按属性将对象列表分组

Question

我需要使用特定对象的属性（位置）对对象列表（Student）进行分组。代码如下：

public class Grouping {
    public static void main(String[] args) {

        List<Student> studlist = new ArrayList<Student>();
        studlist.add(new Student("1726", "John", "New York"));
        studlist.add(new Student("4321", "Max", "California"));
        studlist.add(new Student("2234", "Andrew", "Los Angeles"));
        studlist.add(new Student("5223", "Michael", "New York"));
        studlist.add(new Student("7765", "Sam", "California"));
        studlist.add(new Student("3442", "Mark", "New York"));

    }
}

class Student {
    String stud_id;
    String stud_name;
    String stud_location;

    Student(String sid, String sname, String slocation) {
        this.stud_id = sid;
        this.stud_name = sname;
        this.stud_location = slocation;
    }
}

请给我一个干净的方法。




Answer 1

Map<String, List<Student>> map = new HashMap<String, List<Student>>();

for (Student student : studlist) {
    String key  = student.stud_location;
    if(map.containsKey(key)){
        List<Student> list = map.get(key);
        list.add(student);

    }else{
        List<Student> list = new ArrayList<Student>();
        list.add(student);
        map.put(key, list);
    }

}

Answer 2

这将把students对象添加到HashMap中，并将locationID作为键。

HashMap<Integer, List<Student>> hashMap = new HashMap<Integer, List<Student>>();

遍历此代码并将学生添加到HashMap：

if (!hashMap.containsKey(locationId)) {
    List<Student> list = new ArrayList<Student>();
    list.add(student);

    hashMap.put(locationId, list);
} else {
    hashMap.get(locationId).add(student);
}

如果你想让所有的学生都有特定的位置细节，那么你可以使用这个：

hashMap.get(locationId);

这将使所有学生都具有相同的位置ID。

Answer 3

在Java 8中：

Map<String, List<Student>> studlistGrouped =
    studlist.stream().collect(Collectors.groupingBy(w -> w.stud_location));