Drag and Drop in Java

Question

I have been searching the internet to find examples or how to use java's drag and drop. I have found a few, but all the examples only allow you to drag into a specific location, ie another list box or text area. I want to know if there is a way to drag items onto a jpanel or similar container, having the item put anywhere freely on the container.

Answer 1

As long as the target is a supported drop target for the item you are dragging then you can drop it to containers like JPanel .

You control the way the dragged item is displayed at the drop location. If your panel overrides paintComponent() then you can paint the item however you find appropriate.

Answer 2

'is a way to drag items into a jpanel'

You can set a DropTarget to your JPanel.

public class MyDropTarget extends JPanel implements DropTargetListener {

    private MyImage image;
    private String text;

    public MyDropTarget() {
        setBackground(new Color(30,60,10));
        this.setBorder( BorderFactory.createBevelBorder( BevelBorder.LOWERED, new Color(30,60,10).brighter(), new Color(30,60,10).darker() ) );
        DropTarget dt = new DropTarget(this, DnDConstants.ACTION_COPY_OR_MOVE, this, true, null);
        this.setDropTarget( dt );
    }

    @Override
    public void paintComponent( Graphics g ) {
        super.paintComponent( g );
        if( image != null && image.getImage() != null ) {
            g.drawImage( image.getImage(), 0, 0, null );
            if(image.isError()){
                g.setColor(Color.BLACK);
                g.drawString( text, 0, 0 );
            }
        }
    }

    public void dragEnter( DropTargetDragEvent dtde ) {
        this.setBorder( BorderFactory.createBevelBorder( BevelBorder.RAISED, Color.RED.brighter(), Color.RED.darker() ) );
    }

    public void dragExit( DropTargetEvent dte ) {
        this.setBorder( BorderFactory.createBevelBorder( BevelBorder.LOWERED, UIManager.getColor( "MenuBar.highlight" ), UIManager.getColor( "MenuBar.shadow" ) ) );
    }

    public void dragOver( DropTargetDragEvent dtde ) {
    }

    public void drop( DropTargetDropEvent dtde ) {
        try {           
            text  = (String) dtde.getTransferable().getTransferData( DataFlavor.stringFlavor );
            image = (MyImage)dtde.getTransferable().getTransferData( DataFlavor.imageFlavor );
            this.setBorder( BorderFactory.createBevelBorder( BevelBorder.LOWERED, new Color(30,60,10).brighter(), new Color(30,60,10).darker() ) );
            dtde.dropComplete( true );
            repaint();
        } catch( UnsupportedFlavorException e ) {
            e.printStackTrace();
        } catch( IOException e ) {
            e.printStackTrace();
        } 
        this.setDropTarget( null );
    }

    public void dropActionChanged( DropTargetDragEvent dtde ) {
    }
}

Answer 3

I implemented Drag&Drop in this way:

The quite convenient mechanism for the implementation of Drag&Drop has appeared in Java 6, but it does have its disadvantages. For example, you should explicitly specify a Drop Target, what is not very useful when you need to lay down the object near the Drop Target. Also in the standard implementation there is no guarantee of execution order of listeners' methods. I'll tell you the concept of implementing a more extensible Drag&Drop.

Initially the mouse listeners (Mouse Listener and MouseMotionListener) should be assigned to all Drag Sources. It's need to implement 3 methods: a method of mouse click on the object, a method of moving the mouse while holding the mouse button on the object (mouseDragged in MouseMotionListener) and the mouse up method.

The listeners assignment looks as follows:

component.addMouseListener(new MouseAdapter() {
    @Override
    public void mousePressed(MouseEvent e) {
        //block click right mouse button
        if (MouseEvent.BUTTON1 == e.getButton()) {
            startDrag(e);
        }
    }

    @Override
    public void mouseReleased(MouseEvent e) {
        //block click right mouse button
        if (MouseEvent.BUTTON1 == e.getButton()) {
            endDrag(e);
        }
    }
});

component.addMouseMotionListener(new MouseMotionAdapter() {
    @Override
    public void mouseDragged(MouseEvent e) {
        drag(e);
   }
});

Accordingly when you click on the object Drag&Drop starts, when you move the mouse the object should be moved, when you release the mouse the object should change its position and be moved to a new container. If the object will be moved in the frame of one container, then it is possible to implement only mouseDragged () method, in which the coordinates of the dragged object will be changed:

@Override
public void mouseDragged(MouseEvent e) {
    Point mouseLocation = e.getLocationOnScreen();
    Component draggedComponent = (Component) e.getSource();
    SwingUtilities.convertPointFromScreen(mouseLocation, 
draggedComponent.getParent());
    draggedComponent.setLocation(mouseLocation);
}

But dragged object coordinates can be set relative to the container in which it is located. Accordingly, when the mouse is moved to another container it is necessary to add a component to a new container and to calculate the new coordinates, etc. This method is not very beautiful and extensible, so I suggest using GlassPane to display the dragged object.

The algorithm looks as follows:

• Click on the object.

• Get a screenshot of the object (see how to make a screenshot ). Hide the original object.

• Draw on glassPane a screenshot of the object, based on the coordinates of the mouse.

• When you move the mouse you need to redraw a screenshot according to the new coordinates.

• When you release the mouse you need to place the object on the container under which the cursor is located.

• Display the original object.

With this approach, we have no any dependences on the container on which the cursor should be placed to make Drop and correspondingly the object can be "Dropped" anywhere.

GlassPane with transparency effect:

public class GhostGlassPane extends JPanel {
    private final AlphaComposite composite;
    private BufferedImage ghostImage = null;
    private Point location = new Point(0, 0);

    public GhostGlassPane() {
        setOpaque(false);
        composite = AlphaComposite.getInstance(AlphaComposite.
SRC_OVER, 0.7f);
}
public void paintComponent(Graphics g) {

    if (ghostImage == null)
        return;

    Graphics2D g2 = (Graphics2D) g;
    g2.setComposite(composite);
    g2.drawImage(ghostImage, (int) (location.getX()),
(int) (location.getY()), null);

    }
}

In this response only concept of implementation is given.

This information is taken from my article: Frequently Asked Questions during Java applet development