return key + value;
          }
        };
    Map<String, String> transformed = transformEntries(map, concat);

    assertEquals(ImmutableMap.of("a", "a4", "b", "b9"), transformed);
  }

  @SuppressWarnings("unused")
  public void testTransformEntriesGenerics() {
    Map<Object, Object> map1 = ImmutableMap.<Object, Object>of(1, 2);