今天在测试代码的时候出现一个异常ConcurrentModificationException，该异常网上很多解决方案以及解释，但我还是再记录一遍吧。
代码抽象出来是这样的：
import java.util.ArrayList;
import java.util.List;
public class Test {
public static void main(String[] args) {
List list=new ArrayList();
list.add(1);
list.add(2);
list.add(3);
list.add(4);
list.add(5);
for (Integer i : list) {//这是迭代
if(i==3){
list.remove(new Integer(i));//引起异常的操作
}
}
}
}
该代码在运行期间就出现java.util.ConcurrentModificationException异常。
这个循环其实是对list进行迭代。
1.在迭代的时候怎么判断是否还有下一个(hasNext()方法怎么实现)：
public boolean hasNext() {
return cursor != size();
}
cursor:Index of element to be returned by subsequent call to next
size():是该list的size
所以只要两者不相等，就认为还有元素。
2.迭代的时候怎么取下一个(next()方法怎么实现)：
public E next() {
checkForComodification();
try {
E next = get(cursor);
lastRet = cursor++;
return next;
} catch (IndexOutOfBoundsException e) {
checkForComodification();
throw new NoSuchElementException();
}
}
final void checkForComodification() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
modelCount:The number of times this list has been structurally modified.Structural modifications are those that change the size of the list, or otherwise perturb it in such a fashion that iterations in progress may yield incorrect results.
expectedModCount:期望的modelCount
这2个变量是有迭代器自己来维护的。
上面这段程序出现异常是因为我们使用Collection里面的remove方法进行删除，ArrayList的remove方法实现：
public boolean remove(Object o) {
if (o == null) {
for (int index = 0; index < size; index++)
if (elementData[index] == null) {
fastRemove(index);
return true;
}
} else {
for (int index = 0; index < size; index++)
if (o.equals(elementData[index])) {
fastRemove(index);
return true;
}
}
return false;
}
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; // Let gc do its work
}
modCount+1,导致modCount和expectedModCount不相等。
3.解决方法就是用迭代器自己的remove方法：
public void remove() {
if (lastRet == -1)
throw new IllegalStateException();
checkForComodification();
try {
AbstractList.this.remove(lastRet); //将modCount+1，实现如下
if (lastRet < cursor)
cursor--;
lastRet = -1;
expectedModCount = modCount; //维护
} catch (IndexOutOfBoundsException e) {
throw new ConcurrentModificationException();
}
}
public E remove(int index) {
RangeCheck(index);
modCount++; //***
E oldValue = (E) elementData[index];
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
elementData[--size] = null; // Let gc do its work
return oldValue;
}
)
)
