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public interface Iterable
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{
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public abstract Iterator iterator();
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}
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Collection继承了Iterable接口
Iterator<E> iterator();
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List继承Collection接口
ArrayList实现List接口
????public Iterator<E> iterator() {
????????return new Itr();
????}
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返回一个迭代器对象，这个对象的实现类继承了Iterator接口
public interface Iterator
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{
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public abstract boolean hasNext();
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public abstract Object next();
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public abstract void remove();
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}
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????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;
????????// prevent creating a synthetic??constructor
????????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
????????public void??forEachRemaining(Consumer<? super E> action)??{
????????????Objects.requireNonNull(action);
????????????final int size =??ArrayList.this.size;
????????????int i = cursor;
????????????if (i < size) {
????????????????final Object[] es =??elementData;
????????????????if (i >= es.length)
????????????????????throw new??ConcurrentModificationException();
????????????????for (; i < size && modCount??== expectedModCount; i++)
?????????????????????action.accept(elementAt(es, i));
????????????????// update once at end to??reduce heap write traffic
????????????????cursor = i;
????????????????lastRet = i - 1;
????????????????checkForComodification();
????????????}
????????}
????????final void checkForComodification()??{
????????????if (modCount != expectedModCount)
????????????????throw new??ConcurrentModificationException();
????????}
????}
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另外for each,反编译后
Integer i;
for(Iterator iterator = list.iterator(); iterator.hasNext(); System.out.println(i)){
????i = (Integer)iterator.next();???
??}
其实就是依赖Iterator迭代器
（只要实现了Iterable接口，就可以直接使用foreach）
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一个小问题： 为什么不直接将hasNext()，next()方法放在Iterable接口中，其他类直接实现就可以了？？
答： 原因是有些集合类可能不止一种遍历方式，实现了Iterable的类可以再实现多个Iterator内部类，例如LinkedList中的ListItr和DescendingIterator两个内部类，就分别实现了双向遍历和逆序遍历。通过返回不同的Iterator实现不同的遍历方式，这样更加灵活。如果把两个接口合并，就没法返回不同的Iterator实现类了。
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原文：https://www.cnblogs.com/beeenwei/p/12553639.html