public static <T, U, A, R> Collector<T, ?, R> mapping (Function<? super T, ? extends U> mapper, Collector<? super U, A, R> downstream)

Adapts a Collector accepting elements of type U to one accepting elements of type T by applying a mapping function to each input element before accumulation.

Parameters:

<T>		the type of the input elements
<U>		type of elements accepted by downstream collector
<A>		intermediate accumulation type of the downstream collector
<R>		result type of collector
mapper		a function to be applied to the input elements
downstream		a collector which will accept mapped values

Returns:  a collector which applies the mapping function to the input elements and provides the mapped results to the downstream collector

@apiNote The mapping() collectors are most useful when used in a multi-level reduction, such as downstream of a groupingBy or partitioningBy. For example, given a stream of Person, to accumulate the set of last names in each city:

     Map<City, Set<String>> lastNamesByCity
         = people.stream().collect(groupingBy(Person::getCity,
                                              mapping(Person::getLastName, toSet())));