}

  public void test_skipNulls_useForNull() {
    Joiner j = Joiner.on("x").skipNulls();
    assertThrows(UnsupportedOperationException.class, () -> j.useForNull("y"));
  }

  public void test_useForNull_twice() {
    Joiner j = Joiner.on("x").useForNull("y");
    assertThrows(UnsupportedOperationException.class, () -> j.useForNull("y"));
  }

  public void testMap() {