@Test
public void keys() throws Exception {

    multiMap.putAll(ImmutableListMultimap.of(
            "first", new TestEntity("1", "v1"),
            "second", new TestEntity("2", "v2"),
            "second", new TestEntity("3", "v3")
    ));
    final Multiset<String> keys = multiMap.keys();
    assertThat(keys).hasSize(3);
    assertThat(keys.elementSet()).hasSize(2);
    assertThat(keys.count("first")).isEqualTo(1);
    assertThat(keys.count("second")).isEqualTo(2);
}