JAVA8-GroupingBy

2019-01-21

/posts/2019-01-21-java8stream-groupingby/ map[avatar:https://avatars.githubusercontent.com/u/123456789?v=4 email:iworkvip@gmail.com location:China name:melonkid]

基础

if we want to group a list with key. we can use new JAVA8 API->groupyingBy。this api provides opretor which can group a list like SQL(select … form… where… group by)

this new API has three overload metheds

one param

    class Person {
         private String name;
         private int age;
         // getter setter...
    }

    List<Person> persons = ...
    // classify person by name
    persons.stream.collect(Collectors.groupingBy(Person::getName))

two params

    class Person {
         private String name;
         private int age;
         private int sex; 
         // getter setter...
    }

    List<Person> persons = ...
    // classify by sex(male & female) and caculate the count
    // of sex
    persons.stream.collect(Collectors.groupingBy(Person::getSex), counting())

three params

    class Person {
         private String name;
         private int age;
         private int sex; 
         // getter setter...
    }

    List<Person> persons = ...
    // classify by sex(male & female) and caculate the count
    // of sex
    persons.stream.collect(Collectors.groupingBy(Person::getSex), counting(), toList)