ThisNoExpression

Correction

Here is what's right.

this is an expression that evaluates to an object reference. The type of this is the enclosing class.

class Count {
  private final int value;
  public Count(final int value) {
    this.value = value;
  }
  public Count countDownIfPossible() {
    if (value>0) {
      return new Count(value-1);
    } else {
      return this;  // perfectly fine: type of this is Count
    }
}

Origin

Where could this misconception come from?

Students initially only see this being used to access instance variables (e.g., this.x = 5;). Thus they develop the idea that this. is a magical prefix with the sole purpose of qualifying instance variable accesses.

Symptoms

How do you know your students might have this misconception?

Students may believe that this on its own doesn’t have any meaning. They might express statements that treat pieces of code like return this or m(this) or x = this as wrong.

Another simple related symptom shows when they do not realize that this.m() is a proper way to invoke a method m() on the same object.

Value

How can you build on this misconception?

This misconception provides an opportunity to discuss the concept of expressions and to make students aware of the power of the idea of composing expressions from other expressions.