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1. Overview

Collectors were added in Java 8 which helped accumulate input elements into mutable containers such as Map, List, and Set.

In this article, we're going to explore two new collectors added in Java 9: Collectors.filtering and Collectors.flatMapping used in combination with Collectors.groupingBy providing intelligent collections of elements.

2. Filtering Collector

The Collectors.filtering is similar to the Stream filter(); it's used for filtering input elements but used for different scenarios. The Stream's filter is used in the stream chain whereas the filtering is a Collector which was designed to be used along with groupingBy.

With Stream's filter, the values are filtered first and then it's grouped. In this way, the values which are filtered out are gone and there is no trace of it. If we need a trace then we would need to group first and then apply filtering which actually the Collectors.filtering does.

The Collectors.filtering takes a function for filtering the input elements and a collector to collect the filtered elements:

public void givenList_whenSatifyPredicate_thenMapValueWithOccurences() {
    List<Integer> numbers = List.of(1, 2, 3, 5, 5);

    Map<Integer, Long> result = numbers.stream()
      .filter(val -> val > 3)
      .collect(Collectors.groupingBy(i -> i, Collectors.counting()));

    assertEquals(1, result.size());

    result = numbers.stream()
      .collect(Collectors.groupingBy(i -> i,
        Collectors.filtering(val -> val > 3, Collectors.counting())));

    assertEquals(4, result.size());

3. FlatMapping Collector

The Collectors.flatMapping is similar to Collectors.mapping but has a more fine-grained objective. Both the collectors takes a function and a collector where the elements are collected but flatMapping function accepts a Stream of elements which is then accumulated by the collector.

Let's see the following model class:

class Blog {
    private String authorName;
    private List<String> comments;
    // constructor and getters

Collectors.flatMapping lets us skip intermediate collection and write directly to a single container which is mapped to that group defined by the Collectors.groupingBy:

public void givenListOfBlogs_whenAuthorName_thenMapAuthorWithComments() {
    Blog blog1 = new Blog("1", "Nice", "Very Nice");
    Blog blog2 = new Blog("2", "Disappointing", "Ok", "Could be better");
    List<Blog> blogs = List.of(blog1, blog2);
    Map<String,  List<List<String>>> authorComments1 = blogs.stream()
       Collectors.mapping(Blog::getComments, Collectors.toList())));
    assertEquals(2, authorComments1.size());
    assertEquals(2, authorComments1.get("1").get(0).size());
    assertEquals(3, authorComments1.get("2").get(0).size());

    Map<String, List<String>> authorComments2 = blogs.stream()
        Collectors.flatMapping(blog -> blog.getComments().stream(), 

    assertEquals(2, authorComments2.size());
    assertEquals(2, authorComments2.get("1").size());
    assertEquals(3, authorComments2.get("2").size());

The Collectors.mapping maps all grouped author's comments to the collector's container i.e. List whereas this intermediate collection is removed with flatMapping as it gives a direct stream of the comment list to be mapped to the collector's container.

4. Conclusion

This article illustrates the use of the new Collectors introduced in Java9 i.e. Collectors.filtering() and Collectors.flatMapping() used in combination with Collectors.groupingBy().

These Collectors can also be used along with Collectors.partitioningBy() but it only creates two partitions based on conditions and the real power of the collectors isn't leveraged; hence left out of this tutorial.

The complete source code for the code snippets in this tutorial is available over on GitHub.

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