JPA OneToMany - Collection is null

Question

I\'m trying to set up a bidirectional relationship using JPA. I understand that it\'s the responsability of the application to maintain both sides of the relationship.

Answer 1

It seems that books type of Collection in Library class is not initilized. It is null;

So when class addBook method to add a book object to collection. It cause NullPointerException.

@OneToMany(mappedBy = "library", cascade = CascadeType.ALL)
 private Collection books;

 public void addBook(Book b) {
    this.books.add(b);
    if(b.getLibrary() != this)
    b.setLibrary(this);
 }

Initilize it and have a try.

Change

private Collection books;

To

private Collection books = new ArrayList();