ImmutableMap.of(AnEnum.A, 1, AnEnum.C, 2, AnEnum.E, 3),
            mapEntry(AnEnum.A, 1),
            mapEntry(AnEnum.C, 2),
            mapEntry(AnEnum.E, 3));
  }

  public void testToImmutableMap_exceptionOnDuplicateKey() {
    Collector<Entry<AnEnum, Integer>, ?, ImmutableMap<AnEnum, Integer>> collector =
        Maps.toImmutableEnumMap(Entry::getKey, Entry::getValue);
    try {

    assertMapEquals(zis.combine(zat).build(), "one", 1, "two", 2, "three", 3);
  }

  // TODO(b/172823566): Use mainline testToImmutableMap once CollectorTester is usable to java7.
  public void testToImmutableMap_exceptionOnDuplicateKey_java7_combine() {
    ImmutableMap.Builder<String, Integer> zis =
        ImmutableMap.<String, Integer>builder().put("one", 1).put("two", 2);
    ImmutableMap.Builder<String, Integer> zat =

            ImmutableMap.of("one", 1, "two", 2, "three", 3),
            mapEntry("one", 1),
            mapEntry("two", 2),
            mapEntry("three", 3));
  }

  public void testToImmutableMap_exceptionOnDuplicateKey() {
    Collector<Entry<String, Integer>, ?, ImmutableMap<String, Integer>> collector =
        ImmutableMap.toImmutableMap(Entry::getKey, Entry::getValue);
    try {