The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.
See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases.
See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.
Every drag source (Java based or otherwise) advertises the set of actions it supports when exporting data. If it supports data being copied, it advertises the COPY
action; if it supports data being moved from it, then it advertises the MOVE
action, and so on. For Swing components, the source actions are advertised through the
getSourceActions
method.
When a drag is initiated, the user has some control over which of the source actions is chosen for the transfer by way of keyboard modifiers used in conjunction with the drag gesture — this is called the user action. For example, the default (where no modifiers are used) generally indicates a move action, holding the Control key indicates a copy action, and holding both Shift and Control indicates a linking action. The user action is available via the
getUserDropAction
method.
The user action indicates a preference, but ultimately it is the target that decides the drop action. For example, consider a component that will only accept copied data. And consider a drag source that supports both copy and move. The TransferHandler
for the copy-only target can be coded to only accept data from the source using the
setDropAction
method, even if the user has indicated a preference for a move action.
This work happens in the canImport
method, where the target's TransferHandler
decides whether to accept the incoming data. An implementation that explicitly chooses the COPY
action, if it is supported by the source, might look like this:
public boolean canImport(TransferHandler.TransferSupport support) { // for the demo, we will only support drops (not clipboard paste) if (!support.isDrop()) { return false; } // we only import Strings if (!support.isDataFlavorSupported(DataFlavor.stringFlavor)) { return false; } // check if the source actions (a bitwise-OR of supported actions) // contains the COPY action boolean copySupported = (COPY & support.getSourceDropActions()) == COPY; if (copySupported) { support.setDropAction(COPY); return true; } // COPY is not supported, so reject the transfer return false; }
The code snippet displayed in bold shows where the source's supported drop actions are queried. If copy is supported, the setDropAction
method is invoked to ensure that only a copy action will take place and the method returns true.
Next we will look at a demo that explicitly sets the drop action using setDropAction
.