Java - 将对象列表转换为地图数据结构的地图

Question

我正在尝试将对象数据结构列表转换为地图。

Map<String, Map<Integer, StudentModel>>

字符串是运动变量

整数是 orderId。

我的逻辑返回值低于如下所示。我正在寻找更好的方法来实现这一目标。

@Data
@Builder
public class Student {    
    private String name;
    private int rollno;
    private int studentage;
    private String sports;
    private int orderId;
}   

@Data
@Builder
public class StudentModel {
    private String name;
    private int rollno;
    private int studentage;
    private String sports;
}

创建相同的数据并添加到列表中：

arraylist.add(new Student(223, "Zebra", 26, "cricket", 1));
arraylist.add(new Student(245, "Rahul", 24, "cricket", 2));
arraylist.add(new Student(209, "Ajeet", 32, "cricket", 3));
arraylist.add(new Student(140, "Abhay", 28, "basketball", 4));
arraylist.add(new Student(270, "Ranger", 29, "basketball", 5));
arraylist.add(new Student(250, "Ranger1", 39, "basketball",6));
Collections.sort(arraylist, Comparator.comparing(Student::getOrderId));

我的预期输出：

{
 cricket={
     1=Student(sports=cricket, rollno=223, name=Zebra, studentage=26), 
     2=Student(sports=cricket, rollno=245, name=Rahul, studentage=24), 
     3=Student(sports=cricket, rollno=209, name=Ajeet, studentage=32)
     }, 
 basketball={
     4=Student(sports=basketball, rollno=140, name=Abhay, studentage=28), 
     5=Student(sports=basketball, rollno=270, name=Ranger, studentage=29), 
     6=Student(sports=basketball, rollno=250, name=Ranger1, studentage=39)
     }
 }

逻辑写法：

public Map<String, Map<Integer, StudentModel>> studentModel() {
    Map<Integer, StudentModel> studentMap = new LinkedHashMap<>();
    Map<String, Map<Integer, StudentModel>> inputMap = new LinkedHashMap<>();
    for (Student student : arraylist) {
        StudentModel studentModel = StudentModel.builder().name(student.getName)
        .rollno(student.getRollno)
        .studentage(student.getStudentage)
        .sports(student.getSports)
        studentMap.put(studentModel.getOrderId, studentModel);
        inputMap.put(student.getSports, studentMap);
    }
 }

上面的逻辑返回下面的结果：

 {
 cricket={
     1=Student(sports=cricket, rollno=223, name=Zebra, studentage=26), 
     2=Student(sports=cricket, rollno=245, name=Rahul, studentage=24), 
     3=Student(sports=cricket, rollno=209, name=Ajeet, studentage=32),
     4=Student(sports=basketball, rollno=140, name=Abhay, studentage=28), 
     5=Student(sports=basketball, rollno=270, name=Ranger, studentage=29), 
     6=Student(sports=basketball, rollno=250, name=Ranger1, studentage=39)
     }, 
 basketball={
     1=Student(sports=cricket, rollno=223, name=Zebra, studentage=26), 
     2=Student(sports=cricket, rollno=245, name=Rahul, studentage=24), 
     3=Student(sports=cricket, rollno=209, name=Ajeet, studentage=32),
     4=Student(sports=basketball, rollno=140, name=Abhay, studentage=28), 
     5=Student(sports=basketball, rollno=270, name=Ranger, studentage=29), 
     6=Student(sports=basketball, rollno=250, name=Ranger1, studentage=39)
     }
 }

Answer 1

问题是您对inputMap 的每个条目仅使用studentMap 的单个实例/内存，因此您会看到每个“运动”键的所有6 个学生条目，因为每个inputMap 条目值都指向到studentMap 的相同参考值，从而观察任何运动类型键所做的更改。

您在这里要做的是为每种“运动”类型创建studentMap 的新内存，所以。您可以尝试这样做：

public Map<String, Map<Integer, StudentModel>> studentModel() {

    Map<Integer, StudentModel> studentMap;
    Map<String, Map<Integer, StudentModel>> inputMap = new LinkedHashMap<>();
    for (Student student : arraylist) {
        StudentModel studentModel = StudentModel.builder().name(student.getName)
        .rollno(student.getRollno)
        .studentage(student.getStudentage)
        .sports(student.getSports)

        // This would create new memory for each type of sport
        studentMap = inputMap.getOrDefault(student.getSports, new LinkedHashMap<Integer, StudentModel>());
        studentMap.put(studentModel.getOrderId, studentModel);
        inputMap.put(student.getSports, studentMap);
    }
 }

Answer 2

您需要为每项运动创建一张新地图：

public Map<String, Map<Integer, StudentModel>> studentModel() {
    Map<String, Map<Integer, StudentModel>> inputMap = new LinkedHashMap<>();
    for (Student student : arraylist) {
        StudentModel studentModel = StudentModel.builder().name(student.getName)
        .rollno(student.getRollno)
        .studentage(student.getStudentage)
        .sports(student.getSports)
        // create a new map and insert it into the outer map if it's not already there
        Map<Integer, StudentModel> studentMap = inputMap.computeIfAbsent(student.getSports, k -> new LinkedHashMap<>());
        studentMap.put(studentModel.getOrderId, studentModel);
    }
 }

与问题无关，但应使用 getter 方法访问字段，例如而不是 student.getSports 你应该有一个私有的 sports 字段和一个 getSports() 方法。

Answer 3

groupingBy 方法完全符合您的需要。您想根据他们的运动对原始数组列表中的学生进行分组：

Map<String, List<Student>> inputMap = arraylist.stream().collect(Collectors.groupingBy(Student::getSports));

Answer 4

发生这种情况是因为您在两个类别中使用相同的学生地图实例。

这是你的代码

 public Map<String, Map<Integer, StudentModel>> studentModel() {
    Map<Integer, StudentModel> studentMap = new LinkedHashMap<>();
    Map<String, Map<Integer, StudentModel>> inputMap = new LinkedHashMap<>();
    for (Student student : arraylist) {
        StudentModel studentModel = StudentModel.builder().name(student.getName)
                .rollno(student.getRollno)
                .studentage(student.getStudentage)
                .sports(student.getSports)
        studentMap.put(studentModel.getOrderId, studentModel);//logical error here
        inputMap.put(student.getSports, studentMap);//error here
    }
}

使用下面的代码获得所需的结果

public Map<String, Map<Integer, StudentModel>> studentModel() {
    Map<Integer, StudentModel> studentMap = new LinkedHashMap<>();
    Map<String, Map<Integer, StudentModel>> inputMap = new LinkedHashMap<>();
    for (Student student : arraylist) {
        if (inputMap.get(student.getSports())==null){
            studentMap = new LinkedHashMap<>();
        }else{
            studentMap = inputMap.get(student.getSports());
        }
        StudentModel studentModel = StudentModel.builder().name(student.getName)
                .rollno(student.getRollno)
                .studentage(student.getStudentage)
                .sports(student.getSports)
        studentMap.put(studentModel.getOrderId, studentModel);//error here
        inputMap.put(student.getSports, studentMap);//error here
    }
}`