Java Stream.collect groupingBy and aggregated classes

Question

This code is straightforward :

Map<String, List<BlogPost>> postsPerType = posts.stream()
  .collect(groupingBy(BlogPost::getType));

where BlogPost is :

class BlogPost {
    String title;
    String author;
    String type;
    int likes;
    // getter setter
}

What if BlogPost has some other classes as fields and I want to group by one of those fields, e.g.

class BlogPost {
    String title;
    String author;
    Description description;
    int likes;
    // getter setter
}

class Description {
    String part1;
    String type;
    // getter setter
}

How can I access say

myBlogPost.getDescription().getType()

inside a lambda expression?

Pseudo code:

Map<String, List<BlogPost>> postsPerType = posts.stream()
      .collect(groupingBy(BlogPost::getDescription::getType));

edit : groupingBy refers to :

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

It is a duplicate question indeed. Should I delete it? – Wolf359 Oct 31 '18 at 21:34 — Wolf359, Oct 31 '18 at 21:34

score 3 · Answer 1 · answered Oct 31 '18 at 21:23

3

Just create a lambda returning that expression:

groupingBy(p -> p.getDescription().getType())

answered Oct 31 '18 at 21:23

Andy Turner

137,514
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score 2 · Accepted Answer · answered Oct 31 '18 at 21:23

2

Map<String, List<BlogPost>> postsPerType = posts.stream()
  .collect(groupingBy(bp -> bp.getDescription().getType()));

answered Oct 31 '18 at 21:23

Bernie

2,253
1
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Java Stream.collect groupingBy and aggregated classes

2 Answers2