Question

Say we have the following two entities.

Library

public class Library
{
    private Collection<Book> books = new ArrayList<Book>();

    public boolean add(Book book)
    {
        return books.add(book);
    }
    public boolean remove(Book book)
    {
        return books.remove(book);
    }
}

Book

public class Book
{
    private Library library;
    public setLibrary(Library library)
    {
        this.library = library;
    }
}

This is a bi-directional relationship which will most likely break pretty quick. Even though you add a book to a library, the library of the book is not updated. And even though you update the library of a book, the library isn’t updated.

So, how would you maintain this bi-directional relationship in a good way? I could of course just do an add+remove in the setLibrary and vice versa, but this would as far as I can see cause a loop which would probably end in a StackOverflowError.

Answer 1

Define a “owner” for the relation. Does the library “own” the book or does the book own the library?

The answer to that question tells you who updates the “slave” entity.

I feel that the library owns the book, so the “add book to library” code should go into the library which will then update the back references.