code: @NotNull
      public static <T> Collector<T, ?, Long> summingLong(@NotNull final ToLongFunction<? super T> mapper) {
          return new CollectorsImpl<T, long[], Long>(
  
                  LONG_2ELEMENTS_ARRAY_SUPPLIER,
  
                  new BiConsumer<long[], T>() {
                      @Override
                      public void accept(long[] t, T u) {
                          t[0] += mapper.applyAsLong(u);
                      }
                  },
  
                  new Function<long[], Long>() {
                      @Override
                      public Long apply(long[] value) {
                          return value[0];
                      }
                  }
          );
      }
docs: Returns a {@code Collector} that summing long-valued input elements.
  
  @param <T> the type of the input elements
  @param mapper  the mapping function which extracts value from element to calculate result
  @return a {@code Collector}
  @since 1.1.3