How does Java 8 mapToInt ( mapToInt(e -> e) )improves performance exactly?

Question

I\'m reading the book \"Java 8 Lambdas\", and at some point the author says \"It’s a good idea to use the primitive specialized functions wherever possible because of the perfor

Answer 1

There's an extra level of boxing going on in

int sum1 = list.stream().reduce(0, (acc, e) -> acc + e).intValue();

as the reduction function is a BinaryOperator<Integer> - it gets passed two Integer values, unboxes them, adds them, and then re-boxes the result. The mapToInt version unboxes the Integer elements from the list once and then works with primitive int values from that point on as an IntStream.