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6.2. Cut-and-Paste

In addition to the data transfer framework classes, the java.awt.datatransfer package also defines the Clipboard class and the ClipboardOwner interface, which implement data transfer with the cut-and-paste metaphor.

A typical cut-and-paste transfer works as follows:

  • In the initiating application, the user types Ctrl-C or Ctrl-X or in some other way tells the application that he wants to copy or cut some data.

  • The application takes the selected data and encapsulates it in an appropriate Transferable object. The next step is to call the getSystemClipboard() method of the Toolkit object, to get a Clipboard object. The application then calls the setContents() method of the Clipboard, passing the Transferable object as the new clipboard contents.

  • If the user issued a cut command, the initiating application typically deletes the data after transferring it to the clipboard. If the user issued a copy command, however, the application typically just highlights the data to make it clear to the user what data is available for pasting. Often this data should remain highlighted for as long as the initiating application owns the clipboard. When an application calls setContents(), it becomes the clipboard owner and remains such until some other application transfers data to the clipboard. The application must, in fact, pass a object that implements ClipboardOwner to the setContents() method. This object is used to notify the application when it ceases to be the clipboard owner. Until that happens, however, the application must maintain the Transferable object and be willing to provide the data when it is requested.

  • At some point, the user moves his attention to some other application and issues a paste command in that application. This receiving first application calls Toolkit.getSystemClipboard() to obtain a Clipboard object. Then it calls getContents() to obtain a Transferable object that represents the data available on the clipboard. The application uses getTransferDataFlavors() or isDataFlavorSupported() to see if the clipboard data is available in a format it is willing to accept. If there is such a format, the application calls getTransferData() to transfer the data.

  • At some point after this cut-and-paste operation, the user cuts or copies a new piece of data in an application. At this point, the original application ceases to be the clipboard owner and no longer has to make its data available for pasting. The lostOwnership() method of the originating application's ClipboardOwner object is called to notify the application of this occurrence.

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