Question

我有这个例子。我已从本教程http://www.baeldung.com/java-groupingby-collector中获取了它。但它没有解释如何获得几个分组参数。例如，此代码只获得喜欢的总和

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

此代码仅获得标题。

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

我能以某种方式将这两个陈述合二为一吗？我需要按作者分组。标题和类型应该是一个字符串，用逗号和喜欢的总和分隔。

类BlogPost

    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);

    }
}

结果

{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}]}

所以结果应该是这样的：

{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}]}

使用Java 8 Lambda可以做得更好。

如果它是一个SQL语句。可能会是这样的：

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

怎么做？

Answer 1

我做到了。有用。但我相信这不是最好的解决方案。

我创建了一个新类BlogPostGroup

    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 +
                '}';
    }
}

更改了主类：

    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);
    }
}

结果如下：

{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}]}

如何做得更好？更短，更容易阅读。

按参数映射的Java组值

1 个答案: