Why does returning null (where a boolean is expected) as the result of a ternary operator compile? [duplicate]

Question

A curiosity that I've just noticed, rather than a problem.

I'm not allowed to write

public boolean x() {
  return null;
}

or this:

public boolean x() {
  if (DEBUG) {
    return true;
  } else {
    return null;
  }
}

but I am allowed to write

public boolean x() {
  return DEBUG ? true : null;
}

Why is this? (It appears to throw an NPE if the "else" branch is taken.)

Answer 1

As the jls states:

The type of a conditional expression is determined as follows: If the second and third operands have the same type (which may be the null type), then that is the type of the conditional expression. If one of the second and third operands is of primitive type T, and the type of the other is the result of applying boxing conversion (§5.1.7) to T, then the type of the conditional expression is T.

This means java allows null, since it can be used to generate an instance of Boolean, which can be unboxed to boolean (read the section about boxing in the jls for more info). But since the Boolean instance is initialized null, the call to booleanValue() will result in a NullPointerException.