[JAVA, Stream] 스트림 결과물을 Map으로 만들어보자! Collectors.toMap(), Collectors.GroupingBy()

class BlogPost { String title; String author; BlogPostType type; int likes; //constructor, getter, setter, equals... } enum BlogPostType { NEWS, REVIEW, GUIDE } class Tuple { BlogPostType type; String author; } List<BlogPost> posts = Arrays.asList( new BlogPost("제목1", "작가1", GUIDE, 1), new BlogPost("제목2", "작가1", REVIEW, 2), new BlogPost("제목3", "작가1", REVIEW, 3), new BlogPost("제목4", "작가2", NEWS, 4), new BlogPost("제목5", "작가3", GUIDE, 5), new BlogPost("제목6", "작가4", REVIEW, 6), //동일한 2건 new BlogPost("제목7", "작가4", GUIDE, 7), new BlogPost("제목7", "작가4", GUIDE, 7) );

@DisplayName("keyMapper만 주어서 맵 생성 테스트") @Test void groupingByAuthor() { Map<String, List<BlogPost>> groupByAuthor = posts.stream() .collect(Collectors.groupingBy(BlogPost::getAuthor)); int expectedAuthorsSize = 4; assertEquals(groupByAuthor.size(), expectedAuthorsSize); }

@DisplayName("복잡한 keyMapper 활용하여 맵 생성") @Test void gropingByTuple(){ Map<Tuple, List<BlogPost>> postsPerTypeAndAuthor = posts.stream() .collect(Collectors.groupingBy(post -> new Tuple(post.getType(), post.getAuthor()))); assertEquals(postsPerTypeAndAuthor.get(new Tuple(GUIDE,"작가1")).size(), 1); }

@DisplayName("value를 List가 아니라 원하는 방식으로 mapping") @Test void groupingByModifyingValueType(){ Map<BlogPostType, Set<BlogPost>> postsPerType = posts.stream() .collect(Collectors.groupingBy(BlogPost::getType, Collectors.toSet())); int expectedAuthorsSize = 3; //4개 중 중복건 2개 assertEquals(postsPerType.get(GUIDE).size(), expectedAuthorsSize); }

@DisplayName("여러 칼럼을 기준으로 그룹핑") @Test void groupingByMultipleColumn() { Map<String, Map<BlogPostType, List<BlogPost>>> groupingAuthorThanType = posts.stream() .collect(Collectors.groupingBy(BlogPost::getAuthor, Collectors.groupingBy(BlogPost::getType))); int expectedSize = 2; assertEquals(groupingAuthorThanType.get("작가1").get(REVIEW).size(), expectedSize); }

@DisplayName("그룹한 결과값의 개수 구하기") @Test void gettingGroupingResultsCount() { Map<String, Long> averageLikesPerAuthor = posts.stream() .collect(Collectors.groupingBy(BlogPost::getAuthor, Collectors.counting())); int expectedLikesCount = 3; //[3] assertEquals(averageLikesPerAuthor.get("작가1"), expectedLikesCount); } @DisplayName("그룹한 결과값의 평균 구하기") @Test void gettingGroupingResultsAverage() { Map<String, Double> averageLikesPerAuthor = posts.stream() .collect(Collectors.groupingBy(BlogPost::getAuthor, Collectors.averagingInt(BlogPost::getLikes))); int expectedLikesCount = 2; //[1,2,3] assertEquals(averageLikesPerAuthor.get("작가1"), expectedLikesCount); } @DisplayName("그룹한 결과값의 합계 구하기") @Test void gettingGroupingResultsSum() { Map<String, Integer> averageLikesPerAuthor = posts.stream() .collect(Collectors.groupingBy(BlogPost::getAuthor, Collectors.summingInt(BlogPost::getLikes))); int expectedLikesCount = 6; //[1,2,3] assertEquals(averageLikesPerAuthor.get("작가1"), expectedLikesCount); }

@DisplayName("그룹 결과의 최대,최소 요소 구하기") @Test void gettingGroupingResultsMaxAndMin() { Map<BlogPostType, Optional<BlogPost>> maxLikesPerPostType = posts.stream() .collect(Collectors.groupingBy(BlogPost::getType, Collectors.maxBy(Comparator.comparingInt(BlogPost::getLikes)))); Map<BlogPostType, Optional<BlogPost>> minLikesPerPostType = posts.stream() .collect(Collectors.groupingBy(BlogPost::getType, Collectors.minBy(Comparator.comparingInt(BlogPost::getLikes)))); int max = 7; int min = 1; assertEquals(maxLikesPerPostType.get(GUIDE).get().getLikes(), max); assertEquals(minLikesPerPostType.get(GUIDE).get().getLikes(), min); }

@DisplayName("value를 커스텀하여 저장하기") @Test void mappingGroupedResultsToDifferentType() { Map<BlogPostType, String> postsPerType = posts.stream() .collect(Collectors.groupingBy(BlogPost::getType, Collectors.mapping(BlogPost::getTitle, Collectors.joining(", ", "Post titles: [", "]")))); String expectedResult = "Post titles: [제목2, 제목3, 제목6]"; assertEquals(postsPerType.get(REVIEW), expectedResult); }

@DisplayName("다른 map으로 저장하기") @Test void groupingByAnotherMap() { EnumMap<BlogPostType, List<BlogPost>> postsPerType = posts.stream() .collect(Collectors.groupingBy(BlogPost::getType, () -> new EnumMap<>(BlogPostType.class), Collectors.toList())); //[제목1, 제목5, 제목7, 제목7] int expectedGuideSize = 4; assertEquals(postsPerType.get(GUIDE).size(), expectedGuideSize); }

ConcurrentMap<BlogPostType, List<BlogPost>> postsPerType = posts.parallelStream() .collect(groupingByConcurrent(BlogPost::getType));

class Book { private String name; private int releaseYear; private String isbn; // constructor, getters and setters }

List<Book> bookList = new ArrayList<>(); bookList.add(new Book("The Fellowship of the Ring", 1954, "0395489318")); bookList.add(new Book("The Two Towers", 1954, "0345339711")); bookList.add(new Book("The Return of the King", 1955, "0618129111"));

@DisplayName("Isbn가 key, 책 이름이 value인 맵 생성") @Test void toMapTest() { Map<String, String> IsbnAndName = books.stream() .collect(Collectors.toMap(Book::getIsbn, Book::getName)); int expectedSize = 3; assertEquals(IsbnAndName.size(), expectedSize); }

@DisplayName("동일한 key가 있을 때 오류가 나는지 확인") @Test void whenMapHasDuplicatedKeyTest() { assertThrows(IllegalStateException.class, () -> books.stream() .collect(Collectors.toMap(Book::getReleaseYear, Function.identity()))); }

@DisplayName("키 중복시 병합 로직도 추가한 toMap테스트") @Test void listToMapWithDupKeyTest() { Map<Integer, Book> releaseYearAndBook = books.stream() .collect(Collectors.toMap(Book::getReleaseYear, Function.identity(), (existing, replacement) -> existing)); // (기존 value, 새롭게 들어온 value) → 수행할 행위, 여기선 map.put()과 달리 기존값이 남도록 해보겠습니다. assertEquals(releaseYearAndBook.size(), 2); assertEquals(releaseYearAndBook.get(1954).getName(), "The Fellowship of the Ring"); }

@DisplayName("Hash Map이 아닌 다른 Map으로 생성") @Test void listToConcurrentMapTest() { ConcurrentHashMap<Integer, Book> concurrentHashMap = books.stream() .collect(Collectors.toMap(Book::getReleaseYear, Function.identity(), (o1, o2) -> o1, ConcurrentHashMap::new)); assertTrue(concurrentHashMap instanceof ConcurrentHashMap); }