@Test public void testMapping_downstreamBiCollector() {

  BiStream<String, Integer> salaries = BiStream.of("Joe", 100, "Tom", 200);
  BiCollector<String, Integer, ImmutableMap<Integer, String>> toReverseMap =
      BiCollectors.mapping((k, v) -> v, (k, v) -> k, ImmutableMap::toImmutableMap);
  assertThat(salaries.collect(toReverseMap))
      .containsExactly(100, "Joe", 200, "Tom")
      .inOrder();
}