Caused by: java.util.ConcurrentModificationException at java.util.ArrayList$Itr.checkForComodificat

if(lists.size() < 1){
    lists.add(s);	
}else{
	for (SubmitAnswer sub : lists) {
	if(sub.getExamPaperId() == s.getExamPaperId()){
		continue;
	}else{
		lists.add(s);						
		}
	}
}

原因是因为在增强的for循环中不可以改变lists的大小