JAVA 8 FLATMAP EXAMPLE

A developer knows a set of programming languages:

public class Developer {

    private String name;
    private Set languages;

    public Developer(String name) {
        this.languages = new HashSet<>();
        this.name = name;
    }

    public void add(String language) {
        this.languages.add(language);
    }

    public Set getLanguages() {
        return languages;
    }
}

A team has more than one developers. Now we would like to know the aggregate programming language skills for a given team. Stream#flatMap is perfect for "flattening" collections:

import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;
import static org.junit.Assert.assertTrue;
import org.junit.Test;

public class FlatMapTest {

    @Test
    public void flatMap() {
        List team = new ArrayList<>();
        Developer polyglot = new Developer("esoteric");
        polyglot.add("clojure");
        polyglot.add("scala");
        polyglot.add("groovy");
        polyglot.add("go");

        Developer busy = new Developer("pragmatic");
        busy.add("java");
        busy.add("javascript");

        team.add(polyglot);
        team.add(busy);

        List teamLanguages = team.stream().
                map(d -> d.getLanguages()).
                flatMap(l -> l.stream()).
                collect(Collectors.toList());
        assertTrue(teamLanguages.containsAll(polyglot.getLanguages()));
        assertTrue(teamLanguages.containsAll(busy.getLanguages()));
    }
}