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.