接口 Iterable ，类是 Iterator

public interface Iterable<T> {    /**     * Returns an iterator over a set of elements of type T.     *     * @return an Iterator.     */    Iterator<T> iterator();}public interface Iterator<E> {    /**     * Returns {@code true} if the iteration has more elements.     * (In other words, returns {@code true} if {@link #next} would     * return an element rather than throwing an exception.)     *     * @return {@code true} if the iteration has more elements     */    boolean hasNext();    /**     * Returns the next element in the iteration.     *     * @return the next element in the iteration     * @throws NoSuchElementException if the iteration has no more elements     */    E next();    /**     * Removes from the underlying collection the last element returned     * by this iterator (optional operation).  This method can be called     * only once per call to {@link #next}.  The behavior of an iterator     * is unspecified if the underlying collection is modified while the     * iteration is in progress in any way other than by calling this     * method.     *     * @throws UnsupportedOperationException if the {@code remove}     *         operation is not supported by this iterator     *     * @throws IllegalStateException if the {@code next} method has not     *         yet been called, or the {@code remove} method has already     *         been called after the last call to the {@code next}     *         method     */    void remove();}