I need a if-statement for drag and drop

I need a if-statement for my drag and drop code:

If the user drag the item into a empty box, all is okay. The item will show in the empty box (this can do my code below).

But, if the user drag the item into a non-empty-box, the item in the non-empty-box will exchange with the drag item. My code drag the item in the box also the box is not empty :-(

And my second Problem: If the user drag the item anywhere on the screen, the item disappear :-(

@Override
public boolean onDrag(View v, DragEvent event) {
    switch (event.getAction()) {
        case DragEvent.ACTION_DRAG_STARTED:
            // Do nothing
            break;
        case DragEvent.ACTION_DRAG_ENTERED:
            v.setBackground(enterShape);
            break;
        case DragEvent.ACTION_DRAG_EXITED:
            v.setBackground(normalShape);
            break;
        case DragEvent.ACTION_DROP:
            // view dropped, reassign the view to the new ViewGroup
            View view = (View) event.getLocalState();
            ViewGroup owner = (ViewGroup) view.getParent();
            owner.removeView(view);
            LinearLayout container = (LinearLayout) v;
            container.addView(view);
            view.setVisibility(View.VISIBLE);
            break;
        case DragEvent.ACTION_DRAG_ENDED:
            v.setBackground(normalShape);
        default:
            break;
    }
    return true;
}