TransferHandler (Java Platform SE 8 )
Keywords: javax.swing.TransferHandler class
Date: 2018-07-01

javax.swing

## Class TransferHandler

- java.lang.Object
- - javax.swing.TransferHandler

- All Implemented Interfaces:
    Serializable

      public class TransferHandler
      extends Object
      implements Serializable

  This class is used to handle the transfer of a `Transferable` to and from Swing components. The `Transferable` is used to represent data that is exchanged via a cut, copy, or paste to/from a clipboard. It is also used in drag-and-drop operations to represent a drag from a component, and a drop to a component. Swing provides functionality that automatically supports cut, copy, and paste keyboard bindings that use the functionality provided by an implementation of this class. Swing also provides functionality that automatically supports drag and drop that uses the functionality provided by an implementation of this class. The Swing developer can concentrate on specifying the semantics of a transfer primarily by setting the `transferHandler` property on a Swing component.

  This class is implemented to provide a default behavior of transferring a component property simply by specifying the name of the property in the constructor. For example, to transfer the foreground color from one component to another either via the clipboard or a drag and drop operation a `TransferHandler` can be constructed with the string "foreground". The built in support will use the color returned by `getForeground` as the source of the transfer, and `setForeground` for the target of a transfer.

  Please see How to Use Drag and Drop and Data Transfer, a section in The Java Tutorial, for more information.

  Since:
    1.4

- - ### Nested Class Summary

    Nested Classes
    Modifier and Type | Class and Description
    --- | ---
    `static class` | `TransferHandler.DropLocation` Represents a location where dropped data should be inserted.
    `static class` | `TransferHandler.TransferSupport` This class encapsulates all relevant details of a clipboard or drag and drop transfer, and also allows for customizing aspects of the drag and drop experience.

  - ### Field Summary

    Fields
    Modifier and Type | Field and Description
    --- | ---
    `static int` | `COPY` An `int` representing a "copy" transfer action.
    `static int` | `COPY_OR_MOVE` An `int` representing a source action capability of either "copy" or "move".
    `static int` | `LINK` An `int` representing a "link" transfer action.
    `static int` | `MOVE` An `int` representing a "move" transfer action.
    `static int` | `NONE` An `int` representing no transfer action.

  - ### Constructor Summary

    Constructors
    Modifier | Constructor and Description
    --- | ---
    `protected` | `TransferHandler()` Convenience constructor for subclasses.
    `` | `TransferHandler(String property)` Constructs a transfer handler that can transfer a Java Bean property from one component to another via the clipboard or a drag and drop operation.

  - ### Method Summary

    All Methods Static Methods Instance Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `boolean` | `canImport(JComponent comp, DataFlavor[] transferFlavors)` Indicates whether a component will accept an import of the given set of data flavors prior to actually attempting to import it.
    `boolean` | `canImport(TransferHandler.TransferSupport support)` This method is called repeatedly during a drag and drop operation to allow the developer to configure properties of, and to return the acceptability of transfers; with a return value of `true` indicating that the transfer represented by the given `TransferSupport` (which contains all of the details of the transfer) is acceptable at the current time, and a value of `false` rejecting the transfer.
    `protected Transferable` | `createTransferable(JComponent c)` Creates a `Transferable` to use as the source for a data transfer.
    `void` | `exportAsDrag(JComponent comp, InputEvent e, int action)` Causes the Swing drag support to be initiated.
    `protected void` | `exportDone(JComponent source, Transferable data, int action)` Invoked after data has been exported.
    `void` | `exportToClipboard(JComponent comp, Clipboard clip, int action)` Causes a transfer from the given component to the given clipboard.
    `static Action` | `getCopyAction()` Returns an `Action` that performs copy operations to the clipboard.
    `static Action` | `getCutAction()` Returns an `Action` that performs cut operations to the clipboard.
    `Image` | `getDragImage()` Returns the drag image.
    `Point` | `getDragImageOffset()` Returns an anchor offset for the image to drag.
    `static Action` | `getPasteAction()` Returns an `Action` that performs paste operations from the clipboard.
    `int` | `getSourceActions(JComponent c)` Returns the type of transfer actions supported by the source; any bitwise-OR combination of `COPY`, `MOVE` and `LINK`.
    `Icon` | `getVisualRepresentation(Transferable t)` Returns an object that establishes the look of a transfer.
    `boolean` | `importData(JComponent comp, Transferable t)` Causes a transfer to a component from a clipboard or a DND drop operation.
    `boolean` | `importData(TransferHandler.TransferSupport support)` Causes a transfer to occur from a clipboard or a drag and drop operation.
    `void` | `setDragImage(Image img)` Sets the drag image parameter.
    `void` | `setDragImageOffset(Point p)` Sets an anchor offset for the image to drag.

    - ### Methods inherited from class java.lang.Object

      `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- - ### Field Detail

    - #### NONE

          public static final int NONE

      An `int` representing no transfer action.
      See Also:
        Constant Field Values

    - #### COPY

          public static final int COPY

      An `int` representing a "copy" transfer action. This value is used when data is copied to a clipboard or copied elsewhere in a drag and drop operation.
      See Also:
        Constant Field Values

    - #### MOVE

          public static final int MOVE

      An `int` representing a "move" transfer action. This value is used when data is moved to a clipboard (i.e. a cut) or moved elsewhere in a drag and drop operation.
      See Also:
        Constant Field Values

    - #### COPY_OR_MOVE

          public static final int COPY_OR_MOVE

      An `int` representing a source action capability of either "copy" or "move".
      See Also:
        Constant Field Values

    - #### LINK

          public static final int LINK

      An `int` representing a "link" transfer action. This value is used to specify that data should be linked in a drag and drop operation.
      Since:
        1.6
      See Also:
        `DnDConstants.ACTION_LINK`, Constant Field Values

  - ### Constructor Detail

    - #### TransferHandler

          public TransferHandler(String property)

      Constructs a transfer handler that can transfer a Java Bean property from one component to another via the clipboard or a drag and drop operation.
      Parameters:
        `property` - the name of the property to transfer; this can be `null` if there is no property associated with the transfer handler (a subclass that performs some other kind of transfer, for example)

    - #### TransferHandler

          protected TransferHandler()

      Convenience constructor for subclasses.

  - ### Method Detail

    - #### getCutAction

          public static Action getCutAction()

      Returns an `Action` that performs cut operations to the clipboard. When performed, this action operates on the `JComponent` source of the `ActionEvent` by invoking `exportToClipboard`, with a `MOVE` action, on the component's `TransferHandler`.
      Returns:
        an `Action` for performing cuts to the clipboard

    - #### getCopyAction

          public static Action getCopyAction()

      Returns an `Action` that performs copy operations to the clipboard. When performed, this action operates on the `JComponent` source of the `ActionEvent` by invoking `exportToClipboard`, with a `COPY` action, on the component's `TransferHandler`.
      Returns:
        an `Action` for performing copies to the clipboard

    - #### getPasteAction

          public static Action getPasteAction()

      Returns an `Action` that performs paste operations from the clipboard. When performed, this action operates on the `JComponent` source of the `ActionEvent` by invoking `importData`, with the clipboard contents, on the component's `TransferHandler`.
      Returns:
        an `Action` for performing pastes from the clipboard

    - #### setDragImage

          public void setDragImage(Image img)

      Sets the drag image parameter. The image has to be prepared for rendering by the moment of the call. The image is stored by reference because of some performance reasons.
      Parameters:
        `img` - an image to drag

    - #### getDragImage

          public Image getDragImage()

      Returns the drag image. If there is no image to drag, the returned value is `null`.
      Returns:
        the reference to the drag image

    - #### setDragImageOffset

          public void setDragImageOffset(Point p)

      Sets an anchor offset for the image to drag. It can not be `null`.
      Parameters:
        `p` - a `Point` object that corresponds to coordinates of an anchor offset of the image relative to the upper left corner of the image

    - #### getDragImageOffset

          public Point getDragImageOffset()

      Returns an anchor offset for the image to drag.
      Returns:
        a `Point` object that corresponds to coordinates of an anchor offset of the image relative to the upper left corner of the image. The point `(0,0)` returns by default.

    - #### exportAsDrag

          public void exportAsDrag(JComponent comp,
                                   InputEvent e,
                                   int action)

      Causes the Swing drag support to be initiated. This is called by the various UI implementations in the `javax.swing.plaf.basic` package if the dragEnabled property is set on the component. This can be called by custom UI implementations to use the Swing drag support. This method can also be called by a Swing extension written as a subclass of `JComponent` to take advantage of the Swing drag support.

      The transfer will not necessarily have been completed at the return of this call (i.e. the call does not block waiting for the drop). The transfer will take place through the Swing implementation of the `java.awt.dnd` mechanism, requiring no further effort from the developer. The `exportDone` method will be called when the transfer has completed.

      Parameters:
        `comp` - the component holding the data to be transferred; provided to enable sharing of `TransferHandler`s
        `e` - the event that triggered the transfer
        `action` - the transfer action initially requested; either `COPY`, `MOVE` or `LINK`; the DnD system may change the action used during the course of the drag operation

    - #### exportToClipboard

          public void exportToClipboard(JComponent comp,
                                        Clipboard clip,
                                        int action)
                                 throws IllegalStateException

      Causes a transfer from the given component to the given clipboard. This method is called by the default cut and copy actions registered in a component's action map.

      The transfer will take place using the `java.awt.datatransfer` mechanism, requiring no further effort from the developer. Any data transfer will be complete and the `exportDone` method will be called with the action that occurred, before this method returns. Should the clipboard be unavailable when attempting to place data on it, the `IllegalStateException` thrown by `Clipboard.setContents(Transferable, ClipboardOwner)` will be propagated through this method. However, `exportDone` will first be called with an action of `NONE` for consistency.

      Parameters:
        `comp` - the component holding the data to be transferred; provided to enable sharing of `TransferHandler`s
        `clip` - the clipboard to transfer the data into
        `action` - the transfer action requested; this should be a value of either `COPY` or `MOVE`; the operation performed is the intersection of the transfer capabilities given by getSourceActions and the requested action; the intersection may result in an action of `NONE` if the requested action isn't supported
      Throws:
        `IllegalStateException` - if the clipboard is currently unavailable
      See Also:
        `Clipboard.setContents(Transferable, ClipboardOwner)`

    - #### importData

          public boolean importData(TransferHandler.TransferSupport support)

      Causes a transfer to occur from a clipboard or a drag and drop operation. The `Transferable` to be imported and the component to transfer to are contained within the `TransferSupport`.

      While the drag and drop implementation calls `canImport` to determine the suitability of a transfer before calling this method, the implementation of paste does not. As such, it cannot be assumed that the transfer is acceptable upon a call to this method for paste. It is recommended that `canImport` be explicitly called to cover this case.

      Note: The `TransferSupport` object passed to this method is only valid for the duration of the method call. It is undefined what values it may contain after this method returns.

      Parameters:
        `support` - the object containing the details of the transfer, not `null`.
      Returns:
        true if the data was inserted into the component, false otherwise
      Throws:
        `NullPointerException` - if `support` is `null`
      Since:
        1.6
      See Also:
        `canImport(TransferHandler.TransferSupport)`

    - #### importData

          public boolean importData(JComponent comp,
                                    Transferable t)

      Causes a transfer to a component from a clipboard or a DND drop operation. The `Transferable` represents the data to be imported into the component.

      Note: Swing now calls the newer version of `importData` that takes a `TransferSupport`, which in turn calls this method (if the component in the `TransferSupport` is a `JComponent`). Developers are encouraged to call and override the newer version as it provides more information (and is the only version that supports use with a `TransferHandler` set directly on a `JFrame` or other non-`JComponent`).

      Parameters:
        `comp` - the component to receive the transfer; provided to enable sharing of `TransferHandler`s
        `t` - the data to import
      Returns:
        true if the data was inserted into the component, false otherwise
      See Also:
        `importData(TransferHandler.TransferSupport)`

    - #### canImport

          public boolean canImport(TransferHandler.TransferSupport support)

      This method is called repeatedly during a drag and drop operation to allow the developer to configure properties of, and to return the acceptability of transfers; with a return value of `true` indicating that the transfer represented by the given `TransferSupport` (which contains all of the details of the transfer) is acceptable at the current time, and a value of `false` rejecting the transfer.

      For those components that automatically display a drop location during drag and drop, accepting the transfer, by default, tells them to show the drop location. This can be changed by calling `setShowDropLocation` on the `TransferSupport`.

      By default, when the transfer is accepted, the chosen drop action is that picked by the user via their drag gesture. The developer can override this and choose a different action, from the supported source actions, by calling `setDropAction` on the `TransferSupport`.

      On every call to `canImport`, the `TransferSupport` contains fresh state. As such, any properties set on it must be set on every call. Upon a drop, `canImport` is called one final time before calling into `importData`. Any state set on the `TransferSupport` during that last call will be available in `importData`.

      This method is not called internally in response to paste operations. As such, it is recommended that implementations of `importData` explicitly call this method for such cases and that this method be prepared to return the suitability of paste operations as well.

      Note: The `TransferSupport` object passed to this method is only valid for the duration of the method call. It is undefined what values it may contain after this method returns.

      Parameters:
        `support` - the object containing the details of the transfer, not `null`.
      Returns:
        `true` if the import can happen, `false` otherwise
      Throws:
        `NullPointerException` - if `support` is `null`
      Since:
        1.6
      See Also:
        `importData(TransferHandler.TransferSupport)`, `TransferHandler.TransferSupport.setShowDropLocation(boolean)`, `TransferHandler.TransferSupport.setDropAction(int)`

    - #### canImport

          public boolean canImport(JComponent comp,
                                   DataFlavor[] transferFlavors)

      Indicates whether a component will accept an import of the given set of data flavors prior to actually attempting to import it.

      Note: Swing now calls the newer version of `canImport` that takes a `TransferSupport`, which in turn calls this method (only if the component in the `TransferSupport` is a `JComponent`). Developers are encouraged to call and override the newer version as it provides more information (and is the only version that supports use with a `TransferHandler` set directly on a `JFrame` or other non-`JComponent`).

      Parameters:
        `comp` - the component to receive the transfer; provided to enable sharing of `TransferHandler`s
        `transferFlavors` - the data formats available
      Returns:
        true if the data can be inserted into the component, false otherwise
      See Also:
        `canImport(TransferHandler.TransferSupport)`

    - #### getSourceActions

          public int getSourceActions(JComponent c)

      Returns the type of transfer actions supported by the source; any bitwise-OR combination of `COPY`, `MOVE` and `LINK`.

      Some models are not mutable, so a transfer operation of `MOVE` should not be advertised in that case. Returning `NONE` disables transfers from the component.

      Parameters:
        `c` - the component holding the data to be transferred; provided to enable sharing of `TransferHandler`s
      Returns:
        `COPY` if the transfer property can be found, otherwise returns `NONE`

    - #### getVisualRepresentation

          public Icon getVisualRepresentation(Transferable t)

      Returns an object that establishes the look of a transfer. This is useful for both providing feedback while performing a drag operation and for representing the transfer in a clipboard implementation that has a visual appearance. The implementation of the `Icon` interface should not alter the graphics clip or alpha level. The icon implementation need not be rectangular or paint all of the bounding rectangle and logic that calls the icons paint method should not assume the all bits are painted. `null` is a valid return value for this method and indicates there is no visual representation provided. In that case, the calling logic is free to represent the transferable however it wants.

      The default Swing logic will not do an alpha blended drag animation if the return is `null`.

      Parameters:
        `t` - the data to be transferred; this value is expected to have been created by the `createTransferable` method
      Returns:
        `null`, indicating there is no default visual representation

    - #### createTransferable

          protected Transferable createTransferable(JComponent c)

      Creates a `Transferable` to use as the source for a data transfer. Returns the representation of the data to be transferred, or `null` if the component's property is `null`
      Parameters:
        `c` - the component holding the data to be transferred; provided to enable sharing of `TransferHandler`s
      Returns:
        the representation of the data to be transferred, or `null` if the property associated with `c` is `null`

    - #### exportDone

          protected void exportDone(JComponent source,
                                    Transferable data,
                                    int action)

      Invoked after data has been exported. This method should remove the data that was transferred if the action was `MOVE`.

      This method is implemented to do nothing since `MOVE` is not a supported action of this implementation (`getSourceActions` does not include `MOVE`).

      Parameters:
        `source` - the component that was the source of the data
        `data` - The data that was transferred or possibly null if the action is `NONE`.
        `action` - the actual action that was performed

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