The reason why the member variable of the node to be deleted from the doubly linked list should be initialized to null

Question

public class Node<E> {
    E data;
    Node<E> left;
    Node<E> right;
}

The code below deletes a node at a specific index from a doubly linked list. At the end of the code, left, right, and data of the node to be deleted must all be initialized to null. Is this a necessary process? What can happen if you don't initialize to null

public void remove(int index) {

    Node foundNode = findNode(index);
    Node leftNode = foundNode.left;
    Node rightNode = foundNode.right;

    leftNode.right = rightNode;
    rightNode.left = leftNode;

    foundNode.left = null;
    foundNode.right = null;
    foundNode.data = null;

    --size;
}

Answer 1

In "normal" circumstances the three last assignments with null are not needed. When the function completes, its local variables are deallocated, and so there will usually not be any reference to the node that was referenced by foundNode.

However, it could be that in other code -- not part of this function -- there is still a variable that references that node, and then it is not that bad to indicate that this node is no longer part of a (longer) list. By setting left and right to null, the following invariant is maintained for every node:

1. Either node.left == null, or
2. node.left.right == node

...and similarly:

1. Either node.right == null, or
2. node.right.left == node

If we do not set foundNode.left = null, then the above invariant is not restored, and this could be an issue if there is still a reference to foundNode outside of the function's execution context.

As to foundNode.data = null: that should not really be necessary. If the node referenced by foundNode is not referenced elsewhere, then foundNode can be garbage collected. And if there is also no other reference to its data, then also that data can be garbage collected. Setting this property to null does not really change that process. Secondly, if we consider the case where there still might be a reference to this node, then null might be data that is actually meaningful in the overall algorithm, so setting it to null is not guaranteed to mark it in an unambiguous way as a "deleted" node.

Answer 2

To be entirely clear: new Node() would initialize the node fields left and right with null.

That remove sets all fields of the removed foundNode to null is a misguided attempt to optimize garbage collection.

You do not create lingering objects (other removed nodes), no memory leak. As a garbage collector can deal with collections of unreachable objects.

Mind: for other languages this might not hold.