for(:)增强，Iterator迭代器

1、在JDK5之前对于数组或者list的遍历需要根据长度，以及下标进行遍历于是就有了其遍历方式

List<String> list = new ArrayList<String>();
for (int index = 0; index < list.size(); index++ ) {
    String str = list.get(index);
    System.out.println("str: " + str);
}

2、在JDK5之后引入了：增强特性使list的遍历，进行改变

List<String> list = new ArrayList<String>();
for (String str : list){
    System.out.println("str:" + str);
}

3、反编译class文件可以看到增强for循环会被编译器自动处理如下代码：

List<String> list = new ArrayList<String>();
Iterator var4 = list.iterator();

while(var4.hasNext()) {
    str = (String)var4.next();
    System.out.println("str:" + str);
}

即JDK在编译java文件的时候会将：符号以JDK的特性进行转换成list的迭代器，接着通过获取迭代器的下一元素处理数据

在ArrayList类的里面存着一个内部类Itr 实现了Iterator的接口

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

    /**
     * An optimized version of AbstractList.Itr
     */
    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;

        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();
            }
        }

迭代器的原理是通过记录上一次读取数据的下标，以及下一次要下标，从而通过cursor和lastRet直接对集合的元素进行访问

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