腾讯一面！说说ArrayList的遍历foreach与iterator时remove的区别，我一脸懵逼

​ 运行上面的代码可以看到，使用迭代器的删除操作是不会有问题、能成功删除的；而使用foreach循环进行删除则会抛出ConcurrentModificationException异常，但如果使用foreach循环删除第一个元素“1”的时候又会发现不会抛出异常。那么这到底是为什么呢？

​ 众所周知，foreach循环是一种语法糖，那么其背后到底是如何来实现的呢？将上面反例的代码反编译后的结果如下：

public class com.hys.util.Test {
  public com.hys.util.Test();
    Code:
0: aload_0
1: invokespecial #1    // Method java/lang/Object."":()V
4: return
 
  public static void main(java.lang.String[]);
    Code:
0: new    #2    // class java/util/ArrayList
3: dup
4: invokespecial #3    // Method java/util/ArrayList."":()V
7: astore_1
8: aload_1
9: ldc    #4    // String 1
      11: invokeinterface #5,  2     // InterfaceMethod java/util/List.add:(Ljava/lang/Object;)Z
      16: pop
      17: aload_1
      18: ldc    #6    // String 2
      20: invokeinterface #5,  2     // InterfaceMethod java/util/List.add:(Ljava/lang/Object;)Z
      25: pop
      26: aload_1
      27: invokeinterface #7,  1     // InterfaceMethod java/util/List.iterator:()Ljava/util/Iterator;
      32: astore_2
      33: aload_2
      34: invokeinterface #8,  1     // InterfaceMethod java/util/Iterator.hasNext:()Z
      39: ifeq   72
      42: aload_2
      43: invokeinterface #9,  1     // InterfaceMethod java/util/Iterator.next:()Ljava/lang/Object;
      48: checkcast     #10   // class java/lang/String
      51: astore_3
      52: ldc    #6    // String 2
      54: aload_3
      55: invokevirtual #11   // Method java/lang/String.equals:(Ljava/lang/Object;)Z
      58: ifeq   69
      61: aload_1
      62: aload_3
      63: invokeinterface #12,  2    // InterfaceMethod java/util/List.remove:(Ljava/lang/Object;)Z
      68: pop
      69: goto   33
      72: return
}

public Iterator iterator() {
    return new Itr();
}

可以看到是返回了一个内部类Itr： 

private class Itr implements Iterator {
    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];
    }
 
    //...
}

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

List list = new ArrayList<>();
list.add("1");
list.add("2");
 
for (String item : list) {
    if ("1".equals(item)) {
 list.add(item);
    }
}

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();
    }
}

List list = new ArrayList<>();
list.add("1");
list.add("2");
 
ListIterator iterator = list.listIterator();
while (iterator.hasNext()) {
    String item = iterator.next();
    if ("1".equals(item)) {
 iterator.add(item);
    }
}

同上，首先来看一下第5行代码处的listIterator方法： 

public ListIterator listIterator() {
    return new ListItr(0);
}

private class ListItr extends Itr implements ListIterator {
  ListItr(int index) {
      super();
      cursor = index;
  }

  //...

  public void add(E e) {
      checkForComodification();

      try {
   int i = cursor;
   ArrayList.this.add(i, e);
   cursor = i + 1;
   lastRet = -1;
   expectedModCount = modCount;
      } catch (IndexOutOfBoundsException ex) {
   throw new ConcurrentModificationException();
      }
  }
}