Count number of nested elements using stream api java

Question

Count number of content using stream

class Subject {
  private String id;
  private String name;
  private List<Unit> units;
}

class Unit {
  private String id;
  private String name;
  private List<Topic> topics;
}

class Topic {
  private String id;
  private String name;
  private List<Content> contents;
}

class Content {
  private String id;
  private String contentType;
  private SubTopic subtopic;
}

With Java 8 and Streams I want the count of the Content elements which is of contentType equal to the video.

To count topic I tried this:

int topicCount = subject.getUnits().stream()
    .map(Unit::getTopics)
    .filter(topics -> topics != null)
    .mapToInt(List::size)
    .sum();

are you really willing to check the List being null? when you wrote .filter(topics -> topics != null)? I mean I can read it, but you should avoid that. (assigning null to a List) — Naman
– Naman, Commented Apr 9, 2020 at 14:04

Mureinik · Accepted Answer · 2020-04-09 14:27:13Z

4

You could flat map the nested elements and count them:

long videoContentCount = 
    subject.getUnits()
           .stream()
           .flatMap(u -> u.getTopics().stream())
           .flatMap(t -> t.getContents().stream())
           .filter(c -> c.getCountetType().equals("video"))
           .count();

edited Apr 9, 2020 at 14:27

answered Apr 9, 2020 at 12:01

Mureinik

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3 Comments

Naman Over a year ago

flatMap(Unit::getTopics), wouldn't that require getTopics to return a Stream?

Mureinik Over a year ago

@Naman arg, yes, I forgot to stream it, thanks for noticing! Edited and fixed.

the_tech_maddy Over a year ago

Adding extra point for this. You could use enum for content type, instead of String literals for better type safety which would avoid NPE for getContentType

Vikas · Accepted Answer · 2020-04-09 12:26:56Z

2

You use streams as below,

subject.getUnits()
        .stream()
        .map(Unit::getTopics)
        .flatMap(List::stream)
        .map(Topic::getContents)
        .flatMap(List::stream)
        .map(Content::getContentType)
        .filter("video"::equals)
        .count();

You can avoid map,

subject.getUnits()
        .stream()
        .flatMap(e->e.getTopics().stream())
        .flatMap(e->e.getContents().stream())
        .map(Content::getContentType)
        .filter("video"::equals)
        .count();

answered Apr 9, 2020 at 12:26

Vikas

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