写代码遇到这个问题,很多博客文章都是在反复的强调理论,而没有对应的实例,所以这里从实例出发,后研究理论:
一、错误产生情况
1 、字符型
(1)添加
public static void main(String[] args) {
List<String> stringList = new ArrayList<String>();
stringList.add("张三毛");
stringList.add("李四");
stringList.add("王五");
stringList.add("钱二");
if (stringList!=null) {
if (!stringList.equals(Collections.EMPTY_LIST.isEmpty())){
for (String s : stringList) {
stringList.add("赵大");
}
}
}
System.out.println(stringList);
} 报错
改写为如下即可:
public static void main(String[] args) {
List<String> stringList = new ArrayList<String>();
stringList.add("张三毛");
stringList.add("李四");
stringList.add("王五");
stringList.add("钱二");
if (stringList != null) {
if (!stringList.equals(Collections.EMPTY_LIST.isEmpty())) {
stringList.add("赵大");
Iterator<String> it = stringList.iterator();
while (it.hasNext()) {
it.next();
}
}
}
System.out.println(stringList);
}打印出结果:
然后我们打印其next,就会发现其循环就是通过it.next()方法将数据添加进去的
打印:
(2)、删除
错误写法:
private static String key = "钱二";
public static void main(String[] args) {
List<String> stringList = new ArrayList<String>();
stringList.add("张三毛");
stringList.add("李四");
stringList.add("王五");
stringList.add("钱二");
if (stringList != null) {
if (!stringList.equals(Collections.EMPTY_LIST.isEmpty())) {
for (String s : stringList) {
if (key.equals(s)){
stringList.remove(s);
}
}
}
}
System.out.println(stringList);
}报错:
改写为:
private static String key = "钱二";
public static void main(String[] args) {
List<String> stringList = new ArrayList<String>();
stringList.add("张三毛");
stringList.add("李四");
stringList.add("王五");
stringList.add("钱二");
if (stringList != null) {
if (!stringList.equals(Collections.EMPTY_LIST.isEmpty())) {
Iterator<String> it = stringList.iterator();
while (it.hasNext()) {
String next = it.next();
if (key.equals(next)) {
it.remove();
}
}
}
}
System.out.println(stringList);
}结果:
2、整形
正确添加:
public class ConcurrentBaseApplication {
private static String key = "钱二";
public static void main(String[] args) {
List<Integer> integerList = new ArrayList<Integer>();
integerList.add(1);
integerList.add(2);
integerList.add(3);
integerList.add(4);
Integer next=0;
if (integerList != null) {
if (!integerList.equals(Collections.EMPTY_LIST.isEmpty())) {
integerList.add(5);
Iterator<Integer> it = integerList.iterator();
while (it.hasNext()) {
next = it.next();
System.out.println(next);
}
}
}
System.out.println(integerList);
}
}结果:
正确删除:
public class ConcurrentBaseApplication {
public static void main(String[] args) {
List<Integer> integerList = new ArrayList<Integer>();
integerList.add(1);
integerList.add(2);
integerList.add(3);
integerList.add(4);
if (integerList != null) {
if (!integerList.equals(Collections.EMPTY_LIST.isEmpty())) {
Iterator<Integer> it = integerList.iterator();
while (it.hasNext()) {
Integer next = it.next();
if("2".equals(next.toString())){
it.remove();
}
}
}
}
System.out.println(integerList);
}
}结果:
(3) 实体类
创建实体类
Student
package com.north.big.penguin.pojo; import java.io.Serializable; /**
* @author liuyangos8888
*/
public class Student implements Serializable { /**
* 姓名
*/
private String name; /**
* 年龄
*/
private String age; /**
* 标识
*/
private String id; public Student() {
} public Student(String name, String age, String id) {
this.name = name;
this.age = age;
this.id = id;
} public String getName() {
return name;
} public void setName(String name) {
this.name = name;
} public String getAge() {
return age;
} public void setAge(String age) {
this.age = age;
} public String getId() {
return id;
} public void setId(String id) {
this.id = id;
} @Override
public String toString() {
return "Student{" +
"name='" + name + '\'' +
", age='" + age + '\'' +
", id='" + id + '\'' +
'}';
}
}
正确的添加:
public class ConcurrentBaseApplication {
public static void main(String[] args) {
List<Student> students = new ArrayList<Student>();
Student student1 = new Student();
student1.setName("李雷");
student1.setAge("13");
student1.setId(UUID.randomUUID().toString());
Student student2 = new Student();
student2.setName("韩梅梅");
student2.setAge("14");
student2.setId(UUID.randomUUID().toString());
Student student3 = new Student();
student3.setName("李华");
student3.setAge("15");
student3.setId(UUID.randomUUID().toString());
students.add(student1);
students.add(student2);
students.add(student3);
if (students != null) {
if (!students.equals(Collections.EMPTY_LIST.isEmpty())) {
Student student4 = new Student();
student4.setName("小明");
student4.setAge("16");
student4.setId(UUID.randomUUID().toString());
Iterator<Student> it = students.iterator();
while (it.hasNext()) {
// 添加学生
Student next = it.next();
}
}
}
System.out.println(students);
}
}结果:
正确的删除:
public class ConcurrentBaseApplication {
public static void main(String[] args) {
List<Student> students = new ArrayList<Student>();
Student student1 = new Student();
student1.setName("李雷");
student1.setAge("13");
student1.setId(UUID.randomUUID().toString());
Student student2 = new Student();
student2.setName("韩梅梅");
student2.setAge("14");
student2.setId(UUID.randomUUID().toString());
Student student3 = new Student();
student3.setName("李华");
student3.setAge("15");
student3.setId(UUID.randomUUID().toString());
students.add(student1);
students.add(student2);
students.add(student3);
if (students != null) {
if (!students.equals(Collections.EMPTY_LIST.isEmpty())) {
Student student4 = new Student();
student4.setName("小明");
student4.setAge("16");
student4.setId(UUID.randomUUID().toString());
Iterator<Student> it = students.iterator();
while (it.hasNext()) {
// 添加学生
Student next = it.next();
Integer integerAge = Integer.valueOf(next.getAge());
if(integerAge>14){
it.remove();
}
}
}
}
System.out.println(students);
}
}结果:
结果集:
[Student{name='李雷', age='13', id='617e914f-ed33-472d-bbbd-1a6bf5ef5901'}, Student{name='韩梅梅', age='14', id='cb804e43-4846-4fc6-84c8-9e4a6b17d7f1'}]二、原理补充
继承关系图
根据错误我们知道是Itr出错了
根据添加class,可以看到它跟ArrayList的关系
Itr是一个内部类,还实现了Iterator
其源码
/**
* An optimized version of AbstractList.Itr
*/
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; 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];
} 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();
}
} @Override
@SuppressWarnings("unchecked")
public void forEachRemaining(Consumer<? super E> consumer) {
Objects.requireNonNull(consumer);
final int size = ArrayList.this.size;
int i = cursor;
if (i >= size) {
return;
}
final Object[] elementData = ArrayList.this.elementData;
if (i >= elementData.length) {
throw new ConcurrentModificationException();
}
while (i != size && modCount == expectedModCount) {
consumer.accept((E) elementData[i++]);
}
// update once at end of iteration to reduce heap write traffic
cursor = i;
lastRet = i - 1;
checkForComodification();
} final void checkForComodification() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
}
想要调用iteror转化ArrayList就要找iterator方法
Itr参数解析
| 参数 | 含义 |
| cursor | 一个索引,代表下一个要访问的元素的索引 |
| astRet | 表示上一个访问的元素的索引 |
| expectedModCount | 表示修改次数的 |
| modCount | 表示对List的修改次数 |
其方法主要是hasNext()和next()两个方法,
用其判断是否还有元素未被访问,代码中
while(iter.hasNext()){
}
如果下一个访问的元素下标不等于ArrayList的大小，就表示有元素需要访问，这个很容易理解，如果下一个访问元素的下标等于ArrayList的大小，则肯定到达末尾了。
首先在next()方法中会调用checkForComodification()方法，然后根据cursor的值获取到元素，接着将cursor的值赋给lastRet，并对cursor的值进行加1操作。初始时，cursor为0，lastRet为-1，那么调用一次之后，cursor的值为1，lastRet的值为0。注意此时，modCount为0，expectedModCount也为0。
当判断当前元素的值是否为2，若为2，则调用list.remove()方法来删除该元素。
ArrayList中的remove
通过remove方法删除元素最终是调用的fastRemove()方法，在fastRemove()方法中，首先对modCount进行加1操作（因为对集合修改了一次），然后接下来就是删除元素的操作，最后将size进行减1操作，并将引用置为null以方便垃圾收集器进行回收工作。
执行完删除操作后，继续while循环，调用hasNext方法()判断，由于此时cursor为1，而size为0，那么返回true，所以继续执行while循环，然后继续调用iterator的next()方法.
如果modCount不等于expectedModCount，则抛出ConcurrentModificationException异常。
此时modCount为1，而expectedModCount为0，因此程序就抛出了ConcurrentModificationException异常。