朝花夕拾

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

1. 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))

2. 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())

3. 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)