Question

Run the following Java code:

boolean b = false;
Double d1 = 0d;
Double d2 = null;
Double d = b ? d1.doubleValue() : d2;

Why is there a NullPointerException?

Answer 1

The return type of the conditional expression b ? d1.doubleValue : d2 is double. A conditional expression must have a single return type. Following the rules for binary numeric promotion, d2 is autounboxed to a double, which causes a NullPointerException when d2 == null.

From the language spec, section §15.25:

Otherwise, if the second and third
operands have types that are
convertible (§5.1.8) to numeric types,
then there are several cases: …

Otherwise, binary numeric promotion (§5.6.2) is applied
to the operand types, and the type of
the conditional expression is the
promoted type of the second and third
operands. Note that binary numeric
promotion performs unboxing conversion
(§5.1.8) and value set conversion
(§5.1.13).