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How to drag and drop with Java 2, Part 1

Explore the Java platform's new drag and drop classes

By Gene De Lisa, JavaWorld.com, 03/01/99

Java 2 (formerly JDK 1.2) introduced the ability to transfer data using the familiar drag and drop (D&D) metaphor. In Java 2, D&D utilizes the underlying data-transfer mechanism introduced in JDK 1.1 (java.awt.datatransfer) for use with the clipboard. Although this article discusses D&D operations in the context of GUI components, the specification includes no restrictions that prevent direct programmatic operations.

To develop the D&D metaphor, Java 2 defines several new classes in package java.awt.dnd. Please note: The GUI components used in this article are Swing components. In actuality, any subclass of java.awt.Component may be used.

First, we'll look at how a GUI component representing the data source of a D&D operation maintains an association with a java.awt.dnd.DropSource object.

Second, we'll examine how another GUI component representing the destination of the data of a D&D operation maintains an association with a java.awt.dnd.DropTarget object.

Finally, we'll wrap up with a java.awt.datatransfer.Transferable object that encapsulates the data transferred between the DragSource and DropTarget objects.

To download the source code in either zip or tar formats, see Resources.

DataFlavors and actions

When the Transferable object encapsulates data, it makes the data available to DropTarget in a variety of DataFlavors. For a local transfer within the same JVM (Java virtual machine), Transferable provides an object reference.

However, for transfers to another JVM or to the native system, this wouldn't make any sense, so a DataFlavor using a java.io.InputStream subclass usually is provided. (While a discussion of data transfer classes is beyond the scope of this article, you will find a linked list of previous JavaWorld articles on this topic in the Resources section below.)

When invoking a drag and drop operation, you may request various drag and drop actions. The DnDConstants class defines the class variables for the supported actions:

• ACTION_NONE -- no action taken
• ACTION_COPY -- the DragSource leaves the data intact
• ACTION_MOVE -- the DragSource deletes the data upon successful completion of the drop
• ACTION_COPY or ACTION_MOVE -- the DragSource will perform either action requested by the DropTarget
• ACTION_LINK or ACTION_REFERENCE -- a data change to either the source or the destination propagates to the other location

Creating a draggable component

For a GUI component to act as the source of a D&D operation, it must be associated with five objects:

• java.awt.dnd.DragSource
• java.awt.dnd.DragGestureRecognizer
• java.awt.dnd.DragGestureListener
• java.awt.datatransfer.Transferable
• java.awt.dnd.DragSourceListener

The DragSource

A common way to obtain a DragSource object is to use one instance per JVM. Class method DragSource.getDefaultDragSource will obtain a shared DragSource object that is used for the lifetime of the JVM. Another option is to provide one DragSource per instance of the Component class. With this option, however, you accept responsibility for implementation.