ThisCanBeNull

Correction

Here is what's right.

The this variable (a special local variable) never is null. The variable exists during activations of instance methods, and it always points to the instance on which the method operates. That instance always exists, thus the this variable never is null.

Note that a call like Object o = null; o.m(); would not succeed; it would throw a NullPointerException, and thus it defintely would not lead to a situation where this is null.

Origin

Where could this misconception come from?

It is possible that this misconception originates in learning about recursive traversals over recursive structures. Students may see null as an object, and they may include checks for such a “base case” (if (this == null)) in implementations of methods of classes implementing recursive structures.

Symptoms

How do you know your students might have this misconception?

The following question from a paper-based exam lead to a considerable number of student submissions exhibiting this misconception:

// The following class represents a recursive tree data structure.
// All your methods must be implemented using recursion.
public class Node {
  private final Node left;
  private final Node right;
  private final int value;
  
  public Node(Node left, Node right, int value) {
    this.left = left;
    this.right = right;
    this.value = value;
  }

  /** Return the total number of nodes in this tree. */
  public int count() {
    // this body represents an implementation 
    // similar to those produced by students
    if (this == null) return 0;
    return left.count() + right.count() + 1;
  }
}

The following code was produced by a student as part of a linked list implementation (after seeing a different implementation using a sentinel/null object, and having to produce an implementation that does not use a null object to terminate the list):

public class IntList {

  private int value;
  private IntList tail;

  public int length() {
    if (this==null) {
      return 0;
    } else if (this.tail == null) {
      return 1;
    } else {
      this.tail.length();
    }
  }
}