StringBuilder sb3 = new StringBuilder().append('x');
      joiner.appendTo(sb3, partsArray[0], partsArray[1], rest);
      assertEquals("x" + expected, sb3.toString());
    }
  }

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

  public void test_skipNulls_useForNull() {