SharedPreferences存储set

Set<String> checkAnswer = new HashSet<String>();
checkAnswer=(HashSet<String>)SPUtils.get(context,message.getFrom()+"%%"+message.getTo(),checkAnswer);
Log.d("TrueWordcheckAnswer","checkAnswer:"+checkAnswer.toString());

if (checkAnswer.isEmpty() || (!checkAnswer.contains(message.getMsgId()))){
    checkAnswer = new HashSet<String>(checkAnswer);
    checkAnswer.add(message.getMsgId());
    SPUtils.put(context,message.getFrom()+"%%"+message.getTo(),checkAnswer);
}