为什么不建议用增强for循环删除列表中元素（通俗易懂版）分析然后最后解决方案扩充

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我是靠谱客的博主会撒娇衬衫，这篇文章主要介绍为什么不建议用增强for循环删除列表中元素（通俗易懂版）分析然后最后解决方案扩充，现在分享给大家，希望可以做个参考。

分析

首先

然后

最后

解决方案

扩充

首先我们来看错误案例

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    @Test
    public void doSomething(){
        ArrayList<String> list = new ArrayList<String>();
        list.add("昨天");
        list.add("今天");
        list.add("明天");
        for (String s : list){
            if (s.equals("昨天")){
                list.remove(s);
            }
        }
    }

这个将会报错

java.util.ConcurrentModificationException

分析

首先

调用了三次add，我们来看list中add的源码（不会的可以看我关于源码博文的list讲解），最终调用了modCount++。这个modCount变量就是为了记录操作list的次数，详细可以看源码。

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private void ensureExplicitCapacity(int minCapacity) {
        modCount++;

        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }

然后

我们都知道增强for循环，相当于是迭代器的语法糖。ArrayList中迭代器的方法源码

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public Iterator<E> iterator() {
        return new Itr();
    }

//Itr为内部类
    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; //赋值给expectedModCount

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();
        }
    }

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public Iterator<E> iterator() {
        return new Itr();
    }

//Itr为内部类
    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; //赋值给expectedModCount

        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();
        }
    }

增强for循环相当于在while里调用了hasnext方法以及调用了next的方法，最后调用了外部的remove方法（注意，不是内部类itr的方法，此时expectedModCount为3。

调用remove方法时的源码，实质调用了fastRemove，此刻modCount又加1变为了4

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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
    }

最后

还记得上边循环调用了next方法吗，我们来看一下next方法

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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];
        }

里边有个checkForComodification()

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

而此刻modCount为4，expectedModCount为3，不相等，所以抛出异常。

解决方案

我们注意到，在Itr内部类中有个remove方法，其中有个代码让expectedModCount=modCount（代码注释标出）

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public void remove() {
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();

            try {
                ArrayList.this.remove(lastRet);
                cursor = lastRet;
                lastRet = -1;
                expectedModCount = modCount; //是这个代码
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }

所以我们需要调用内部类里边的remove。标准代码如下：

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    @Test
    public void doSomething(){
        ArrayList<String> list = new ArrayList<String>();
        list.add("昨天");
        list.add("今天");
        list.add("明天");
        Iterator<String> iterator = list.iterator();
        while (iterator.hasNext()){
            if (iterator.next().equals("昨天")){
                iterator.remove();
            }
        }
        System.out.println(list);
    }