매개 변수 별 맵의 Java 그룹 값

Question

이 예제가 있습니다. 이 튜토리얼 http://www.baeldung.com/java-groupingby-collector에서 가져 왔습니다. 그러나 그룹화 된 여러 매개 변수를 얻는 방법은 설명하지 않습니다. 는 예를 들어 다음과 같은 코드는

Map<BlogPostType, Integer> likesPerType = posts.stream() 
    .collect(groupingBy(BlogPost::getType, summingInt(BlogPost::getLikes))); 

이 코드는 타이틀을 얻을 좋아의 합계를 얻을.

Map<BlogPostType, String> postsPerType = posts.stream() 
    .collect(groupingBy(BlogPost::getType, 
    mapping(BlogPost::getTitle, joining(", ", "Post titles: [", "]")))); 

어떻게 든이 두 문장을 하나로 결합 할 수 있습니까? 저자별로 그룹화해야합니다. 제목과 유형은 각각 쉼표로 구분 된 하나의 문자열이어야하며 좋아하는 것의 유형이어야합니다.

클래스 블로그 게시물

public class BlogPost { 

    String title; 
    String author; 
    String type; 
    int likes; 

    public BlogPost(String title, String author, String type, int likes) { 
     this.title = title; 
     this.author = author; 
     this.type = type; 
     this.likes = likes; 
    } 

    public String getTitle() { 
     return title; 
    } 

    public void setTitle(String title) { 
     this.title = title; 
    } 

    public String getAuthor() { 
     return author; 
    } 

    public void setAuthor(String author) { 
     this.author = author; 
    } 

    public String getType() { 
     return type; 
    } 

    public void setType(String type) { 
     this.type = type; 
    } 

    public int getLikes() { 
     return likes; 
    } 

    public void setLikes(int likes) { 
     this.likes = likes; 
    } 

    @Override 
    public String toString() { 
     return "BlogPost{" + 
       "title='" + title + '\'' + 
       ", author='" + author + '\'' + 
       ", type=" + type + 
       ", likes=" + likes + 
       '}'; 
    } 
} 

Main.java

import java.util.Arrays; 
import java.util.HashMap; 
import java.util.List; 
import java.util.Map; 

public class Main { 
    public static void main(String[] args) { 
     Map<Integer, List<BlogPost>> map = new HashMap<>(); 
     map.put(2017, Arrays.asList(
       new BlogPost("title1", "author1", "Guide", 10), 
       new BlogPost("title2", "author1", "News", 15), 
       new BlogPost("title3", "author2", "News", 5), 
       new BlogPost("title4", "author2", "Review", 10) 
       )); 
     map.put(2018, Arrays.asList(
       new BlogPost("title5", "author1", "Guide", 3), 
       new BlogPost("title6", "author1", "News", 2), 
       new BlogPost("title7", "author2", "News", 1), 
       new BlogPost("title8", "author2", "Review", 11) 
     )); 

     System.out.println(map); 

    } 
} 

{2017=[BlogPost{title='title1', author='author1', type=Guide, likes=10}, BlogPost{title='title2', author='author1', type=News, likes=15}, BlogPost{title='title3', author='author2', type=News, likes=5}, BlogPost{title='title4', author='author2', type=Review, likes=10}],2018=[BlogPost{title='title5', author='author1', type=Guide, likes=3}, BlogPost{title='title6', author='author1', type=News, likes=2}, BlogPost{title='title7', author='author2', type=News, likes=1}, BlogPost{title='title8', author='author2', type=Review, likes=11}]} 

그래서 결과 결과는 다음과 같이해야합니다 :

,

{2017=[BlogPost{title='title1, title2', author='author1', type='Guide,News', likes=25}, BlogPost{title='title3, title4' author='author2', type='News, Review', likes=15}],2018=[BlogPost{title='title5, title6', author='author1', type='Guide,News', likes=5}, BlogPost{title='title7, title8', author='author2', type='News, Review', likes=12}]} 

자바 8 람다로 할 수있을 때 더 좋습니다.

SQL 문이면. 이

SELECT author, title, type, SUM(likes) FROM BlogPost GROUP BY author; 

을 수행 할 수 있습니다 방법 : 아마 그것은 같은 것?

Answer 1

나는 그것을 만들었습니다. 그것은 작동합니다. 그러나 나는 그것이 최선의 해결책이 아니라고 믿는다.

import java.util.*; 
import java.util.stream.Collectors; 

import static java.util.stream.Collectors.groupingBy; 
import static java.util.stream.Collectors.joining; 
import static java.util.stream.Collectors.mapping; 

public class Main { 
    public static void main(String[] args) { 
     Map<Integer, List<BlogPost>> map = new HashMap<>(); 
     map.put(2017, Arrays.asList(
       new BlogPost("title1", "author1", "Guide", 10), 
       new BlogPost("title2", "author1", "News", 15), 
       new BlogPost("title3", "author2", "News", 5), 
       new BlogPost("title4", "author2", "Review", 10) 
       )); 
     map.put(2018, Arrays.asList(
       new BlogPost("title5", "author1", "Guide", 3), 
       new BlogPost("title6", "author1", "News", 2), 
       new BlogPost("title7", "author2", "News", 1), 
       new BlogPost("title8", "author2", "Review", 11) 
     )); 

     System.out.println(map); 

     Map<Integer, List<BlogPostGroup>> mapPostGroup = new HashMap<>(); 

     for (Map.Entry<Integer, List<BlogPost>> entry :map.entrySet()) { 
      List<BlogPost> blogPosts = entry.getValue(); 

      Map<String, Integer> postsLikesSum = blogPosts.stream().collect(groupingBy(BlogPost::getAuthor, Collectors.summingInt(BlogPost::getLikes))); 

      Map<String, String> postsTitles = blogPosts.stream().collect(groupingBy(BlogPost::getAuthor, mapping(BlogPost::getTitle, joining(",")))); 

      Map<String, String> postsTypes = blogPosts.stream().collect(groupingBy(BlogPost::getAuthor, mapping(BlogPost::getType, joining(",")))); 


      List<BlogPostGroup> blogPostGroups = new ArrayList<>(); 

      blogPosts.forEach(
        p -> blogPostGroups.add(
          new BlogPostGroup(
            postsTitles.get(p.getAuthor()), 
            p.getAuthor(), 
            postsTypes.get(p.getAuthor()), 
            postsLikesSum.get(p.getAuthor()) 
          ) 
        ) 
      ); 

      // delete duplicates 
      Set<BlogPostGroup> blogPostGroupSet = new HashSet<>(blogPostGroups); 
      blogPostGroups.clear(); 
      blogPostGroups.addAll(blogPostGroupSet); 
      // sort by likes DESC order  
      blogPostGroups.sort((p1, p2) -> p2.likes - p1.likes); 

      mapPostGroup.put(entry.getKey(), blogPostGroups); 
     } 

     System.out.println(mapPostGroup); 
    } 
} 

결과가 이것이다 :

나는

import java.util.Objects; 

public class BlogPostGroup { 

    String titles; 
    String author; 
    String types; 
    int likes; 

    public BlogPostGroup(String titles, String author, String types, int likes) { 
     this.titles = titles; 
     this.author = author; 
     this.types = types; 
     this.likes = likes; 
    } 

    public String getTitles() { 
     return titles; 
    } 

    public void setTitles(String titles) { 
     this.titles = titles; 
    } 

    public String getAuthor() { 
     return author; 
    } 

    public void setAuthor(String author) { 
     this.author = author; 
    } 

    public String getTypes() { 
     return types; 
    } 

    public void setTypes(String types) { 
     this.types = types; 
    } 

    public int getLikes() { 
     return likes; 
    } 

    public void setLikes(int likes) { 
     this.likes = likes; 
    } 

    @Override 
    public boolean equals(Object o) { 
     if (this == o) return true; 
     if (o == null || getClass() != o.getClass()) return false; 
     BlogPostGroup that = (BlogPostGroup) o; 
     return likes == that.likes && 
       Objects.equals(titles, that.titles) && 
       Objects.equals(author, that.author) && 
       Objects.equals(types, that.types); 
    } 

    @Override 
    public int hashCode() { 

     return Objects.hash(titles, author, types, likes); 
    } 

    @Override 
    public String toString() { 
     return "BlogPostGroup{" + 
       "titles='" + titles + '\'' + 
       ", author='" + author + '\'' + 
       ", types='" + types + '\'' + 
       ", likes=" + likes + 
       '}'; 
    } 
} 

는 메인 클래스를 변경 BlogPostGroup 새로운 클래스를 생성 한 더 나은

{2017=[BlogPostGroup{titles='title1,title2', author='author1', types='Guide,News', likes=25}, BlogPostGroup{titles='title3,title4', author='author2', types='News,Review', likes=15}], 2018=[BlogPostGroup{titles='title7,title8', author='author2', types='News,Review', likes=12}, BlogPostGroup{titles='title5,title6', author='author1', types='Guide,News', likes=5}]} 

을 수행하는 방법은? 더 짧고 읽기 쉽습니다.