In java 8 using stream API, how to return instance from Map with multiple calculations required

Question

Suppose there is class like this:

class A {

    long sent;
    long received;
    double val; // given as max {(double)sent/someDenominator,(double)received/someDenominator}
}

of which there are number of instance references in Map<String , A>.

Is it possible in one go, using stream API, to return instance of class A with following properties:

• sent = sum of sent fields from all instances
• received = sum of received fields from all instances in Map
• val = maximum value of val, given all entries where val = max {sent/someDenominator,received/someDenominator}

What would be trivial task using standard for loop and one iteration, i don't have a clue how to achieve with stream API.

Answer 1

You could use reduce:

Optional<A> a = map.values()
                   .stream()
                   .reduce((a1, a2) -> new A(a1.sent + a2.sent, a1.received + a2.received, Math.max(a1.val, a2.val)));