filtered.put("b", 2);
    assertEquals(ImmutableMap.of("a", 1, "b", 2), filtered);

    assertThrows(IllegalArgumentException.class, () -> filtered.put("yyy", 3));
  }

  public void testFilteredKeysIllegalPutAll() {
    Map<String, Integer> unfiltered = createUnfiltered();
    Map<String, Integer> filtered = Maps.filterKeys(unfiltered, NOT_LENGTH_3);
    filtered.put("a", 1);
    filtered.put("b", 2);