Java 8: Iterate and group objects from list based on object attributes

Question

I have a list (GlobalBooks) which looks similar to the below

import java.util.ArrayList;
import java.util.List;

public class Main {

    public static void main(String[] args) {

        GlobalBooks globalBooks = new GlobalBooks();

        List<Book> bookList  = new ArrayList<Book>();


        Book book = new Book();
        List<BookContent> bookContents = new ArrayList<BookContent>();

        book.setBookName("A");
        BookContent content = new BookContent();
        content.setDescription("December 2016");
        content.setComponentID(20l);
        bookContents.add(content);

        content = new BookContent();
        content.setDescription("January 2016");
        content.setComponentID(30l);
        bookContents.add(content);

        content = new BookContent();
        content.setDescription("Febuary 2016");
        content.setComponentID(40l);
        bookContents.add(content);
        book.setContents(bookContents);

        bookList.add(book);


        book = new Book();
        bookContents = new ArrayList<BookContent>();

        book.setBookName("B");
        content = new BookContent();
        content.setDescription("December 2016");
        content.setComponentID(20l);
        bookContents.add(content);

        content = new BookContent();
        content.setDescription("January 2016");
        content.setComponentID(30l);
        bookContents.add(content);

        content = new BookContent();
        content.setDescription("Febuary 2016");
        content.setComponentID(40l);
        bookContents.add(content);
        book.setContents(bookContents);

        bookList.add(book);

        globalBooks.setBooks(bookList);


        System.out.println(globalBooks);


    }

}

I am looking for java 8 functions which could stream the globalBooks response and collect a map of List of Books based on the description field in the BooksContent.

Map<String,List<Books>> i.e. all the books with the same description should be grouped together by description ?

I can do this via regular java but the code becomes too messy and untidy.

since each book has multiple contents with different descriptions, you would need each book to appear in multiple map entries, correct? — Patrick Parker
– Patrick Parker, Commented Feb 3, 2017 at 5:52
Similar question using Guava MultiMap: Cleanest way to index a Collection by a property of the item's that itself is a collection — MikaelF
– MikaelF, Commented Feb 4, 2017 at 6:16

Patrick Parker · Accepted Answer · 2017-02-03 07:45:22Z

6

Since each book has multiple contents with different descriptions, you would need each book to appear in multiple map entries. So I would use a flatMap to create a stream element for each BookContent, specifically a Description+Book pair. Then collect into a map as usual with Collectors.groupingBy:

    Map<String,List<Book>> result = globalBooks.getBooks().stream().flatMap(
        (Book b) -> b.getContents().stream().map(
            (BookContent bc) -> new Pair<>(bc.getDescription(), b)
        )
    ).collect(Collectors.groupingBy(
        Pair::getKey,
        Collectors.mapping(Pair::getValue, Collectors.toList())
    ));

Note:

The Pair class I have used above is from javafx.util.Pair. However you could just as easily use AbstractMap.SimpleEntry, your own Pair class, or any collection data type capable of holding two Objects.

edited Feb 3, 2017 at 7:45

answered Feb 3, 2017 at 6:17

Patrick Parker

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Comments

Rajiv Kapoor · Accepted Answer · 2017-02-03 07:25:57Z

1

Another solution could be

    Map<String,List<Book>> result = new HashMap<String, List<Book>>();

    globalBooks.getBooks().stream().forEach(book->{

        book.getContents().stream().forEach(bookContent->{

            result.computeIfAbsent(bookContent.getDescription(),(list)->new ArrayList<Book>()).add(book);
        });
    });

answered Feb 3, 2017 at 7:25

Rajiv Kapoor

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2 Comments

Patrick Parker Over a year ago

This code unnecessarily uses side-effects. If executed in parallel, the non-thread-safety of HashMap would cause incorrect results, and adding needed synchronization would cause contention.

Patrick Parker Over a year ago

I think this answer is good because it shows that a standard Java loop using computeIfAbsent is shorter to write, easier to read, and (probably) faster to execute. However you should not be using Stream.forEach, just a standard for(item:collection) loop, as explained in my previous comment.

MikaelF · Accepted Answer · 2017-02-03 17:50:17Z

1

This will do the job:

Map<String, List<String>> result = bookList.stream().flatMap(b -> b.getContents().stream())
    .collect(Collectors.toMap(str -> str, str -> bookList.stream()
    .filter(b -> b.getContents().contains(str))
    .collect(Collectors.toList()), (a, b) -> {a.addAll(b);return a;}));

If you want to execute this in parallel, simple replace the collector with Collectors.toConcurrentMap.

edited Feb 3, 2017 at 17:50

answered Feb 3, 2017 at 17:11

MikaelF

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