assertThat(mapCache.unmodifiableKeySet()).hasSize(3);
    for (String key : mapCache.unmodifiableKeySet()) {
      assertThat(mapCache.get(key)).isEqualTo(key + "_value");
    }
  }

  @Test
  public void testPutNewValue() {
    assertThat(mapCache.put("key", "value")).isNull();
    assertThat(mapCache.get("key")).isEqualTo("value"); // ensure key/value is cached
    assertThat(mapCache.put("key", "new value")).isEqualTo("value");