英文:
Grouping a List by two fields in Java
问题
// 我有一个名为 Student 的类。它包含城市、姓名和国家字段。我有一个包含 Student 对象的 ArrayList。
// 我想做的是根据城市和国家字段对学生对象进行分组。我可以像这样使用一个属性来做到：
for (Student student : studentList) {
Long key = student.country;
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);
}
}
英文:
I have a Student class. It contains the city, name and country fields.I have an ArrayList that contains Student objects. What I want to do is to group student objects with city and country fields. I can do it with one attribute like this
But how can i do both name and country? Do you have any idea? I am using java 7.
for (Student student: studentList) {
Long key = student.country;
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);
}
}
答案1
得分: 1
使用一些允许您支持多个值的关键类型，例如：
List<Object> key = Arrays.asList(city, country);
将您的 Long key ... 行替换为以下内容（并相应更新 map 的类型），例如：
Map<List<Object>, List<Student>> map = new HashMap<>();
（List<Object> 实际上在这里不是一个很好的选择，这只是一种权宜之计：我会使用类似于特定的 CityAndCountry 类，例如使用 Google 的 AutoValue 进行定义。）
英文:
Use some key type which allows you to support multiple values, for example:
List<Object> key = Arrays.asList(city, country);
Replace your Long key ... line with this (and update map's type accordingly), e.g.
Map<List<Object>, List<Student>> map = new HashMap<>();
(List<Object> is actually not a great choice here, it's merely expedient: I'd use something like a specific CityAndCountry class, defined for example using Google's AutoValue)
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