通过 while 循环在 hashmap 中添加 <Key, Value> 但不起作用

Question

我有以下 CSV 文件，我想创建一个哈希图，其唯一键应该是每行的第一个值，值应该是 arraylist，其中包含该行第一个值下的每一行的信息. CSV 示例如下：

1,0
1,1
1,2
1,3
1,4
2,0
2,1
2,2
4,0
10,0
10,1
10,2
10,3
10,4
10,5

理想情况下，我希望最终的 ArrayList 类似于：

{1=[1,0, 1,1, 1,2, 1,3, 1,4], 2=[2,0, 2,1, 2,2], 4=[4,0], 10=[10,0, 10,1, 10,2, 10,3, 10,4, 10, 5]}

下面是我用来尝试通过while循环解决这个问题的代码

public static void main(String[] args) {
        int activityRecord = 1;
        String activitiesFile = "scenarios/BrusselsPopulationFromR/BrusselsActivities.csv";

        HashMap<Integer, ArrayList<String>> idAndAllActivities = new HashMap<>();
        try {
            BufferedReader activityReader = new BufferedReader(new FileReader(activitiesFile));
            String agentActivity = null;
            ArrayList activities = new ArrayList();

            while ((agentActivity = activityReader.readLine()) != null){
                String activityWithAllInfo = agentActivity;
                String[] activitySpilted = agentActivity.split(",");
                int activityAgentID = Integer.parseInt(activitySpilted[0]);

                if (activityAgentID == activityRecord){
                    activities.add(activityWithAllInfo);
                } else if (activityAgentID != activityRecord){
                    idAndAllActivities.put(activityRecord, activities);
                    activityRecord = activityAgentID;
                    activities.clear();
                    activities.add(activityWithAllInfo);
                }
            }

        } catch (IOException e) {
            e.printStackTrace();
        }

        System.out.println(idAndAllActivities);
    }

基本上，我会尝试证明activityAgentID == activityRecord是否（原本为1），如果不同，我会将添加到hashmap并清除activities arrayList。然而，这个程序的输出是这样的：

{1=[10,0, 10,1, 10,2, 10,3, 10,4, 10,5], 2=[10,0, 10,1, 10,2, 10,3, 10,4, 10,5], 4=[10,0, 10,1, 10,2, 10,3, 10,4, 10,5]}

这不是我想要的，我试图调试它，但我找不到原因，因为我对 Java 真的很陌生......我想知道你是否知道如何解决这个问题？任何提示将不胜感激！

谢谢！

Answer 1

您的代码中的问题是您重用了activities 数组。活动数组和HashMap 中分配的数组都指向同一个引用，所以如果你更新activities 数组，地图中的所有条目也会更新。

假设文件有排序的键，你可以像这样调整你的代码

HashMap<Integer, ArrayList<String>> idAndAllActivities = new HashMap<>();
try {
    BufferedReader activityReader = new BufferedReader(new FileReader(activitiesFile));
    String agentActivity = null;
    ArrayList<String> activities = new ArrayList<>();

    while ((agentActivity = activityReader.readLine()) != null){
          String[] activitySpilted = agentActivity.split(",");
          int activityAgentID = Integer.parseInt(activitySpilted[0]);

          if (activityAgentID == activityRecord){
               activities.add(agentActivity);
          } else {
               idAndAllActivities.put(activityRecord, activities);
               activityRecord = activityAgentID;
               activities = new ArrayList<>();
               activities.add(agentActivity);
          }
     }

     idAndAllActivities.put(activityRecord, activities);

} catch (IOException e) {
        e.printStackTrace();
}

System.out.println(idAndAllActivities);

请注意，您不需要这个：String activityWithAllInfo = agentActivity;，因为String 在 Java 中是不可变的

Answer 2

你可以用另一种方式来做。如果CSV文件的顺序发生如下变化也不会中断的方式：

1,0
1,1
2,0
1,2
10,0
10,1
10,2
1,3
1,4
2,1
2,2
4,0
10,3
10,4
10,5

使用此 CSV，您的代码将失败，因为您依赖 CSV 文件将所有 1 活动记录放在一起。为了解决这个问题，请尝试以下操作（注意您不再依赖activityRecord）：

public static void main(String[] args) {
    String activitiesFile = "scenarios/BrusselsPopulationFromR/BrusselsActivities.csv";

    HashMap<Integer, ArrayList<String>> idAndAllActivities = new HashMap<>();
    try {
        BufferedReader activityReader = new BufferedReader(new FileReader(activitiesFile));
        String agentActivity = null;

        while ((agentActivity = activityReader.readLine()) != null){
            String activityWithAllInfo = agentActivity;
            String[] activitySpilted = agentActivity.split(",");
            int activityAgentID = Integer.parseInt(activitySpilted[0]);

            if (idAndAllActivities.containsKey(activityAgentID)){
                ArrayList existentActivities = idAndAllActivities.get(activityAgentID);
                existentActivities.add(activityWithAllInfo);
            } else {
                ArrayList activities = new ArrayList();
                activities.add(activityWithAllInfo);
                idAndAllActivities.put(activityAgentID, activities);
            }
        }

    } catch (IOException e) {
        e.printStackTrace();
    }

    System.out.println(idAndAllActivities);
}

Answer 3

假设您使用的是 Java 8 或更高版本，在 Files.lines 和 Collectors.groupingBy 的帮助下，任务可以大大简化并且代码更具可读性：

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class Example {

    public static void main(String[] args) {
        String activitiesFile = "scenarios/BrusselsPopulationFromR/BrusselsActivities.csv";
        Map<Integer,List<String>> idAndAllActivities = null;
        try (Stream<String> content = Files.lines(Paths.get(activitiesFile))) {
            idAndAllActivities = content.collect(Collectors.groupingBy(row -> Integer.parseInt(row.split(",")[0])));
        } catch (IOException e) {
            e.printStackTrace();
        }
        System.out.println(idAndAllActivities);
    }
}