java集合的遍历删除

java集合的遍历删除。

使用普通for循环遍历集合

//以下代码不会有任何问题   
	List list = new ArrayList();
    list .add(1);
	list.add(2);
    list.add(3);
    for(int i = 0 ; i < list.size();i++){
        list.remove(list.get(i));//或者 list.remove(i);或者使用iterator.remove();
    }

当你使用集合的forEach()遍历集合时，

//这里会报异常： java.util.ConcurrentModificationException   
	List list = new ArrayList();
    list .add(1);
	list.add(2);
    list.add(3);
	for (Integer integer : list) {
         list.remove(integer);
    }

这是因为forEach底层用的是iterator迭代器

当使用iterator迭代器遍历集合时

        List integerList=new ArrayList<>();
        Iterator iterator = integerList.iterator();
        while (iterator.hasNext()){
            Integer integer=iterator.next();
            integerList.remove(integer);//报异常java.util.ConcurrentModificationException  
            iterator.remove(); //没有问题
        }

原因：

这里查看ArrayList实现iterator的源码，注意iterator的next()和remove()方法。
你会发现，iterator的hasnext()和remove()方法，都调用了checkForComodification()，当ArrayList的modCount （修改次数）!= expectedModCount（迭代器修改次数）时，会抛异常，当你调用iterator的remove()方法，会同步两个修改次数，而ArrayList不会同步，所以会抛异常

  private class Itr implements Iterator {
        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();//检查集合修改次数
            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();//检查集合修改次数

            try {
                ArrayList.this.remove(lastRet);
                cursor = lastRet;
                lastRet = -1;
                expectedModCount = modCount;//这里同步了两个修改次数
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }

        @Override
        @SuppressWarnings("unchecked")
        public void forEachRemaining(Consumer 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();
        }
    }