Because the Java Language Specification says so
Binary numeric promotion is performed on the operands (§5.6.2).
Note that binary numeric promotion performs value set conversion
(§5.1.13) and may perform unboxing conversion (§5.1.8).
The type of an additive expression on numeric operands is the promoted
type of its operands.
and, regarding numeric promotion,
Widening primitive conversion (§5.1.2) is applied to convert either or
both operands as specified by the following rules:
- [...]
- Otherwise, both operands are converted to type
int.
So the byte values are promoted to int values and added up. The result of the expression is the promoted type, therefore an int.
You can simply cast the result
byte z = (byte) (b + a);
but careful with overflow/underflow.
