迭代器模式的作用：提供一种方法来顺序访问聚合对象中的一系列数据，而不暴露聚合对象的内部表示。

案例

迭代器模式最经典的应用就是 JDK 中工具类 Iterator 接口的设计，定义了对集合的访问方法。

public interface Iterator<E> {

	boolean hasNext();

	E next();

	default void remove() {
		throw new UnsupportedOperationException("remove");
	}

	default void forEachRemaining(Consumer<? super E> action) {
		Objects.requireNonNull(action);
		while (hasNext())
			action.accept(next());
	}
}

每种集合中都有 iterator() 方法，返回各自的迭代器，迭代器按照各自算法实现了 Iterator 接口，满足遍历需求。

如 ArrayLis 中

public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable {

	public Iterator<E> iterator() {
		return new Itr();
	}

	//ArrayList 迭代器的内部实现类
	private class Itr implements Iterator<E> {
		int cursor;	   // index of next element to return
		int lastRet = -1; // index of last element returned; -1 if no such
		int expectedModCount = modCount;

		Itr() {}

		public boolean hasNext() {
			return cursor != size;
		}

		@SuppressWarnings("unchecked")
		public E next() {
			checkForComodification();
			int i = cursor;
			if (i >= size)
				throw new NoSuchElementException();
			Object[] elementData = ArrayList.this.elementData;
			if (i >= elementData.length)
				throw new ConcurrentModificationException();
			cursor = i + 1;
			return (E) elementData[lastRet = i];
		}

		public void remove() {
			if (lastRet < 0)
				throw new IllegalStateException();
			checkForComodification();

			try {
				ArrayList.this.remove(lastRet);
				cursor = lastRet;
				lastRet = -1;
				expectedModCount = modCount;
			} catch (IndexOutOfBoundsException ex) {
				throw new ConcurrentModificationException();
			}
		}

		@Override
		@SuppressWarnings("unchecked")
		public void forEachRemaining(Consumer<? super E> consumer) {
			Objects.requireNonNull(consumer);
			final int size = ArrayList.this.size;
			int i = cursor;
			if (i >= size) {
				return;
			}
			final Object[] elementData = ArrayList.this.elementData;
			if (i >= elementData.length) {
				throw new ConcurrentModificationException();
			}
			while (i != size && modCount == expectedModCount) {
				consumer.accept((E) elementData[i++]);
			}
			// update once at end of iteration to reduce heap write traffic
			cursor = i;
			lastRet = i - 1;
			checkForComodification();
		}

		final void checkForComodification() {
			if (modCount != expectedModCount)
				throw new ConcurrentModificationException();
		}
	}

}