Help for this page

Select Code to Download

Select or download this

Exception in thread "main" java.lang.IllegalStateException: A book has
+ a null property
        at com.example.myproject.Author.getBookIds(Author.java:38)
...
        at com.example.myproject.Book.getId(Book.java:22)
        at com.example.myproject.Author.getBookIds(Author.java:35)
        ... 1 more