The operators + and - are called the additive operators. They have the same precedence and are syntactically left-associative (they group left-to-right).
AdditiveExpression:
MultiplicativeExpression
AdditiveExpression+MultiplicativeExpression
AdditiveExpression-MultiplicativeExpression
If the type of either operand of a + operator is String, then the operation is string concatenation.
Otherwise, the type of each of the operands of the + operator must be a primitive numeric type, or a compile-time error occurs.
In every case, the type of each of the operands of the binary - operator must be a primitive numeric type, or a compile-time error occurs.