Cannot use add() method for object list-CodePudding

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".

CodePudding user response：

   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.)