-a
--a1
--a2
-b
--b1
--b2

tree.setOnDragDetected...
tree.setOnDragOver...
tree.setOnDragDropped ( new EventHandler<DragEvent>() {
 @Override public void handle(DragEvent event) {
  Dragboard db=event.getDragboard();
   boolean success=false;
   if ( event.getDragboard().hasString() ) {
System.out.println("déplacer "+db.getString());
    success=true;
   }
   event.setDropCompleted(success);
   event.consume();
  }
 }
);