Cannot use add() method for object list

Question

I have a Node class and I want each Node to have a list of its leaves. For this, I create an instance variable ArrayList(). When I try to add a Node to this list I get a NullPointerException: Cannot invoke "java.util.ArrayList.add(Object)" because "this.nextSibling" is null. Why is this happening?

This is my code:

public class Node {

   private String name;
   private Node firstChild;
   private ArrayList<Node> nextSibling;

   Node (String n, Node d, ArrayList<Node> r) {
      this.name = n;
      this.firstChild = d;
      this.nextSibling = r;
   }

   Node() {
      this("", null, null);
   }

   Node (String s) {
      this(s, null, null);
   }

   Node (String s, Node p) {
      this(s, p, null);
   }

   Node(String s, ArrayList<Node> lst) {
      this(s, null, lst);
   }

   public void setFirstChild (Node d) {
      firstChild = d;
   }

   public void addNextSibling (Node r) {
         nextSibling.add(r);
   }

   public static Node parsePostfix (String s) {
      if (checkForNoData(s)) {
         throw new RuntimeException("Invalid input: " + s);
      }
      Stack<Node> nodeStack = new Stack<>();

      List<String> nodeNamesCheck = new ArrayList<>();
      List<String> nodeNames = getNodeNames(s);
      int y = nodeNames.size() - 1;

      Node root = new Node(nodeNames.get(y));

      nodeStack.push(root);
      nodeNamesCheck.add(nodeNames.get(y));

      for (int i = s.length() - 1; i >= 0; i--) {
         if (s.charAt(i) == ')') {
            y--;
            Node firstChild = new Node(nodeNames.get(y), new ArrayList<>());
            nodeNamesCheck.add(nodeNames.get(y));
            nodeStack.peek().setFirstChild(firstChild);
            nodeStack.push(firstChild);
         } if (s.charAt(i) == ',') {
            if (!nodeStack.empty())
               nodeStack.pop();
            y--;
            Node nextSibling = new Node(nodeNames.get(y), new ArrayList<>());
            nodeNamesCheck.add(nodeNames.get(y));
            nodeStack.peek().addNextSibling(nextSibling);
            nodeStack.push(nextSibling);
         } else if (s.charAt(i) == '(') {
            if (!nodeStack.empty()) {
               nodeStack.pop();
            }
         }
      }
      if (nodeNames.size() != nodeNamesCheck.size() && nodeNames.size() != 1)
         throw new RuntimeException("Invalid input: " + s);
      return root;
   }
}

The parsePostfix(String s) method takes a tree as a string and re-creates this with Nodes. An example input would be "(H,G,F)E,(D,C,)B)A".

This is confusing. You say you want each node to store an array of its leaves, but then your node instances have a reference called nextSibling. To me the concept of next sibling has really nothing to do with the concept "all my leaves". What would it mean for the first entry in that array list to be a node that also has an array list? Are you really going to duplicate all sibling references in all their array lists? And how does this relate with leaves? Please clarify. — trincot
– trincot, Commented Oct 31, 2022 at 18:50

Louis Wasserman · Accepted Answer · 2022-10-31 18:57:58Z

2

   private ArrayList<Node> nextSibling;

   Node (String n, Node d, ArrayList<Node> r) {
      this.name = n;
      this.firstChild = d;
      this.nextSibling = r;
   }

   Node() {
      this("", null, null);
   }

   Node (String s) {
      this(s, null, null);
   }

   Node (String s, Node p) {
      this(s, p, null);
   }

You are, very explicitly, setting nextSibling to null in the last three constructors. So of course this.nextSibling is null.

If you don't want that to happen, you will need to set it to something other than null, like new ArrayList<>().

(As @trincot mentions, your variable names also seem confusing, as nextSibling is a strange name for a list of leaves -- which would be children, not siblings.)

answered Oct 31, 2022 at 18:57

Louis Wasserman

200k26 gold badges367 silver badges437 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Cannot use add() method for object list

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related