为什么循环删除List元素会报错
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描述
大家在工作中应该都会遇到从List集合中删除某一个或多个元素的业务场景
相信大家都会避开在循环里面删除元素，使用其他方式处理
很多面试官也都会问为什么循环里面不能删除元素？
示例
public static void main(String[] args) {
List<String> list = new ArrayList<>();
list.add("test0");
list.add("test1");
list.add("test2");
list.add("test3");
list.add("test4");
list.add("test5");
System.out.println(list);
list.remove(3);
System.out.println(list);
list.remove("test1");
System.out.println(list);
for (int i = 0; i < list.size(); i++) {
if (i == 2) {
list.remove(i);
}
}
System.out.println(list);
Iterator<String> it = list.iterator();
while (it.hasNext()) {
if (it.next().equals("test2")) {
it.remove();
}
}
System.out.println(list);
for (String s : list) {
if ("test5".equals(s)) {
list.remove(s);
}
}
System.out.println(list);
}
打印结果
[test0, test1, test2, test3, test4, test5]
[test0, test1, test2, test4, test5]
[test0, test2, test4, test5]
[test0, test2, test5]
[test0, test5]
Exception in thread "main" java.util.ConcurrentModificationException
at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:909)
at java.util.ArrayList$Itr.next(ArrayList.java:859)
at com.fc.store.Test.main(Test.java:46)
从打印结果可以看到
- 根据索引删除 --正常
- 根据元素删除 --正常
- 循环根据索引删除 --正常
- 迭代删除原始 --正常
- 循环根据元素删除 --不正常
执行过程
List<String> list = new ArrayList<>();
集合初始化时
transient Object[] elementData; 空数组
private int size; 长度为0
protected transient int modCount = 0; 修改次数为0
添加元素
list.add("test0");
elementData[0] = "test0";
size = 1;
modCount = 1;
list.add("test1");
elementData[0] = "test0";
elementData[1] = "test1";
size = 2;
modCount = 2;
list.add("test2");
elementData[0] = "test0";
elementData[1] = "test1";
elementData[2] = "test2";
size = 3;
modCount = 3;
list.add("test3");
elementData[0] = "test0";
elementData[1] = "test1";
elementData[2] = "test2";
elementData[3] = "test3";
size = 4;
modCount = 4;
list.add("test4");
elementData[0] = "test0";
elementData[1] = "test1";
elementData[2] = "test2";
elementData[3] = "test3";
elementData[4] = "test4";
size = 5;
modCount = 5;
list.add("test5");
elementData[0] = "test0";
elementData[1] = "test1";
elementData[2] = "test2";
elementData[3] = "test3";
elementData[4] = "test4";
elementData[5] = "test5";
size = 6;
modCount = 6;
可以发现每添加一个元素，集合size会增加1， 修改次数会增加1
根据索引删除
list.remove(3);
elementData[0] = "test0";
elementData[1] = "test1";
elementData[2] = "test2";
elementData[4] = "test4";
elementData[5] = "test5";
size = 5;
modCount = 7;
根据元素删除
list.remove("test1");
elementData[0] = "test0";
elementData[1] = "test2";
elementData[2] = "test4";
elementData[5] = "test5";
size = 4;
modCount = 8;
循环根据索引删除
for (int i = 0; i < list.size(); i++) {
if (i == 2) {
list.remove(i);
}
}
elementData[0] = "test0";
elementData[1] = "test2";
elementData[5] = "test5";
size = 3;
modCount = 9;
循环根据索引删除
Iterator<String> it = list.iterator();
while (it.hasNext()) {
if (it.next().equals("test2")) {
it.remove();
}
}
elementData[0] = "test0";
elementData[5] = "test5";
size = 2;
modCount = 10;
可以发现每删除一个元素后，集合size会减1，修改次数会增加1
循环根据元素删除
for (String s : list) {
if ("test2".equals(s)) {
list.remove(s);
}
}
就抛异常了，因为Iterator的next方法会校验是否修改
此时：expectedModCount = 10， modCount = 11
public E next() {
checkForComodification();
}
final void checkForComodification() {
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
}
源码解析
List 的add和remove方法每次操作都会对modCount++
public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
public E remove(int index) {
rangeCheck(index);
modCount++;
E oldValue = elementData(index);
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
return oldValue;
}
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; // clear to let GC do its work
}
private void ensureCapacityInternal(int minCapacity) {
ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
}
private void ensureExplicitCapacity(int minCapacity) {
modCount++;
}
Iterator 会继承List的modCount，并赋值给expectedModCount
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;
}
Iterator 的next方法会校验modCount和expectedModCount 是否相同
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();
}
由于在Iterator的便利中使用了List的remove方法，导致modCount增加了 所以在下次next方法中判断modCount和expectedModCount不一致就直接抛出了异常
解决方案
第一种使用迭代器删除
Iterator<String> it = list.iterator();
while (it.hasNext()) {
if (it.next().equals("test2")) {
it.remove();
}
}
第二种for循环删除后要立即退出
for (String s : list) {
if ("test5".equals(s)) {
list.remove(s);
return;
}
}
JAVA8语法
list.removeIf("test5"::equals);
Set，Map 同理
同样Set，Map循环里面删除报错也是同样的原理
往往大家在遇到问题后，都只是找到了主要原因，但是并没有找到根本原因。
只有通过深入分析找到根本原因，制定预防措施（加入CR清单，添加扫码规则，制定奖惩措施），才能够真正避免问题