this.value = checkNotNull(value);
    }

    @Override
    public String toString() {
      return "value=" + value;
    }
  }

  @SuppressWarnings("ComparableType")
  private static class ComparableSubClass extends NonComparableSuperClass
      implements Comparable<NonComparableSuperClass> {

    ComparableSubClass(Integer value) {
      super(value);
    }

  }

  // Compare by toString() to satisfy 2 properties:
  // 1. compareTo(null) should throw NullPointerException
  // 2. the order is deterministic and easy to understand, for debugging purpose.
  @SuppressWarnings("ComparableType")
  private static final class ByToString implements Comparable<Object>, Serializable {
    private static final ByToString INSTANCE = new ByToString();

    @Override
    public int compareTo(Object o) {