[Stream] 스트림 : collect - Collectors - toList, toCollection, 사용자 정의 수집

 

 

 

1. toList()

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class ToListExam {

    public static void main(String[] args) {

        List<Person> personList = Arrays.asList(
                new Person("lee",9, Person.Gender.MALE),
                new Person("kim",5, Person.Gender.FEMALE),
                new Person("park",6, Person.Gender.FEMALE),
                new Person("sin",3, Person.Gender.FEMALE),
                new Person("hong",4, Person.Gender.MALE),
                new Person("ahn",7, Person.Gender.MALE),
                new Person("moo",1, Person.Gender.MALE)
        );

        // 남학생만 필터링 후 List 생성 
        List<Person> maleList = personList.stream()
                .filter(s->s.getGender()==Person.Gender.MALE)
                .collect(Collectors.toList());
        /*
            Collector<T, ?, List<T>> toList()
            T : 참조타입
            ? : 수집기 List로 받기 때문에 수집기가 필요하지 않음
            List<T> : 리턴값
        */

      maleList.forEach(System.out::println);

    }
}

 

2. toCollection

import java.util.Arrays;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
import java.util.function.Supplier;
import java.util.stream.Collectors;

public class ToListExam {

    public static void main(String[] args) {

        List<Person> personList = Arrays.asList(
                new Person("lee",9, Person.Gender.MALE),
                new Person("kim",5, Person.Gender.FEMALE),
                new Person("park",6, Person.Gender.FEMALE),
                new Person("sin",3, Person.Gender.FEMALE),
                new Person("hong",4, Person.Gender.MALE),
                new Person("ahn",7, Person.Gender.MALE),
                new Person("moo",1, Person.Gender.MALE)
        );

        Set<Person> femaleSet =  personList.stream()
                .filter(s->s.getGender()==Person.Gender.FEMALE)
                .collect(Collectors.toCollection(new Supplier<Set<Person>>() {
                    @Override
                    public Set<Person> get() {
                        return new HashSet<>();
                    }
                }));

        femaleSet.forEach(System.out::println);

    }
}

 

메소드 참조  

Set<Person> femaleSet =  personList.stream()
              .filter(s->s.getGender()==Person.Gender.FEMALE)
              .collect(Collectors.toCollection(HashSet::new));

 

3. 사용자 정의 수집

/* 수집 대상 객체 */
public class Person {
    enum Gender{ MALE, FEMALE }
    enum City{ Seoul, Pusan }

    private  String name;
    private Gender gender;
    private  int score;
    private City city;

    public Person(String name, int score, Gender gender) {
        this.name = name;
        this.score = score;
        this.gender = gender;
    }

    public String getName() {
        return name;
    }

    public int getScore() {
        return score;
    }

    public Gender getGender() {
        return gender;
    }

    public City getCity() {
        return city;
    }

    @Override
    public String toString() {
        return "Person{name='" + name + '\'' + ", gender=" + gender + ", score=" + score + ", city=" + city + '}';
    }
}

 

/* 사용자 정의 수집기 */
import java.util.ArrayList;
import java.util.List;

public class MalePerson {

    private List<Person> list;

    public MalePerson() {
        list = new ArrayList<>();
    }

    public void accumulate(Person person){
        list.add(person);
    }

    public List<Person> getList() {
        return list;
    }

    public void combine(MalePerson other){
        list.addAll(other.getList());
    }
}

 

import java.util.Arrays;
import java.util.List;
import java.util.function.BiConsumer;
import java.util.function.Supplier;

public class CollectExam {

    public static void main(String[] args) {

        List<Person> personList = Arrays.asList(

                new Person("lee",9, Person.Gender.MALE),
                new Person("kim",5, Person.Gender.FEMALE),
                new Person("park",6, Person.Gender.FEMALE),
                new Person("sin",3, Person.Gender.FEMALE),
                new Person("hong",4, Person.Gender.MALE),
                new Person("ahn",7, Person.Gender.MALE),
                new Person("moo",1, Person.Gender.MALE)
        );

        MalePerson malePerson =
        personList.stream().filter(i->i.getGender()== Person.Gender.MALE)
                .collect(
                        // @Param1
                        new Supplier<MalePerson>() { //  supplier : 사용자정의 수집기
                            @Override
                            public MalePerson get() {
                                return new MalePerson();
                            }
                        },
                        // @Param2
                        new BiConsumer<MalePerson, Person>() { // @param2 accumulator :  수집기에 누적
                            @Override
                            public void accept(MalePerson malePerson, Person person) {
                                malePerson.accumulate(person);
                            }
                        },
                        // @Param3
                        new BiConsumer<MalePerson, MalePerson>() { // @param2 combiner : 병렬처리에서 서로 다른 수집기 결합
                            @Override
                            public void accept(MalePerson malePerson1, MalePerson malePerson2) {
                                malePerson1.combine(malePerson2);
                            }
                        }
                );

        malePerson.getList().stream().forEach(System.out::println);

    }
}

/* 람다 */
MalePerson malePerson =
	personList.stream().filter(i->i.getGender()== Person.Gender.MALE)
	.collect(
            	() -> new MalePerson(),
           	 (mp, p) -> mp.accumulate(p),
            	(mp1, mp2) -> mp1.combine(mp2)
            );

 

/* 메소드 참조 */
MalePerson malePerson =
        personList.stream().filter(i->i.getGender()== Person.Gender.MALE)
        .collect(MalePerson::new, MalePerson::accumulate, MalePerson::combine);