android/guava/src/com/google/common/collect/Ordering.java
@GwtCompatible(serializable = true)
  public <S extends T> Ordering<S> nullsFirst() {
    return new NullsFirstOrdering<S>(this);
  }
@GwtCompatible(serializable = true)
  public <S extends T> Ordering<S> nullsFirst() {
    return new NullsFirstOrdering<S>(this);
  }
Ordering<String> o = Ordering.<String>natural().nullsFirst();