java - 如何获得给定数组列表的最大国家/地区

Question

如何将结果集设置为 {GERMANY=3} 而不是 {GERMANY=3, POLAND=2, UK=3}

public class Student {
    private final String name;
    private final int age;
    private final Country country;
    private final int score;

    // getters and setters (omitted for brevity)
}

public enum Country { POLAND, UK, GERMANY }


//Consider below code snippet 

public static void main(String[] args) {
    List<Student> students = Arrays.asList(
            /*          NAME       AGE COUNTRY          SCORE */
            new Student("Jan",     13, Country.POLAND,  92),
            new Student("Anna",    15, Country.POLAND,  95),
            new Student("Helga",   14, Country.GERMANY, 93),
            new Student("Leon",    14, Country.GERMANY, 97),
            new Student("Chris",    15, Country.GERMANY, 97),
            new Student("Michael", 14, Country.UK,      90),
            new Student("Tim",     15, Country.UK,      91),
            new Student("George",  14, Country.UK,      98)
    );

// Java 8 code to get all countries code but 
// How do I get the only country that has maximum students from ArrayList given above.

    Map<Country, Long> numberOfStudentsByCountry =
            students.stream()
                    .collect(groupingBy(Student::getCountry, counting()));
    System.out.println(numberOfStudentsByCountry);
}

结果如下

 {GERMANY=3, POLAND=2, UK=3}

我想要像下面这样的。

 {GERMANY=3}

Answer 1

您可以使用 Stream.max 比较以下值进一步获取 map 中出现频率最高的国家/地区:

Country mostFrequent = numberOfStudentsByCountry.entrySet()
        .stream()
        .max(Map.Entry.comparingByValue())
        .map(Map.Entry::getKey)
        .orElse(Country.POLAND) // some default country

如果您只对单个 Map.Entry 感兴趣，您可以使用

Map.Entry<Country,Long> mostFrequentEntry = numberOfStudentsByCountry.entrySet()
        .stream()
        .max(Map.Entry.comparingByValue()) // extensible here
        .orElse(null); // you can default according to service

注意:这两个都应该具有足够的可扩展性，以便在您想要打破平局时向 Comparator 添加自定义逻辑，例如频率对两个国家来说是平等的。举例来说，这可能发生在示例数据中的 GERMANY 和 UK 之间。