Java autoboxing and ternary operator madness
Ternary expressions, like any expression, have a type that is determined by the compiler. If the two sides of the ternary expression have what looks like different types, then the compiler will try and find a common base type using the least ambiguous of the two options. In your case, the -1
is least ambiguous, and so the type of the ternary expression is int
. Sadly, the compiler doesn't use type inference based on the receiving variable.
The expression rsrqs.get(boxedPci.toString())
is then evaluated and forced into type int
to match the ternary expression, but because it's null
it throws the NPE.
By boxing the -1
, the value of the ternary expression is Integer
, and so you're null-safe.
The explanation can be concluded from the information in java language specification: 15.25. Conditional Operator ? :.
From the table there, you get the information, that, if the second operand (rsrqs.get(boxedPci.toString())
) is of type Integer
and the third operand is of type int
, the result will be of type int
.
That however means, that
Integer boxedRsrq = boxedPci != null ? rsrqs.get(boxedPci.toString()) : -1;
is semantically the same as
Integer boxedRsrq = boxedPci != null ? ((int)rsrqs.get(boxedPci.toString())) : -1;
But that means you get a NullPointerException
, if you get null
from the map, which obviously happens.
If you cast the third operand to Integer
, the second operand will never be cast to int
and no NPE happens.