问题

// Java 6 code
List<UserVO> userList = new ArrayList<UserVO>(); // Your list of UserVO objects

Map<String, List<UserVO>> studentsByCountry = new HashMap<String, List<UserVO>>();

// Grouping the users by country
for (UserVO user : userList) {
    String country = user.getCountry();
    if (!studentsByCountry.containsKey(country)) {
        studentsByCountry.put(country, new ArrayList<UserVO>());
    }
    studentsByCountry.get(country).add(user);
}

// Printing the grouped data
for (Map.Entry<String, List<UserVO>> entry : studentsByCountry.entrySet()) {
    System.out.print(entry.getKey() + " - ");
    List<UserVO> countryUsers = entry.getValue();
    for (int i = 0; i < countryUsers.size(); i++) {
        System.out.print(countryUsers.get(i).getName()); // Assuming UserVO has a getName() method
        if (i < countryUsers.size() - 1) {
            System.out.print(", ");
        }
    }
    System.out.println();
}

// Shuffling the map based on display order
List<String> displayOrder = Arrays.asList("CountryC", "CountryB", "CountryA");
Map<String, List<UserVO>> shuffledMap = new LinkedHashMap<String, List<UserVO>>();
for (String country : displayOrder) {
    if (studentsByCountry.containsKey(country)) {
        shuffledMap.put(country, studentsByCountry.get(country));
    }
}

// Printing the shuffled map
for (Map.Entry<String, List<UserVO>> entry : shuffledMap.entrySet()) {
    System.out.print(entry.getKey() + " - ");
    List<UserVO> countryUsers = entry.getValue();
    for (int i = 0; i < countryUsers.size(); i++) {
        System.out.print(countryUsers.get(i).getName());
        if (i < countryUsers.size() - 1) {
            System.out.print(", ");
        }
    }
    System.out.println();
}

Please note that the above code assumes you have a class named UserVO with appropriate methods like getCountry() and getName() for retrieving the country and name of a user respectively. Additionally, it assumes that you have the list of UserVO objects named userList. The code first groups the users by country and then shuffles the map based on the specified display order.

Map<String, List<UserVO>> studentsByCountry
= resultList.stream().collect(Collectors.groupingBy(UserVO::getCountry));
for (Map.Entry<String, List<UserVO>> entry: studentsByCountry.entrySet())
System.out.println("Student with country = " + entry.getKey() + " value are " + entry.getValue());

CountryA - UserA, UserB, UserC
CountryB - UserM, User
CountryC - UserX, UserY

CountryC - UserX, UserY
CountryB - UserM, User
CountryA - UserA, UserB, UserC

答案1

得分: 4

这是您在普通Java中的做法。请注意，Java 6不支持菱形操作符，因此您必须始终明确地使用<String，List<UserVO>>。

Map<String, List<UserVO>> studentsByCountry = new HashMap<String, List<UserVO>>();
for (UserVO student : resultList) {
  String country = student.getCountry();
  List<UserVO> studentsOfCountry = studentsByCountry.get(country);
  if (studentsOfCountry == null) {
    studentsOfCountry = new ArrayList<UserVO>();
    studentsByCountry.put(country, studentsOfCountry);
  }
  studentsOfCountry.add(student);
}

使用流会更简短，对吗？所以尝试升级到Java 8！

要根据反向字母顺序的特定顺序，如评论中所述，您可以将第一行替换为以下内容：

Map<String, List<UserVO>> studentsByCountry = new TreeMap<String, List<UserVO>>(Collections.reverseOrder());

Map<String, List<UserVO>> studentsByCountry = new HashMap<String, List<UserVO>>();
for (UserVO student: resultList) {
String country = student.getCountry();
List<UserVO> studentsOfCountry = studentsByCountry.get(country);
if (studentsOfCountry == null) {
studentsOfCountry = new ArrayList<UserVO>();
studentsByCountry.put(country, studentsOfCountry);
}
studentsOfCountry.add(student);
}

Map<String,List<UserVO>> studentsByCountry = new TreeMap<String,List<UserVO>>(Collections.reverseOrder());