Problem with dragging and dropping JLabel

[LianaN]

Hi!

I'm trying to add dragging and dropping behaviour to JLabel. It works only when I press the mouse for the first time. If I continue pressing the mouse, the error message shown below is generated. Please, help me to find an error.

JLabel bkg = new JLabel(icon);
jLayeredPane.add(bkg, new Integer(2), 0);
DragDropListener ddList = new DragDropListener();
bkg.addMouseListener(ddList);
bkg.addMouseMotionListener(ddList);

...

   public class DragDropListener extends MouseAdapter implements MouseListener, MouseWheelListener, MouseMotionListener {
        public Component dragComponent;
        private int xAdjustment;
        private int yAdjustment;

        public DragDropListener(){
            super();
            //addMouseListener(this);
            //addMouseMotionListener(this);
        }

        @Override
        public void mouseClicked(MouseEvent e){
            System.out.println("clicked: " + dragComponent);
            if (e.getClickCount() == 2)
            {
                if (e.getComponent()!=null) {
                   MainClass.getBackgroundPane().remove(e.getComponent());
                   MainClass.getBackgroundPane().repaint();
                }
            }
        }

        @Override
        public void mouseMoved(MouseEvent e) {

        }

        @Override
        public void mousePressed(MouseEvent e)
        {      
            dragComponent = null;
            Component c = MainClass.getBackgroundPane().findComponentAt(e.getComponent().getX(), e.getComponent().getY());
            if (c instanceof LayeredPane) return;
            dragComponent = c;
            xAdjustment = dragComponent.getLocation().x - e.getX();
            yAdjustment = dragComponent.getLocation().y - e.getY();
            dragComponent.setLocation(e.getX() + xAdjustment, e.getY() + yAdjustment);
            MainClass.getBackgroundPane().moveToFront(dragComponent);
        }

        @Override
        public void mouseDragged(MouseEvent e)
        {
            if (dragComponent == null) return;
            dragComponent.setLocation(e.getX() + xAdjustment, e.getY() + yAdjustment);
        }

        @Override
        public void mouseReleased(MouseEvent e)
        {
            dragComponent = null;
        }
   }

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
        at swarovsky_api_pack.DragDropListener.mousePressed(DragDropListener.java:56)
        at java.awt.Component.processMouseEvent(Component.java:6260)
        at javax.swing.JComponent.processMouseEvent(JComponent.java:3267)
        at java.awt.Component.processEvent(Component.java:6028)
        at java.awt.Container.processEvent(Container.java:2041)
        at java.awt.Component.dispatchEventImpl(Component.java:4630)
        at java.awt.Container.dispatchEventImpl(Container.java:2099)
        at java.awt.Component.dispatchEvent(Component.java:4460)
        at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4574)
        at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4235)
        at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4168)
        at java.awt.Container.dispatchEventImpl(Container.java:2085)
        at java.awt.Window.dispatchEventImpl(Window.java:2478)
        at java.awt.Component.dispatchEvent(Component.java:4460)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)

Line 56 in the DragDropListener class is the following:

xAdjustment = dragComponent.getLocation().x - e.getX();

[quuba]

Look at standard java \jdk1.7.0\demo\applets\GraphLayout folder
used variables:
- boolean pickfixed;
- int numMouseButtonsDown;
and methods:
- removeMouseMotionListener(this);
- addMouseMotionListener(this);

or alternatively in place of clicked use left and right button (with key modifiers)

what is mouseClicked?
mousePressed
mouseReleased
mousePressed
mouseReleased

Test this on simple mouse program similar to line 22

[LianaN]

Ok, thanks. I'll try your variants a bit later. Now I want to arrange all details with my code snippet. After some experiments, I found out that the problem occurs when I try to drag JLabel component outside of the JLayeredPane area. I tried "try..catch", but it did not help.

@Override
        public void mousePressed(MouseEvent e)
        {      
            dragComponent = null;
            Component c = MainClass.getBackgroundPane().findComponentAt(e.getComponent().getX(), e.getComponent().getY());
            if (c instanceof LayeredPane) return;
            dragComponent = c;
            try {
              xAdjustment = dragComponent.getLocation().x - e.getX();
              yAdjustment = dragComponent.getLocation().y - e.getY();
              dragComponent.setLocation(e.getX() + xAdjustment, e.getY() + yAdjustment);
              MainClass.getBackgroundPane().moveToFront(dragComponent);
            } catch (Exception ex) {
                dragComponent.setLocation(e.getComponent().getX(), e.getComponent().getY());
                MainClass.getBackgroundPane().moveToFront(dragComponent);
            }
        }

So, how could I prevent this error?

[LianaN]

Well, now it's working. But the problem is that when I add multiple JLabel components on JPanel, then I can drag and drop only the last one (that is on top). Someone knows how to solve this problem??!

public class DragDropListener extends MouseAdapter implements MouseListener, MouseMotionListener
{
	Component dragComponent;
	int xAdjustment;
	int yAdjustment;

	public DragDropListener() {}

        @Override
	public void mousePressed(MouseEvent e)
	{
		dragComponent = null;
		Component c =  MainClass.getBeadBoard().findComponentAt(e.getX(), e.getY());

		if (c instanceof JPanel) return;

		dragComponent = c;
		xAdjustment = dragComponent.getLocation().x - e.getX();
		yAdjustment = dragComponent.getLocation().y - e.getY();
		dragComponent = (JLabel)c;
		dragComponent.setLocation(e.getX() + xAdjustment, e.getY() + yAdjustment);
		dragComponent.setSize(dragComponent.getWidth(), dragComponent.getHeight());
		MainClass.getBackgroundPane().add(dragComponent, JLayeredPane.DRAG_LAYER);
	}

        @Override
	public void mouseDragged(MouseEvent me)
	{
		if (dragComponent == null) return;

		dragComponent.setLocation(me.getX() + xAdjustment, me.getY() + yAdjustment);
	 }

        @Override
	public void mouseReleased(MouseEvent e)
	{
		if (dragComponent == null) return;

		//dragComponent.setVisible(false);
		Component c =  MainClass.getBeadBoard().findComponentAt(e.getX(), e.getY());

		if (c instanceof JLabel)
		{
			Container parent = c.getParent();
			parent.remove(0);
			parent.add( dragComponent );
		}
		else
		{
			Container parent = (Container)c;
			parent.add( dragComponent );
		}

		dragComponent.setVisible(true);
	}

        @Override
	public void mouseClicked(MouseEvent e) {}
        @Override
	public void mouseMoved(MouseEvent e) {}
        @Override
	public void mouseEntered(MouseEvent e) {}
        @Override
	public void mouseExited(MouseEvent e) {}

}

[LianaN]

I changed mouseReleased method, and now I can add multiple JLabel components to JPanel and drag them separately:

@Override
	public void mouseReleased(MouseEvent e)
	{
		if (dragComponent == null) return;
	Component c =  MainClass.getBeadBoard().findComponentAt(e.getX(), e.getY());
                Container parent = (Container)c;
		parent.add( dragComponent );
		dragComponent.setVisible(true);
	}