Booleans, conditional operators and autoboxing

Question

Why does this throw NPE

public static void main(String[] args) throws Exception {
    Boolean b = true ? returnsNull() : false; // NPE on this line.
    System.out.println(b);
}

public static Boolean returnsNull() {
    return null;
}

while this doesn't

public static void main(String[] args) throws Exception {
    Boolean b = true ? null : false;
    System.out.println(b); // null
}

?

The solution is by the way to replace false by Boolean.FALSE to avoid null being unboxed to boolean --which isn't possible. But that isn't the question. The question is why? Are there any references in JLS which confirms this behaviour, especially of the 2nd case?

Answer 1

The line:

    Boolean b = true ? returnsNull() : false;

is internally transformed to:

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

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

This is one of the major pitfalls when using autoboxing. This behavious 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; 

Answer 2

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).

Answer 3

The difference is static typing of the expressions at compile time:

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

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

See Java Language Specification, section 15.25 Conditional Operator ? :

• For E1, 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.

  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, 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).

  The compiler inserts auto-boxing code for the 3rd operand (false). The 2nd operand is already Boolean, so no auto-boxing NPE.

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This question needs a similar type analysis:

http://stackoverflow.com/questions/2615498/java-conditional-operator-result-type