java - 如何对 Map> 进行排序？

Question

我对这个问题做了很多研究，但我还没有找到一种方法来对自定义对象列表 ( Map<String, List<CustomObj>> ) 的映射进行排序，基于 CustomObj 的比较。属性(如 SORT_BY_NAME 、 SORT_BY_DATE 等)。

我的问题的一个激励性示例是:

• 我有一个自定义对象:Person (属性为 Name 、 DateOfBith 等...)；
• 我有一个 Map的 Person对象 List如:Map<String, List<Person>> .映射键是 String用于其他目的；
• 我想创建一个比较器和一个排序方法，根据 Person 的属性之间的比较按升序对 map 进行排序。对象(姓名、日期等..)

为简单起见，我报告了真实代码，但适用于 Person 的简化案例对象，因为它已经代表了实体的概念。

Person.java -> 自定义对象

public class Person {

     private String name;
     private Date dateOfBirth;
     ...

     // Empty and Full attrs Constructors
     ...

     // Getter and Setter
     ...

     // Comparator by name
     public static Comparator<Person> COMPARE_BY_NAME = Comparator.comparing(one -> one.name);
     // Comparator by date
     public static Comparator<Person> COMPARE_BY_DATE = Comparator.comparing(one -> one.dateOfBirth);

}

Sorter.java -> 排序器对象

public class Sorter {

     // List Comparator of Person by Date 
     public static final Comparator<? super List<Person>> COMPARATOR_BY_DATE = (Comparator<List<Person>>) (p1, p2) -> {
          for (Persontab person1: p1) {
              for (Person person2: p2) {
                  return Person.COMPARE_BY_DATE.compare(person1, person2);
              }
          }
          return 0;
     };

     // List Comparator of Person by Name
     public static final Comparator<? super List<Person>> COMPARATOR_BY_NAME = (Comparator<List<Person>>) (p1, p2) -> {
          for (Persontab person1: p1) {
              for (Person person2: p2) {
                  return Person.COMPARE_BY_NAME.compare(person1, person2);
              }
          }
          return 0;
     };

     // Sorting method
     public Map<String, List<Person>> sort(Map<String, List<Person>> map, Comparator<? super List<Person>> comparator) {
          return map.entrySet()
            .stream()
            .sorted(Map.Entry.comparingByValue(comparator))
            .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (v1, v2) -> v1, LinkedHashMap::new));
     }

}

Main.java -> 开始代码

public class MainApp {

     public static void main(String[] args) {

          Map<String, List<Person>> exampleMap = new HashMap<>();
          List<Person> personList = new ArrayList<>();
          personList.add(new Person("name1", new Date("2022-01-01")));              
          personList.add(new Person("name12", new Date("2022-01-05")));
          personList.add(new Person("name13", new Date("2022-01-03")));
          map.put("2022-01", personList);

          personList.clear();
          personList.add(new Person("name14", new Date("2021-02-01")));              
          personList.add(new Person("name3", new Date("2021-02-05")));
          personList.add(new Person("name4", new Date("2021-02-03")));
          map.put("2021-02", personList);

          Sorter sorter = new Sorter();

          // Example of sorting by date
          map = sorter.sort(exampleMap, Sorter.COMPARATOR_BY_DATE);
          // In this case the sorting works correctly, or rather it sorts the items by date as I expect
         
          // Example of sorting by name
          map = sorter.sort(exampleMap, Sorter.COMPARATOR_BY_NAME);
          // In this case, I don't think sorting works correctly. Sort each list of elements for each key in ascending order. But it doesn't sort the map elements.

          /* I expect to have the following map when sort by date:
             "2021-02": [
               Person("name14", new Date("2021-02-01")),
               Person("name4", new Date("2021-02-03")),
               Person("name3", new Date("2021-02-05"))
             ], 
             "2022-01": [
               Person("name14", new Date("2021-02-01")),
               Person("name13", new Date("2022-01-03")),
               Person("name12", new Date("2022-01-05"))
             ]
             
     }

}

Answer 1

首先，让我们重申:HashMap 是无序的，所以您需要其他东西。您的 Sorter.sort() 方法实际上将值收集到 LinkedHashMap 中，它提供基于插入顺序的迭代顺序，并且适合您的用例。明确一点(也是为了其他人着想):这不会对 map 本身进行排序，而是创建一个新的 LinkedHashMap。

现在你的比较器:如果你想比较 2 个列表，你可能想比较相同索引的元素。因此你的比较器需要是这样的:

Comparator<List<Person>> = (l1, l2) -> {
   Iterator<Person> itr1 = l1.iterator();
   Iterator<Person> itr2 = l2.iterator();

   while( itr1.hasNext() && itr2.hasNext() ) {
     Person p1 = itr1.next();
     Person p2 = itr1.next();

     int result = Person.COMPARE_BY_DATE.compare(p1, p2);
     if( result != 0 ) {
       return result;
     }
   }

   return 0;
};
 

但是，列表也可能有不同的长度，因此您可能也想处理它:

Comparator<List<Person>> = (l1, l2) -> {
   //iterators and loop here

   //after the loop it seems all elements at equal indices are equal too
   //now compare the sizes

   return Integer.compare(l1.size(), l2.size());
}