Booleans, conditional operators and autoboxing

The difference is that the explicit type of the returnsNull() method affects the static typing of the expressions at compile time:

E1: `true ? returnsNull() : false` - boolean (auto-unboxing 2nd operand to boolean)

E2: `true ? null : false` - Boolean (autoboxing of 3rd operand to Boolean)

See Java Language Specification, section 15.25 Conditional Operator ? :

• For E1, the types of the 2nd and 3rd operands are Boolean and boolean respectively, so this clause applies:

  If one of the second and third operands is of type boolean and the type of the other is of type Boolean, then the type of the conditional expression is boolean.

  Since the type of the expression is boolean, the 2nd operand must be coerced to boolean. The compiler inserts auto-unboxing code to the 2nd operand (return value of returnsNull()) to make it type boolean. This of course causes the NPE from the null returned at run-time.

• For E2, types of the 2nd and 3rd operands are <special null type> (not Boolean as in E1!) and boolean respectively, so no specific typing clause applies (go read 'em!), so the final "otherwise" clause applies:

  Otherwise, the second and third operands are of types S1 and S2 respectively. Let T1 be the type that results from applying boxing conversion to S1, and let T2 be the type that results from applying boxing conversion to S2. The type of the conditional expression is the result of applying capture conversion (§5.1.10) to lub(T1, T2) (§15.12.2.7).

  • S1 == <special null type> (see §4.1)
  • S2 == boolean
  • T1 == box(S1) == <special null type> (see last item in list of boxing conversions in §5.1.7)
  • T2 == box(S2) == `Boolean
  • lub(T1, T2) == Boolean

  So the type of the conditional expression is Boolean and the 3rd operand must be coerced to Boolean. The compiler inserts auto-boxing code for the 3rd operand (false). The 2nd operand doesn't need the auto-unboxing as in E1, so no auto-unboxing NPE when null is returned.

This question needs a similar type analysis:

Java conditional operator ?: result type

The line:

    Boolean b = true ? returnsNull() : false;

is internally transformed to:

    Boolean b = true ? returnsNull().booleanValue() : false; 

to perform the unboxing; thus: null.booleanValue() will yield a NPE

This is one of the major pitfalls when using autoboxing. This behavior is indeed documented in 5.1.8 JLS

Edit: I believe the unboxing is due to the third operator being of boolean type, like (implicit cast added):

   Boolean b = (Boolean) true ? true : false; 

From Java Language Specification, section 15.25:

• If one of the second and third operands is of type boolean and the type of the other is of type Boolean, then the type of the conditional expression is boolean.

So, the first example tries to call Boolean.booleanValue() in order to convert Boolean to boolean as per the first rule.

In the second case the first operand is of the null type, when the second is not of the reference type, so autoboxing conversion is applied:

• Otherwise, the second and third operands are of types S1 and S2 respectively. Let T1 be the type that results from applying boxing conversion to S1, and let T2 be the type that results from applying boxing conversion to S2. The type of the conditional expression is the result of applying capture conversion (§5.1.10) to lub(T1, T2) (§15.12.2.7).