It looks to be an oddity in the Java Language Specification:

"If an integer operator other than a shift operator has at least one operand of type long, then the operation is carried out using 64-bit precision, and the result of the numerical operator is of type long. If the other operand is not long, it is first widened (§5.1.5) to type long by numeric promotion (§5.6).

Otherwise, the operation is carried out using 32-bit precision, and the result of the numerical operator is of type int. If either operand is not an int, it is first widened to type int by numeric promotion."

Which I take to mean:

short + byte = int

short + short = int

int + short = int

int + int = int

int + long = long

long + long = long

I find that a bit inconsistent, to be honest, but that is the way that it's defined.

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