java-在HashMap中包含适当的数据 [英] java - content appropriate data in HashMap

问题描述

想象一下，您有一本学生评价日记。每个学生在日记中对每个主题都有一些标记。我想将其存储在 HashMap<> 中，但我不知道为什么标记会合并。

Imagine you have a journal of student evaluations. Each student have some marks of each subject in a journal. I want to store this in HashMap<> but I can't figure out why do marks combine.

在日记类中：

public class Journal {
    private static HashMap<String, HashMap<String, ArrayList<Integer>>> journal = new HashMap<>(); // "Student" -> "Subject", mark[]

    private HashMap<String, ArrayList<Integer>> journalContainer = new HashMap<>();
    private ArrayList<Integer> marks = new ArrayList<>();

    public void addMark(String student, String subject, int mark) {
        marks.add(mark);
        journalContainer.put(subject, mark);
        journal.put(student, journalContainer);
    }

    public static void outputMarks() {
        for(HashMap.Entry<String, HashMap<String, ArrayList<Integer>>> entry : journal.entrySet())
        {
            System.out.println(entry.getKey() + "/" + entry.getValue());
        }
    }
}

在主要班级：

public class Main {
    public static void main(String[] argc) {
        getJournal().addMark("Alex", "math", 4); // name, subject, mark
        getJournal().addMark("Alex", "math", 2);
        getJournal().addMark("George", "english", 2);
        getJournal().addMark("George", "english", 2);

        Journal.outputMarks();
    }
}

因此输出为：

Alex/{english=[4, 2, 2, 2], math=[4, 2, 2, 2]}
George/{english=[4, 2, 2, 2], math=[4, 2, 2, 2]}

但是正确的输出应该是：

But the right output should be:

Alex/{math=[4, 2]}
George/{english=[2, 2]}

我不知道我是什么做错了。有人可以帮忙吗？

I can't figure out what am I doing wrong. Anyone can help?

推荐答案

问题是您只有一个 marks = new ArrayList<> （）; 实例，您将其用作所有内部 Map 中的值，以及单个 journalContainer = new HashMap<>（）; 实例，该实例用作外部 Map 中的值。

The problem is that you have a single marks = new ArrayList<>(); instance that you use as values in all your inner Maps, as well as a single journalContainer = new HashMap<>(); instance that you use as values in your outer Map.

我将删除这两个实例变量，而是使用局部变量。

I'd remove these two instance variables, and instead use local variables.

您应该使用不同的 ArrayList s和内部 HashMap s作为 Map s的值：

You should use distinct ArrayLists and inner HashMaps as values of your Maps:

public void addMark(String student, String subject, int mark) {
    HashMap<String, ArrayList<Integer>> journalContainer = journal.get(student);
    if (journalContainer == null) {
        journalContainer = new HashMap<>();
        journal.put(student,journalContainer);
    }
    ArrayList<Integer> marks = journalContainer.get(subject);
    if (marks == null) {
        marks = new ArrayList<>();
        journalContainer.put(subject, marks);
    }
    marks.add(mark);
}

BTW，让实例方法修改a静态成员（新闻 地图）。将方法设为静态，或将 Map 设为非静态。

BTW, it doesn't make sense to have an instance method that modifies a static member (the journal Map). Either make the method static, or make the Map non static.

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