[Chapter 8] 8.4 Data Transfer with Cut-and-Paste
Author: David Flanagan
Date: Thu Jul 31 15:56:03 1997

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| Chapter 8 New AWT Features |

## 8.4 Data Transfer with Cut-and-Paste

Java 1.1 adds cut-and-paste capabilities to Java applications through the classes and interfaces of the `java.awt.datatransfer` package. The `DataFlavor` class is perhaps the most central of these classes. It represents the type of data to be transferred. Every data flavor consists of a human-readable name and a data type specification. The data type can be specified in one of two ways: with a Java `Class` object or with a MIME type string. These two different ways of specifying the data type reflect two different ways of transferring the data. When the data type is specified as a class object, objects of that type are transferred using the object serialization mechanism (which is discussed in Chapter 9, Object Serialization). In Example 8.1, for example, the `DataFlavor` is specified using the `Class` object for `java.util.Vector`. This means that data is transferred as a serialized `Vector` object. It also means that the `DataFlavor` object has an implicit MIME type of:

    application/x-java-serialized-object; class=java.util.Vector

The data type of a `DataFlavor` can also be specified as a MIME type. In this case, data is transferred through a stream--the recipient of the data receives a `Reader` stream from which it can read textual data. In this case, the recipient usually has to parse the data according to the rules of the specified MIME type.

The `Transferable` interface is another important piece of the AWT data transfer picture. This interface specifies methods that must be implemented by any object that wants to make data available for transfer. One of its methods returns an array of all the `DataFlavor` types it can use to transfer its data. Another method checks whether the `Transferable` object supports a given method. The most important method, `getTransferData()`, actually returns the data in a format appropriate for the requested `DataFlavor`.

While `DataFlavor` and `Transferable` provide the underlying infrastructure for data transfer, it is the `Clipboard` class and `ClipboardOwner` interface that support the cut-and-paste style of data transfer. A typical cut-and-paste scenario works like this:

- When the user issues a command to "copy" or "cut" something, the initiating application first obtains the system `Clipboard` object by calling the `getSystemClipboard()` method of the `Toolkit` object. Next, it creates a `Transferable` object that represents the data to be transferred. Finally, it passes this transferable object to the clipboard by calling the `setContents()` method of the clipboard. The initiating application must also pass an object that implements the `ClipboardOwner` interface to `setContents()`. By doing so, it becomes the "clipboard owner" and must maintain its `Transferable` object until it ceases to be the clipboard owner.

- When the user issues a command to "paste," the receiving application first obtains the system `Clipboard` object in the same way that the initiating application did. Then, it calls the `getContents()` method of the system clipboard to receive the `Transferable` object stored there. Now it can use the methods defined by the `Transferable` interface to choose a `DataFlavor` for the data transfer and actually transfer the data.

- When the user copies or cuts some other piece of data, a new data transfer is initiated, and the new initiating application (it may be the same one) becomes the new clipboard owner. The previous owner is notified that it is no longer the clipboard owner when the system invokes the `lostOwnership()` method of the `ClipboardOwner` object specified in the initiating call to `setContents()`.

Note that untrusted applets are not allowed to work with the system clipboard because there might be sensitive data on it from other applications. This means that applets cannot participate in inter-application cut-and-paste. Instead, an applet must create a private clipboard object that it can use for intra-applet data transfer.

The `cut()`, `copy()`, and `paste()` methods of Example 8.1 implement cut-and-paste functionality for scribbled lines. They rely on the nested `SimpleSelection` class that implements the `Transferable` and `ClipboardOwner` interfaces. Note the definition of a `DataFlavor` object that serves to specify the type of data transfer. [1]

> [1] Although the example application uses the system clipboard, scribbles can only be pasted between windows of the same application, not between separate instances of the application running in separate Java interpreters. In Java 1.1.1, inter-application cut-and-paste only works with the pre-defined `DataFlavor.stringFlavor` and `DataFlavor.textFlavor` data flavors. Custom types like the one used in the example do not correctly interface with the system clipboard.

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