Java 8 Stream conversion of Array of Array of Class

jasmeet24 :

I'm new to Java 8. I need to create a Array of Strings in Java 8 using below:

I've Pojos of School, Subject, Publisher, Book as described below -

public class School {
    private Subject[] subjects;

    public Subject[] getSubjects() {
         return subjects;
    }

    public void setSubjects(Subject[] subjects) {
         this.subjects = subjects;
    }
}

public class Subject {
    private String subjectName;
    private String subjectId;
    private Publisher publisher;

    public String getSubjectName() {
        return subjectName;
    }

    public void setSubjectName(String subjectName) {
        this.subjectName = subjectName;
    }

    public String getSubjectId() {
        return subjectId;
    }

    public void setSubjectId(String subjectId) {
        this.subjectId = subjectId;
    }

    public Publisher getpublisher() {
        return publisher;
    }

    public void setPublisher(Publisher publisher) {
        this.publisher = publisher;
    }
}

public class Publisher{
    private String name;
    private String sinNo;
    private Book[] books;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getSinNo() {
        return sinNo;
    }

    public void setSinNo(String sinNo) {
        this.sinNo = sinNo;
    }

    public Book[] getBooks() {
        return books;
    }

    public void setBooks(Book[] books) {
        this.books = books;
    }
}

public class Book {
    private String bookName;
    private String bookId;

    public String getBookName() {
        return bookName;
    }

    public void setBookName(String bookName) {
        this.bookName = bookName;
    }

    public String getBookId() {
        return bookId;
    }

    public void setBookId(String bookId) {
        this.bookId = bookId;
    }

}

Now my ask is to make an Array which has all the bookIds from the School object I have. Below old java code is the required code

public static void main(String[] args) {
    List<String> list = new ArrayList<>();
    School first = <<School Object>>;
    for(Subject sub : first.getSubjects()){
        Publisher p = sub.getpublisher();
        for(Book b : p.getBooks()) {
            list.add(b.getBookId());
        }

        }
    //desired result
    String[] bookIds = (String[]) list.toArray();
}

}

Can this be done using Java 8 streams, Collectors, mappings? Also, if we want to get only those book ids which are odd/even ?

m.antkowicz :

Yes it is - you need a flatMap to achieve this

Stream.of(first.getSubjects())
    .map(Subject::getPublisher)
    .map(Publisher::getBooks)
    .flatMap(Arrays::stream)
    .map(Book::getBookId)
    .collect(Collectors.toList());

If you want to have it null safe you need to add additional filtering like

List<String> collect = Stream.of(first.getSubjects())
    .map(Subject::getPublisher)
    .filter(Objects::nonNull) // filter all null publishers
    .map(Publisher::getBooks)
    .filter(Objects::nonNull) // filter all null book lists
    .flatMap(Arrays::stream)
    .map(Book::getBookId)
    .filter(Objects::nonNull) // filter all null book ids
    .collect(Collectors.toList());