说说ArrayList的ConcurrentModificationException，

说说ArrayList的ConcurrentModificationException，

试用for-earch遍历ArrayList,调用remove方法时候，常常回报ConcurrentModificationException异常，今天来从源码上解析这个问题．

列出来问题

代码１：

     ArrayList<String> list = new ArrayList<>();
        list.add("1");
        list.add("2");
        list.add("2");
        list.add("2");
        for (String e : list) {
            System.out.println(e);
            if (e.equals("1")) {
                list.remove(e);
            }
        }

output:

1
Exception in thread "main" java.util.ConcurrentModificationException
    at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)
    at java.util.ArrayList$Itr.next(ArrayList.java:851)
    at com.own.learn.jdk.ref.Test.main(Test.java:17)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:147)

``
**代码２：**

    ArrayList<String> list = new ArrayList<>();
    list.add("1");
    list.add("2");
    list.add("2");
    list.add("2");
    for (int i=0; i< list.size(); i++) {
        String e =  list.get(i);
        System.out.println(e);
        if (e.equals("2")) {
            list.remove(e);
        }
    }

output:

1
2
2

**代码三：**

    ArrayList<String> list = new ArrayList<>();
    list.add("1");
    list.add("2");
    list.add("2");
    list.add("2");
    for (Iterator<String> it = list.iterator(); it.hasNext();) {
        String next = it.next();
        if(next.equals("2"))
            it.remove();
        System.out.println(next);
    }

output:

1
2
2
2

###解决问题
１，checkForComodification报错，查看源码：

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

我们可以清楚看到，modCount和expectedModCount不相等的时候，才有这个问题
list自带的reomve方法调用了fastRemove是：

  private void fastRemove(int index) {
    modCount++;
    int numMoved = size - index - 1;
    if (numMoved > 0)
        System.arraycopy(elementData, index+1, elementData, index,
                         numMoved);
    elementData[--size] = null; // clear to let GC do its work
}

modCount和expectedModCount方法不同步
而Itr中的remove方法：

  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();
        }
    }

“`
modCount和expectedModCount方法同步
到这里ConcurrentModificationException解决了
２，代码２中输出少了一个数字，为啥呢．答案在上面的fastRemove中