implementazione di linked tree. manca il tostring

tree/TreeNode.java

@@ -1,5 +1,6 @@
 package tree;
 
+import position.NodePositionList;
 import position.Position;
 import position.PositionList;
 
@@ -13,13 +14,15 @@ public class TreeNode<E> implements TreePosition<E> {
 
     private E element = null;
     private TreePosition<E> parent = null;
-    private PositionList<Position<E>> children= null;
+    private PositionList<Position<E>> children=  new NodePositionList<Position<E>>();
 
 
     public TreeNode(E in_element, TreePosition<E> in_parent, PositionList<Position<E>> in_children ){
         element = in_element;
         parent = in_parent;
-        children = in_children;
+        if (in_children != null){
+            children = in_children;
+        }
     }
 
 
@@ -35,7 +38,9 @@ public class TreeNode<E> implements TreePosition<E> {
 
     @Override
     public void setChildren(PositionList<Position<E>> c) {
-        children = c;
+        if (c != null){
+            children = c;
+        }
     }
 
     @Override