Collectors

groupingBy

// classifier를 이용해서 key를 뽑아내고, 해당하는 value를 list로 가짐
static <T,K> Collector<T,?,Map<K,List<T>>> 
  groupingBy(Function<? super T,? extends K> classifier);

@Test
void 학습_테스트() {
	Admin admin = new Admin("test@test.com", "abcd1234!@", 
									Role.ADMIN, organization);
	Admin admin2 = new Admin("test@test.com", "abcd1234!@",
									Role.ADMIN, organization);
	Admin admin3 = new Admin("test@test.com", "abcd1234!@",
									Role.SUPERVISOR, organization);

	List<Admin> admins = List.of(admin, admin2, admin3);

	Map<Role, List<Admin>> results = admins.stream().collect(
											Collectors.groupingBy(Admin::getRole));
	assertThat(results.get(Role.ADMIN)).hasSize(2);
	assertThat(results.get(Role.SUPERVISOR)).hasSize(1);
}

// classifier를 이용해서 key를 추출, The downstream collector 
// operates on elements of type T and produces a result of type D.
// The resulting collector produces a Map<K, D>.
// downstream collector : 해당하는 stream에 대해 그 다음 연산을 적용함
static <T,K,A,D> Collector<T,?,Map<K,D>>
  groupingBy(Function<? super T,? extends K> classifier, 
    Collector<? super T,A,D> downstream);

@Test
void 학습_테스트() {
	Admin admin = new Admin("test@test.com", "abcd1234!@", Role.ADMIN, organization);
	Admin admin2 = new Admin("test2@test.com", "abcd1234!@", Role.ADMIN, organization);
	Admin admin3 = new Admin("test3@test.com", "abcd1234!@", Role.SUPERVISOR, organization);

	List<Admin> admins = List.of(admin, admin2, admin3);

	Map<Role, List<String>> results = admins.stream().collect(Collectors.groupingBy(
		Admin::getRole, Collectors.mapping(Admin::getEmail, Collectors.toList()))
  );
	assertThat(results.get(Role.ADMIN)).contains(admin.getEmail(), admin2.getEmail());
	assertThat(results.get(Role.SUPERVISOR)).contains(admin3.getEmail());
}

static <T,K,D,A,M extends Map<K,D>> Collector<T,?,M>
  groupingBy(Function<? super T,? extends K> classifier, 
    Supplier<M> mapFactory, Collector<? super T,A,D> downstream)

key를 Multi value로 하는 방법

public class BlogPost {
    private String title;
    private String author;
    private BlogPostType type;
    private int likes;
    record AuthPostTypesLikes(String author, BlogPostType type, int likes) {};
    
    // constructor, getters/setters
}

Map<BlogPost.AuthPostTypesLikes, List<BlogPost>> postsPerTypeAndAuthor = 
	posts.stream().collect(groupingBy(
		post -> new BlogPost.AuthPostTypesLikes(
			post.getAuthor(), post.getType(), post.getLikes()
		)
	));