing the drag-and-drop interfaces

DragSourceListener,

DropTargetListener, and DragGestureListener. It has an inner class imple-

menting

Tranferable to hold the item being dropped, although ...