list删除时java.util.ConcurrentModificationException

        for(Question q:questionList) {
            if(!qIds.contains(q.getQuestionId())){
                questionList.remove(q);
            }
        }

        for(int k=0;k<questionList.size();k++) {
            if(!qIds.contains(questionList.get(k).getQuestionId())){
                questionList.remove(k);
                k--;
            }
        }

        for(int k=0;k<questionList.size();k++) {
            if(!qIds.contains(questionList.get(k).getQuestionId())){
                questionList.remove(questionList.get(k));
                k--;
            }
        }