How to map frequencies to a new object with Collectors groupingBy?

Question

To get the description frequencies of class objects:

public class Tag {  
    private int excerptID;
    private String description;
}

I use Collectors groupingBy + counting functions:

Map<String, Long> frequencyMap = rawTags.stream().map(Tag::getDescription).collect(Collectors.groupingBy(e -> e, Collectors.counting()));

But I want to return the result as a new object of the class

 public class Frequency {
    private String Description;
    private Long frequency;
    }

instead of Map<String, Long>. What is a simple way to do it?

Answer 1

You can get entrySet of map and transform into Frequency class and collect as List.

rawTags.stream()
       .map(Tag::getDescription)
       .collect(Collectors.groupingBy(e -> e, Collectors.counting()))
       .entrySet()
       .stream()
       .map(e -> new Frequency(e.getKey(), e.getValue()))
       .collect(Collectors.toList());

Or using Collectors.collectingAndThen

rawTags.stream()
    .map(Tag::getDescription)
    .collect(Collectors.groupingBy(e -> e,
              Collectors.collectingAndThen(Collectors.toList(),
                                e -> new Frequency(e.get(0), Long.valueOf(e.size())))));