Iterator it = list.iterator();
while(it.hasNext()) {
　　Object obj = it.next();
}

//Set<Integer> set = map.keySet(); //得到所有key的集合
for (Integer in : map.keySet()) {
  String str = map.get(in);//得到每个key多对用value的值
}

Iterator<Map.Entry<Integer, String>> it = map.entrySet().iterator();
while (it.hasNext()) {
   Map.Entry<Integer, String> entry = it.next();
    System.out.println("key= " + entry.getKey() + " and value= " + entry.getValue());
}

for (Map.Entry<Integer, String> entry : map.entrySet()) {
  //Map.entry<Integer,String> 映射项（键-值对） 有几个方法：用上面的名字entry
  //entry.getKey() ;entry.getValue(); entry.setValue();
  //map.entrySet() 返回此映射中包含的映射关系的 Set视图。
  System.out.println("key= " + entry.getKey() + " and value= " + entry.getValue());
}

for (String v : map.values()) {
  System.out.println("value= " + v);
}

for(Iterator iterator = list.iterator();iterator.hasNext();){          
  int i = (Integer) iterator.next();          
  System.out.println(i);        
}

Iterator iterator = list.iterator();
while(iterator.hasNext()){
  int i = (Integer) iterator.next();
  System.out.println(i);
}

for (Object object : list) { 
  System.out.println(object); 
}

for(int i = 0 ;i<list.size();i++) { 
  int j= (Integer) list.get(i);
  System.out.println(j); 
}

transient Object[] elementData;

public E get(int index) {
  rangeCheck(index);
  return elementData(index);
}

E elementData(int index) {
  return (E) elementData[index];
}

transient int size = 0;
transient Node<E> first;
transient Node<E> last;

public E get(int index) {
  checkElementIndex(index);
  return node(index).item;
}

Node<E> node(int index) {
  if (index < (size >> 1)) {  //查询位置在链表前半部分，从链表头开始查找
    Node<E> x = first;
    for (int i = 0; i < index; i++)
      x = x.next;
    return x;
  } else {           //查询位置在链表后半部分，从链表尾开始查找
    Node<E> x = last;
    for (int i = size - 1; i > index; i--)
      x = x.prev;
    return x;
  }
}

public E next() {
  checkForComodification();
  if (!hasNext())
    throw new NoSuchElementException();

  lastReturned = next;
  next = next.next;
  nextIndex++;
  return lastReturned.item;
}

public E previous() {
  checkForComodification();
  if (!hasPrevious())
    throw new NoSuchElementException();

  lastReturned = next = (next == null) ? last : next.prev;
  nextIndex--;
  return lastReturned.item;
}

//使用Iterator的字节码：
  Code:
    0: new      #16         // class java/util/ArrayList
    3: dup
    4: invokespecial #18         // Method java/util/ArrayList."<init>":()V
    7: astore_1
    8: aload_1
    9: invokeinterface #19, 1      // InterfaceMethod java/util/List.iterator:()Ljava/util/Iterator;
   14: astore_2
   15: goto     25
   18: aload_2
   19: invokeinterface #25, 1      // InterfaceMethod java/util/Iterator.next:()Ljava/lang/Object;
   24: pop
   25: aload_2
   26: invokeinterface #31, 1      // InterfaceMethod java/util/Iterator.hasNext:()Z
   31: ifne     18
   34: return
 
 
//使用foreach的字节码：
  Code:
    0: new      #16         // class java/util/ArrayList
    3: dup
    4: invokespecial #18         // Method java/util/ArrayList."<init>":()V
    7: astore_1
    8: aload_1
    9: invokeinterface #19, 1      // InterfaceMethod java/util/List.iterator:()Ljava/util/Iterator;
   14: astore_3
   15: goto     28
   18: aload_3
   19: invokeinterface #25, 1      // InterfaceMethod java/util/Iterator.next:()Ljava/lang/Object;
   24: checkcast   #31         // class loop/Model
   27: astore_2
   28: aload_3
   29: invokeinterface #33, 1      // InterfaceMethod java/util/Iterator.hasNext:()Z
   34: ifne     18
   37: return

if (list instanceof RandomAccess) {
  //使用传统的for循环遍历。
} else {
  //使用Iterator或者foreach。
}