How can I copy a file and paste it to the clipboard using Java?

Question

How can I copy a file and paste it to the clipboard using Java? My program can copy but it cannot paste. It gives

Exception in thread "main" java.lang.ClassCastException:
java.util.Arrays$ArrayList cannot be cast to java.io.File

My code:

class FileTransferable implements Transferable {
    private final File file;

    public FileTransferable(File file) {
        this.file = file;
    }

    @Override
    public DataFlavor[] getTransferDataFlavors() {
        return new DataFlavor[] { DataFlavor.javaFileListFlavor };
    }

    @Override
    public boolean isDataFlavorSupported(DataFlavor flavor) {
        return DataFlavor.javaFileListFlavor.equals(flavor);
    }

    @Override
    public Object getTransferData(DataFlavor flavor) throws UnsupportedFlavorException, IOException {
        final ArrayList<File> files = new ArrayList<File>();
        files.add(file);
        return files;
    }
}

Answer 1

Here is a full example taken from the DGuitar source code were I have removed the custom or specific Details based on your comment

/*
 * ADropTargetListener
 * Created on 20/04/2005
 *
 *
 */

import java.awt.datatransfer.DataFlavor;
import java.awt.datatransfer.Transferable;
import java.awt.datatransfer.UnsupportedFlavorException;
import java.awt.dnd.DnDConstants;
import java.awt.dnd.DropTargetDragEvent;
import java.awt.dnd.DropTargetDropEvent;
import java.awt.dnd.DropTargetEvent;
import java.awt.dnd.DropTargetListener;
import java.io.File;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;


/**
 * The Drop action happens in this order: 1. dragEnter =
when the mouse enters
 * the component 2. dragOver = after the mouse has entered
but has not been
 * released 3. drop = when the mouse is released
 * 
 * @author Mauricio Gracia G
 */
public class ADropTargetListener implements DropTargetListener {
    /**
     * creates a DropTargetListener 
     */
    public ADropTargetListener() {

    }

    /**
     * overrides the DROP method.
     * 
     * @see java.awt.dnd.DropTargetListener#drop(java.awt.dnd.DropTargetDropEvent)
     */
    public void drop(DropTargetDropEvent dtde) {
        Transferable transfer;

        transfer = dtde.getTransferable();

        //we must accept the transfer to process it
        dtde.acceptDrop(DnDConstants.ACTION_COPY_OR_MOVE);

        importData(transfer);
    }
    /**
     * Imports the data of the Drop action
     * @param t the transferable object
     */

    public void importData(Transferable t) {
        if (canImport(t.getTransferDataFlavors())) {
            try {
                DataFlavor[] flavors = t.getTransferDataFlavors();
                for (int i = 0; i < flavors.length; i++) {
                    //       Drop from Windows
                    if (DataFlavor.javaFileListFlavor.equals(flavors[i])) {

                        Object gtd ;

                        gtd = t.getTransferData(DataFlavor.javaFileListFlavor) ;
                        if (gtd instanceof List) {
                            @SuppressWarnings("unchecked")
                            List<File> fileList = (List<File>) gtd;

                                                        //YOUR CODE here to process the list of Files

                        }

                    }
                    //       Drop from GNOME or kde
                    // delimited by \n  (\r\n) on gnome
                    // will need to remove file:// at start
                    else if (DataFlavor.stringFlavor.equals(flavors[i])) {
                        if (t.getTransferData(DataFlavor.stringFlavor)
instanceof String) {
                            String path = (String) t
                                    .getTransferData(DataFlavor.stringFlavor);
                            List<File> fileList = convertStringsToFileList(path);
                            //YOUR CODE here to process the list of Files

                        }

                    }
                }
            } catch (UnsupportedFlavorException e) {
                e.printStackTrace();

            } catch (IOException e) {
                e.printStackTrace();

            }
        }
    }
    /**
     * converts Strings in certain format to a FileList 
     * @param filestr   the list of files in a single string
     * @return          a list of files
     */

    private ArrayList<File> convertStringsToFileList(String filestr) {
        ArrayList<File> files = new ArrayList<File>();
        String[] tokenizedFiles = filestr.split("\n");
        for (int i = 0; i < tokenizedFiles.length; i++) {
            String path = tokenizedFiles[i];
            if (path.startsWith("file://")) {
                if (path.endsWith("\r")) {
                    //appears to be the case for gnome but not kde
                    path = path.substring(7);
                    path = path.replaceAll("\r", "");

                    path = path.replaceAll("%20", " ");
                } else {
                    path = path.substring(7);
                }
            }
            files.add(new File(path));

        }
        return files;
    }
    /**
     * Determine if it can import certain flavor
     * 
     * @param flavors   an array of DataFlavors
     * @return true or not depending on the flavor
     */
    public boolean canImport(DataFlavor[] flavors) {
        for (int i = 0; i < flavors.length; i++) {
            if (DataFlavor.javaFileListFlavor.equals(flavors[i])) {
                return true;
            } else if (DataFlavor.stringFlavor.equals(flavors[i])) {
                return true;
            }
        }
        return false;
    }


    /**
     * method that is called when the drag starts or enters the window
     * @param dtde a DropTargetDragEvent
     * 
     * @see java.awt.dnd.DropTargetListener#dragEnter(java.awt.dnd.DropTargetDragEvent)
     */
    public void dragEnter(DropTargetDragEvent dtde) {
        //DEBUG
        //System.out.println("dragEnter" + ":" + dtde) ;
    }

    /**
     * a method that is called when the DRAG is OVER ??
     * 
     * @see java.awt.dnd.DropTargetListener#dragOver(java.awt.dnd.DropTargetDragEvent)
     */

    public void dragOver(DropTargetDragEvent dtde) {
        //      DEBUG
        //System.out.println("dragOver" + ":" + dtde) ;
    }

    /**
     * when the drop action is changes this method is called
     * 
     * @see java.awt.dnd.DropTargetListener#dropActionChanged(java.awt.dnd.DropTargetDragEvent)
     */
    public void dropActionChanged(DropTargetDragEvent dtde) {
        //      DEBUG
        //System.out.println("dropActionChanged" + ":" + dtde) ;
    }

    /**
     * method called when the drag exits the window
     * 
     * @see java.awt.dnd.DropTargetListener#dragExit(java.awt.dnd.DropTargetEvent)
     */
    public void dragExit(DropTargetEvent dte) {
        //      DEBUG
        //System.out.println("dragExit" + ":" + dte) ;
    }

}

Answer 2

I can't detect any problem in the actual code. I guess, that the cast error occurs where you actually receive the content from the copy/paste operation, where you receive a file, wrapped in a list and may cast the list to File.

Answer 3

Java cannot handle specific operating system-dependent operations like interfering clipboard etc. since it is a platform-independent language. In JLS and Java API there are no methods for clipboard interference, however there are methods to handle file system operations declared in Java API as a part of JLS (java language specification) which is implemented in most of JVM implementations.

To do such things, you should develop something in C/C++ (using Win32 API etc.) natively using JNI (Java Native Interface) or use off-the-shelf binaries and call them within your Java application.