问集合扩展了Iterable，但为什么集合声明Iterable函数(与Iterable相同)

EN

/**
 * Returns an iterator over the elements in this collection.  There are no
 * guarantees concerning the order in which the elements are returned
 * (unless this collection is an instance of some class that provides a
 * guarantee).
 *
 * @return an <tt>Iterator</tt> over the elements in this collection
 */
Iterator<E> iterator();

/**
 * Returns an iterator over elements of type {@code T}.
 *
 * @return an Iterator.
 */
Iterator<T> iterator();