행복한디벨로퍼

자바 Stream에서 Map, Object내부에 또 다른 Map, Set, List와 같은 Collection이 있을 때 각 value를 개별된 값으로 처리하고 싶은 경우

Student POJO

public class Student {

    private String name;
    private Set<String> book;

    public void addBook(String book) {
        if (this.book == null) {
            this.book = new HashSet<>();
        }
        this.book.add(book);
    }
    //getters and setters

}

flatMap() and Set example

1차적으로 map으로 POJO에 Set에 담긴 Book데이터를 가져와 flatmap으로 펼친 후 list로 collect

import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;

public class TestExample2 {

    public static void main(String[] args) {

        LexiconMain.Student obj1 = new LexiconMain.Student();
        obj1.setName("mkyong");
        obj1.addBook("Java 8 in Action");
        obj1.addBook("Spring Boot in Action");
        obj1.addBook("Effective Java (2nd Edition)");

        LexiconMain.Student obj2 = new LexiconMain.Student();
        obj2.setName("zilap");
        obj2.addBook("Learning Python, 5th Edition");
        obj2.addBook("Effective Java (2nd Edition)");

        List<LexiconMain.Student> list = new ArrayList<>();
        list.add(obj1);
        list.add(obj2);

        List<String> collect =
                list.stream()
                        .map(x -> x.getBook())      //Stream<Set<String>>
                        .flatMap(x -> x.stream())   //Stream<String>
                        .distinct()
                        .collect(Collectors.toList());

        collect.forEach(x -> System.out.println(x));
    }

}

Output

Spring Boot in Action
Effective Java (2nd Edition)
Java 8 in Action
Learning Python, 5th Edition

Stream관련 자세한 예제가 더 궁금하다면 아래 링크로

참고 : https://www.mkyong.com/java8/java-8-flatmap-example/