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Java in a Nutshell

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19. The java.awt.datatransfer Package

This small package contains classes and interfaces that support a generic inter-application data transfer mechanism. It also provides support for cut-and-paste data transfer on top of that mechanism. This package, and all of its classes and interfaces, are new in Java 1.1. Future releases of Java are likely to extend this package with support for data transfer through drag-and-drop.

Figure 19.1 shows the class hierarchy for java.datatransfer. DataFlavor and Transferable define the basic data transfer mechanism. Clipboard and ClipboardOwner provide support for cut-and-paste. StringSelection is a convenience class that makes it particularly easy to transfer textual data between applications.

19.1 java.awt.datatransfer.Clipboard (JDK 1.1)

This class represents a clipboard on which data may be transferred using the cut-and-paste metaphor. When data is "cut," it should be encapsulated in a Transferable object and registered with a Clipboard object by calling setContents(). A Clipboard can only hold a single piece of data at a time, so a ClipboardOwner object must be specified when data is placed on the clipboard. This object is notified that it no longer "owns" the clipboard when the data is replaced by other, more recent, data.

When a "paste" is requested by the user, an application requests the data on the Clipboard by calling getContents(), which returns a Transferable object. The methods of this object can be used to negotiate a mutually-compatible data format and to actually transfer the data.

A clipboard name is passed to the Clipboard() constructor, and may be retrieved with getName(). This name is not actually used in Java 1.1, however.

Note that while applications can create their own private Clipboard objects for intra-application cut-and-paste, it is more common for them to use the system clipboard to enable cut-and-paste between applications. You can obtain the system clipboard by calling the getSystemClipboard() method of the current Toolkit object. Untrusted applet code is not allowed to access the system clipboard, so untrusted applets cannot participate in inter-application cut-and-paste.

public class Clipboard extends Object {
    // Public Constructor
            public Clipboard(String name);
    // Protected Instance Variables
            protected Transferable contents;
            protected ClipboardOwner owner;
    // Public Instance Methods
            public synchronized Transferable getContents(Object requestor);
            public String getName();
            public synchronized void setContents(Transferable contents, ClipboardOwner owner);
}

Passed To:

ClipboardOwner.lostOwnership(), StringSelection.lostOwnership()

Returned By:

Toolkit.getSystemClipboard()


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