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  1. #1
    f1gh is offline Member
    Join Date
    Nov 2010
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    Default adding children to a node that has List of Nodes

    Hi All:

    Okay i am trying to add elements to a List<Node <T>>, i.e. the list is a compilation of generic Nodes. Each node has a <T> data field, has a List<Node <T>> children list, and has Node<T> parent. Here is the code i am trying to write but keep getting NullPointerException:

    Java Code:
        public Node<T> addChild(T child) {
    
            Node toInsert = new Node(child);
           int numchild = getNumberOfChildren();
           System.out.println("This node has: "+numchild);
    
           //basically if this node doesn't have a parent Node
           if(this.parent ==null){
                //this line of code is where i am getting NullPointerException
                this.children.add(toInsert);
                toInsert.setParentNode(this);
                toInsert.children=null;
           }
           else
           {
            this.children.add(toInsert);
    
           }
    
           return this;
        }
    somewhere in my main method i am creating the node:
    Node<String> s1 = new Node<String>(new String("hello"));
    Node<String> s2 = s1.addChild(new String("jack"));

    and than ofcourse i further add nodes and childs so that end i almost have a list that branches out like a tree i.e.
    Java Code:
               hello
              /      \
            jack    nancy
             |       |
            John    jacob
    any help suggestions would be welcomed as i can't figure out the cause of the NullPointer unless the "this" reference is wrong.
    Last edited by f1gh; 11-29-2010 at 05:10 AM.

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