Condition always true when reached in while loop

Question

I created a method to insert a Linked List node in the correct place of a sorted (increasing) linked list but I am having some problems.

public static void sortedInsert(LinkedListNode root, int value) {
        LinkedListNode newNode = new LinkedListNode(value);
        if (root == null || root.data >= value) {
            newNode.next = root;
        } else {
            LinkedListNode curr = root;
            while (curr.next.data < value && curr.next != null) {
                curr = curr.next;
            }
            newNode.next = curr.next;
            curr.next = newNode;
        }
    }

errors:

Exception in thread "main" java.lang.NullPointerException
    at LinkedLists.LinkedListProblems.sortedInsert(LinkedListProblems.java:188)

The curr.next != null portion is highlighted in intellij so I'm assuming that is causing the error. This error only occurs when the value I added is bigger than the last value of the sorted linked list

However when the iterate to the last node of the linked list and the value of that node is still less than the value of the parameter. Shouldn't that exit the while loop?

Answer 1

I think the problem is here

while (curr.next.data < value && curr.next != null) {
  curr = curr.next;
}

You are testing in the wrong order, first check for null -

while (curr.next != null && curr.next.data < value) {
  curr = curr.next;
}

Otherwise when curr.next is null it will have already tested curr.next.data < value which will throw a NullPointerException .

Answer 2

看起来你的第一个应该是！= null，它应该是&&，否则即使root为null也可以对root.data进行求值。

if (root != null && root.data >= value) {

Answer 3

You are getting an exception on that line because cur.next.data gets tested before cur.next!=null .

Swap them over and the && will be short circuited correctly avoiding the exception.