【java】报错java.util.ConcurrentModificationException: null

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我是靠谱客的博主勤恳大白，最近开发中收集的这篇文章主要介绍【java】报错java.util.ConcurrentModificationException: null，觉得挺不错的，现在分享给大家，希望可以做个参考。

概述

异常：java.util.ConcurrentModificationException: null

原因：Itr里面的expectedModCount会与ArrayList的modCount进行比较，二者不相等，所以会抛错。删除的时候是通过ArrayList的remove方法去操作的，不是Itr内部的那个删除方法去操作的。

解决方法：

1.使用itr删除

import java.util.ArrayList;

class MyTest{
    public static void main(String[] args){
        ArrayList<Integer> arr = new ArrayList<Integer>();
        for(int i=0; i<10; i++){
            arr.add(i);
        }

        for(Iterator<Integer> it=arr.iterator(); it.hasNext();){
            Integer i = it.next();
            if(i == 5){
                it.remove();
            }
            else{
                System.out.println(i);
            }
        }
    }
}

2.每次删除的时候都让i减1

import java.util.ArrayList;

class MyTest{
    public static void main(String[] args){
        ArrayList<Integer> arr = new ArrayList<Integer>();
        for(int i=0; i<10; i++){
            arr.add(i);
        }

        for(int i=0; i<arr.size(); i++){
            if(i == 5){
                arr.remove(i);
                i -= 1;
            }
            else{
                System.out.println(i);
            }
        }
    }
}

3.倒序删除，先遍历一遍记录要删除的下标，然后从末尾开始删除

报错源码位置：ArrayList.java里的Itr.next()和Itr.checkForComodification()。

报错就是在Itr.next()中调用checkForComodification()以后产生的，在checkForComodification()函数中，判断了modCount和expectedModCount是否相等。如果不相等，就会抛出我们遇到的这个异常。

报错源代码：

		... ...
		@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();
        }

底层源代码分析：

这里的Itr是在ArrayList的内部类，实现了Iterator接口，用于遍历。

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
	... ...
	        
	/**
     * 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;

        // prevent creating a synthetic constructor
        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
        public void forEachRemaining(Consumer<? super E> action) {
            Objects.requireNonNull(action);
            final int size = ArrayList.this.size;
            int i = cursor;
            if (i < size) {
                final Object[] es = elementData;
                if (i >= es.length)
                    throw new ConcurrentModificationException();
                for (; i < size && modCount == expectedModCount; i++)
                    action.accept(elementAt(es, i));
                // update once at end to reduce heap write traffic
                cursor = i;
                lastRet = i - 1;
                checkForComodification();
            }
        }

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

remove调用的源码

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
    ... ...
	public boolean remove(Object o) {
        final Object[] es = elementData;
        final int size = this.size;
        int i = 0;
        found: {
            if (o == null) {
                for (; i < size; i++)
                    if (es[i] == null)
                        break found;
            } else {
                for (; i < size; i++)
                    if (o.equals(es[i]))
                        break found;
            }
            return false;
        }
        fastRemove(es, i);
        return true;
    }
    
    private void fastRemove(Object[] es, int i) {
        modCount++;
        final int newSize;
        if ((newSize = size - 1) > i)
            System.arraycopy(es, i + 1, es, i, newSize - i);
        es[size = newSize] = null;
    }
    ... ...

remove()函数会调用fastRemove()函数，使得modCount的值自增1.

然而，for循环下一次调用Itr.next()，Itr.next()调用Itr.checkForComodification()时，会发现，modCount和expectedModCount两个值不相等！因为在这个删除操作的过程中没有对expectedModCount重新赋值，所以就抛出异常了。

参考链接：https://blog.csdn.net/qq_35056292/article/details/79751233