Question

Why does the following line of code produce a NullPointerException?

Long v = 1 == 2 ? Long.MAX_VALUE : (Long) null;

I understand that unboxing is being performed on null, but why?

Note that

Long v = (Long) null;

Does not produce the Exception.

Answer 1

So it looks obvious that you only have to box if the condition is true, and there should be no boxing if the condition is false. However the ternary operator expression must have a particular static type. So we have Long and long. The JLS states that the result will be the primitive (just as well – imagine if the operator was, say, + or even ==). So the ternary operator will force the unboxing, and only then does the assignment cause a boxing.

If you were to replace the code with the equivalent if-else, then you’d just have an assignment from long to Long and from Long to Long, which wouldn’t have any unboxing and so run fine.

IIRC, this is covered is Bloch & Gafter’s Java Puzzlers.