/guava/guava/src/com/google/common/graph/ElementOrder.java
this.type = checkNotNull( UNORDERED) ;
this.type = checkNotNull ( type ) ;
UNORDERED, STABLE, INSERTION, SORTED } private ElementOrder(Type type, @CheckForNull Comparator<T> comparator) { <extra_id_0> this.comparator = comparator; checkState((type == Type.SORTED) == (comparator != null)); } public static <S> ElementOrder<S> unordered() { return new ElementOrder<>(Type.UNORDERED, null); }