#OneToMany new Arraylist() is null - spring-boot

Question

Hi I have a ManyToOne relationship between book and Student entity. I created this relationship as a bi-directional relationship. See classes below
Book class
#Getter
#Setter
#Builder
#AllArgsConstructor
#NoArgsConstructor
#Entity(name = "Book")
#Table(name = "book")
public class Book {
#Id
#SequenceGenerator(
name = "book_sequence",
sequenceName = "book_sequence",
allocationSize = 1
)
#GeneratedValue(
strategy = GenerationType.SEQUENCE,
generator = "book_sequence"
)
#Column(
name = "id",
updatable = false
)
private Long id;
#Column(
name = "book_name",
nullable = false
)
private String bookName;
#ManyToOne
#JoinColumn(
name = "student_id",
nullable = false,
referencedColumnName = "id",
foreignKey = #ForeignKey(
name = "student_book_fk"
)
)
private Student student;
}
Student class
#Getter
#Setter
#Builder
#AllArgsConstructor
#NoArgsConstructor
#Entity(name = "Student")
#Table(name = "student",
uniqueConstraints = {
#UniqueConstraint(name = "student_email_unique", columnNames = "email")
}
)
public class Student {
#Id
#SequenceGenerator(name = "student_seq", sequenceName = "student_seq", allocationSize = 1)
#GeneratedValue(strategy = GenerationType.SEQUENCE,generator = "student_seq")
#Column(name = "id",updatable = false)
private Long id;
#Column(name = "first_name", nullable = false, columnDefinition = "TEXT")
private String firstName;
#OneToMany(
mappedBy = "student",
orphanRemoval = true,
cascade = { CascadeType.PERSIST, CascadeType.REMOVE }
)
private List<Book> books = new ArrayList<>();
public void addBook(Book book){
// if(books == null){
// this.books = new ArrayList<>();
// }
if(!this.books.contains(book)){
this.books.add(book);
book.setStudent(this);
}
}
public void removeBook(Book book){
if(this.books.contains(book)){
this.books.remove(book);
book.setStudent(null);
}
}
#Override
public String toString() {
return "Student{" +
"id=" + id +
", firstName='" + firstName + '\'' +
", lastName='" + lastName + '\'' +
", email='" + email + '\'' +
", age=" + age +
", studentIdCard=" + studentIdCard.getCardNumber() +
'}';
}
}
When I run the application I am getting an error
Cannot invoke "java.util.List.contains(Object)" because "this.books" is null
This is available inside the addBook method in my student entity. In my student entity I initialized my books variable as below. I tried solutions provided in other similar issues raised in stackoverflow but unable to get this issue sorted.
#OneToMany(
mappedBy = "student",
orphanRemoval = true,
cascade = { CascadeType.PERSIST, CascadeType.REMOVE }
)
private List<Book> books = new ArrayList<>();
I am not sure whether use of lombok has something to do with this. But if I write my code inside addBook method as below, issue is sorted
public void addBook(Book book){
if(books == null){
this.books = new ArrayList<>();
}
if(!this.books.contains(book)){
this.books.add(book);
book.setStudent(this);
}
}
Any ideas what might cause this issue?
Cheers

Answer 1

I used the #Builder.Default annotation for the books variable.
#OneToMany(
mappedBy = "student",
orphanRemoval = true,
cascade = { CascadeType.PERSIST, CascadeType.REMOVE }
)
#Builder.Default
private List<Book> books = new ArrayList<>();
Reference - use Lombok #Builder #Default #Singular to initialise List<>