Java 流 : multiple grouping

Question

我可以按一个字段分组，但我想同时按两个字段分组

//persons grouped by gender
Map<String, Long> personsGroupedByGender = persons.stream().collect(Collectors.groupingBy(Person::getGender, Collectors.counting()));

//persons grouped by age
Map<Int, Long> personsGroupedByAge = persons.stream().collect(Collectors.groupingBy(Person::getAge, Collectors.counting()));

//persons grouped by gender and age 

    ???

Answer 1

您可以分组两次，结果将如您所愿，但包装略有不同。

persons.stream().collect(Collectors.groupingBy(Person::getGender,  
              Collectors.groupingBy(Person::getAge, Collectors.counting())));