357      * Adapts a {@code Collector} to perform an additional finishing

   358      * transformation.  For example, one could adapt the {@link #toList()}

   359      * collector to always produce an immutable list with:

   360      * <pre>{@code

   361      *     List<String> people

   362      *         = people.stream().collect(collectingAndThen(toList(), Collections::unmodifiableList));

   363      * }</pre>

   364      *

   365      * @param <T> the type of the input elements

   366      * @param <A> intermediate accumulation type of the downstream collector

   367      * @param <R> result type of the downstream collector

   368      * @param <RR> result type of the resulting collector

   369      * @param downstream a collector

   370      * @param finisher a function to be applied to the final result of the downstream collector

   371      * @return a collector which performs the action of the downstream collector,

   372      * followed by an additional finishing step

   373      */

   374     public static<T,A,R,RR> Collector<T,A,RR> collectingAndThen(Collector<T,A,R> downstream,

   375                                                                 Function<R,RR> finisher) {

   376         Set<Collector.Characteristics> characteristics = downstream.characteristics();

   377         if (characteristics.contains(Collector.Characteristics.IDENTITY_FINISH)) {

   378             if (characteristics.size() == 1)

   379                 characteristics = Collectors.CH_NOID;

   380             else {

   381                 characteristics = EnumSet.copyOf(characteristics);

   382                 characteristics.remove(Collector.Characteristics.IDENTITY_FINISH);

   383                 characteristics = Collections.unmodifiableSet(characteristics);

   384             }

   385         }

   386         return new CollectorImpl<>(downstream.supplier(),

   387                                    downstream.accumulator(),

   388                                    downstream.combiner(),

   389                                    downstream.finisher().andThen(finisher),

   390                                    characteristics);

   391     }

   392 

   393     /**