ReferenceToBooleanCoercion

DRAFT

Correction

Here is what's right.

Java separates reference types and boolean types. There is no implicit coercion from arbitrary reference types to boolean.

Pacman pac = ...;

boolean b = pac; // compiler error

As Section 4.2.5 of the Java Language Specification describes, to convert an arbitrary reference to a boolean, one can use comparison operators:

if (i != null) {
  ...
}

Exception: Auto-Unboxing

The one exception is the auto-unboxing of a reference to a Boolean object to boolean:

boolean b;

b = new Boolean(true);                  // b == true
b = new Boolean(false);                 // b == false

This even works for expressions of type Object, which is a supertype of Boolean:

boolean b;

Object o = new Boolean(true);
b = (boolean)o;               // b == true

However, when auto-unboxing null will not be converted to false, but instead the unboxing will cause a NullPointerException:

boolean b;

b = (Boolean)null;         // throws NullPointerException
b = (boolean)null;         // compiler error
b = (boolean)(Object)null; // throws NullPointerException

Origin

Where could this misconception come from?

Students may incorrectly transfer their understanding of Python or JavaScript, where conditional statements to test for nullness of reference variables (like if o: in Python or if (o) in JavaScript) are common.

Symptoms

How do you know your students might have this misconception?

This misconception may surface especially in the context of conditionals.

In the following code, the condition in the if-statement seems to be used as a null check. Students may incorrectly believe that this code compiles.

void m(Ghost g) {
  if (g) {
    g.work();
  }
}