j.appendTo(sb2, ImmutableMultimap.of(1, 2, 3, 4, 5, 6, 1, 3, 5, 10).entries().iterator());
    assertEquals("1:2;1:3;3:4;5:6;5:10", sb2.toString());
  }

  public void test_skipNulls_onMap() {
    Joiner j = Joiner.on(",").skipNulls();
    assertThrows(UnsupportedOperationException.class, () -> j.withKeyValueSeparator("/"));
  }

  private static class DontStringMeBro implements CharSequence {