Populating an N-ary Tree- Java

Question

I'm implementing a N-ary tree in Java; each node can have as many nodes as possible. The problem comes when I try to build a tree. I have a function that recursively creates a tree of a specific height, and assigns the child nodes based from a List of Nodes. When I call the function the root node dose not contain any data; it returns a null once it is finished. I have never implemented an N-ary tree so I'm somewhat sure of the implementation, any pointers on it would really be appreciated!

//Creates a tree 
void createTree(int height) throws InvalidMoveException,   
  Modified_Tree.InvalidMoveException {
    root = new ListNode();
    root = populateTree(moves.root,height,true,0);  
}

 //Function called by Create tree to populate the tree
 //It takes in a ListNode, an int height that determines the height of the tree, 
 //and a boolean, which is used
 //To know whether the node is a black piece/max or a white piece/min

 public ListNode populateTree(ListNode root, int height, boolean black, int score) 
  {

    ListNode node = root;
     List<ListNode> nodes = new List<ListNode>(10);

     //Sets the size of List in node to the int taken
              node.setChildNumber(nodes.size());

    //return if reached the pre-maximum height
    if(height == 1){

        for(int i = 0; i < nodes.size(); i++){

            //Add the child to the last node
            node.addChild(nodes.get(i));

        }

        return node;
    }

    else{

              for(int j =0; j < node.getChildNumber(); j++){
      node = populateTree(node.getChildAt(j),height-1,!black,score);

        }   
    }
    return node;
 }

Any help is really appreciated!

Answer 1

Your problem is here:

List<ListNode> nodes = new List<ListNode>(10);

First, I assume you meant new ArrayList<ListNode>(10); or some other concrete implementation of List<T> . Second, the argument 10 only ensures that you will have 10 locations initially. It does not mean that you will have 10 ListNode instances automatically initialized inside nodes . Then you have:

    for(int i = 0; i < nodes.size(); i++){
        //Add the child to the last node
        node.addChild(nodes.get(i));
    }

This loop will never execute because nodes.size() is zero due to it not containing any nodes at all. So you need to initialized your list with ListNode instances first.