CODEWITHSUNDEEP
javalinked-list
Liondancer
Liondancer

Reputation: 16469

Condition always true when reached in while loop

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?

Upvotes: 5

Views: 4067

Answers (3)

kristianp
kristianp

Reputation: 5895

It looks like your first if should be != null, and it should be an &&, otherwise you can have root.data being evaluated even when root is null.

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

Upvotes: 2

aa333
aa333

Reputation: 2576

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.

Upvotes: 2

Elliott Frisch
Elliott Frisch

Reputation: 201447

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.

Upvotes: 6

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