Assumptions for .equals() method in java - compare object's instance or object's state

Paweł Kubik :

What exactly recognise .equals() method in assumption: object's instance or object's state?

I am learning Hibernate and JPA. When I want to use bidirectional OneToMany association there is a problem because when I override equal and HashCode method with all variables there is become infinity loop. The only way to solve the problem which I see is to not use in one of the class the other class instance in equals() and hashCode(). So I am considering if it is compatible with assumptions in Java for .equals() method.

For example we have a Person class(). I know that email is unique value. So I can compare person by email and then I know that it is the same person (instance) but I don't know if the object's state is the same (firstName and lastName fields can differ). So it is my question, what should we expect during compare object with .equals()?

@Entity
public class Person {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
    private String email="";
    private String firstName = "";
    private String lastName = "";

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Person)) return false;
        Person person = (Person) o;
        return email.equals(person.email);
    }

    @Override
    public int hashCode() {
        return Objects.hash(email);
    }
}

mate00 :

When I think of implementing equals for a given class I ask myself a question: what makes an object unique? What differs instance of one Person different from other? Then I use that properties for implementing equals. In your case, or in case of @Entity objects with @XtoY relationship, in my opinion there is no need to include this relationship to equals.

If for example I have a Person that has a relation to Department, the fact that a person belongs to some departments doesn't make his unique. It's his name/surname/PESEL or email and so on, and that's what I would include in equals.

Hope this helps.