Question

我有一个班级：Question

它有id和content。

我的HashMap Questions为键，asnwers(Strings)为值。

我尝试从地图中查找答案，使用Question作为密钥。

然而它总是提供null 值，我不知道为什么，因为地图确实包含我的问题。

代码：

            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：

我的HashMap 包含问题我正在寻找。
确实有答案（价值）
当两个问题具有相同的ID时，等于方法的问题确实返回。

E D I T：

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;
    }
 }

播种为什么我为什么得到null作为答案？

Answer 1

您的主要问题是您只实施equals()方法。严格禁止使用HashMap的值，因为hashCode()始终必须与--jars /path/to/spark-csv.jar,/path/to/commons-csv.jar一起实现。

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（）

Answer 2

您必须覆盖在HashMap中用作键的类的hashCode方法。

可能很简单：

@Override
public int hashCode() {
    return questionId.hashCode();
}

Answer 3

正如其他人所说，HashMap可能无法检索与您的密钥关联的条目，因为您的Question类缺少hashCode（）函数的覆盖（请参阅此参考：https://docs.oracle.com/javase/7/docs/api/java/lang/Object.html#hashCode()）。 / p>

我建议按如下方式实现该功能：

@Override
public int hashCode() {
     return questionId.hashCode() + content.hashCode();
}

一般规则是If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result.（来自上面的链接）。

HashMap没有按键找到值 - 为什么？

所以你可以看到：

3 个答案: