HashMap没有按键找到价值 - 为什么？

Question

我有一节课: Question

它有id和content.

我有一个HashMap与Questions作为键和asnwers(Strings)作为价值.

我尝试使用for键从我的地图中查找答案Question.

但是它总是给出null值,我不知道为什么因为map确实包含了我的问题.

码:

            Log.i("info", "map:");
            Log.i("info", questionAnswers.toString());
            Log.i("info", "================================");

            Log.i("info", "question we are looking for:");
            Log.i("info", question.toString());
            Log.i("info", "================================");

            Log.i("info", "anwer is null: " + questionAnswers.get(question));
            Log.i("info", "================================");

            Iterator it = questionAnswers.entrySet().iterator();

            //Iterating the map just to get information for test's sake
            while (it.hasNext()) {

                Map.Entry<Question, String> entry = (Map.Entry<Question, String>) it.next();

                //Question in the map
                Question questionInCont = entry.getKey();

                //Testing the equals method
                if (questionInCont.equals(question)) {


                    Log.i("info", "================================");
                    Log.i("info", "question we are looking for is equals to a question in the map");
                    Log.i("info", questionInCont.getQuestionId() + " == " + question.getQuestionId());

                    String answer = questionAnswers.get(question);

                    //Great, equals working but still no answer back, only null
                    Log.i("info", "anwer is still null: " + answer);
                    Log.i("info", "================================");
                }
            }

日志输出:

11-21 13:22:13.083: I/info(18350): map:
11-21 13:22:13.083: I/info(18350): {Question{questionId='9', content='What is your name?'}=John, Question{questionId='8', content='How old are you?'}=33}
11-21 13:22:13.083: I/info(18350): ================================
11-21 13:22:13.083: I/info(18350): question we are looking for:
11-21 13:22:13.084: I/info(18350): Question{questionId='8', content='How old are you?'}
11-21 13:22:13.084: I/info(18350): ================================
11-21 13:22:13.084: I/info(18350): anwer is null: null
11-21 13:22:13.084: I/info(18350): ================================
11-21 13:22:13.084: I/info(18350): ================================
11-21 13:22:13.084: I/info(18350): question we are looking for is equals to a question in the map
11-21 13:22:13.084: I/info(18350): 8 == 8
11-21 13:22:13.084: I/info(18350): anwer is still null: null
11-21 13:22:13.084: I/info(18350): ================================

---

所以你可以看到:

1. 我的HashMap 确实包含了我正在寻找的问题.
2. 它确实有答案(价值)
3. 当两个问题具有相同的id时,问题equals方法确实返回true.

编辑:

感谢您的评论,我的问题类没有hashCode()功能.这是问题吗？

Question class:


public class Question implements Serializable {

    String questionId;
    String content;


    public Question(String id, String content) {
        this.questionId = id;
        this.content = content;
    }


    @Override
    public boolean equals(Object obj) {

        if (obj instanceof Question) {
            Question question = (Question) obj;
            return questionId.equals(question.getQuestionId());
        }

        return false;
    }
 }

播下为什么我得到null作为答案？

Answer 1

您的主要问题是您只实现了该equals()方法.这对于HashMap的值是严格禁止的,因为equals()总是必须与它一起实现hashCode().

HashMap中的每个条目(m1)和第二个条目(m2)必须满足以下条件:m1.equals(m2)表示m1.hashCode()== m2.hashCode()