Should I write equals() and hashCode() methods in JPA entities?

Question

I want to check if entity is in a Collection member (@OneToMany or @ManyToMany) of another entity:

if (entity2.getEntities1().conta         


        

Answer 1

Yes, you should!

If you don't override the default Java.lang.Object equals and hashCode implementation:

@Entity(name = "Book")
public class Book implements Identifiable {
 
    @Id
    @GeneratedValue
    private Long id;
 
    private String title;
 
    //Getters and setters omitted for brevity
}

the merge operation will return a different object instance and the equality contract is going to be broken as explain in this post.

The best way is to use a business key, like this:

@Entity
public class Book implements Identifiable {
 
    @Id
    @GeneratedValue
    private Long id;
 
    private String title;
 
    @NaturalId
    private String isbn;
 
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Book)) return false;
        Book book = (Book) o;
        return Objects.equals(getIsbn(), book.getIsbn());
    }
 
    @Override
    public int hashCode() {
        return Objects.hash(getIsbn());
    }
 
    //Getters and setters omitted for brevity
}

You can also use the identifier for equality, but mind that the hashCode implementation should always return the same value as explained in the same post that I already mentioned:

@Entity
public class Book implements Identifiable {
 
    @Id
    @GeneratedValue
    private Long id;
 
    private String title;
 
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Book)) return false;
        Book book = (Book) o;
        return Objects.equals(getId(), book.getId());
    }
 
    @Override
    public int hashCode() {
        return getClass().hashCode();
    }
 
    //Getters and setters omitted for brevity
}