@Test public void testMapping_downstreamCollector() {

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