Question

I have a domain object called User. Properties of user include ssoId, name, email, createdBy, createdDate and userRole. Of these, ssoId must be unique in the sense no two users can have the same sso id. So my equals method checks for the sso id and returns either true or false.

@Override public boolean equals(Object o) {
    if (!(o instanceof User))
      return false;
    return user.getSsoId().equals((User)o.getSsoId());
}

What I feel is that this is an incorrect implementation, though it is correct as far as the business rules are concerned. The above implementation will return true for two objects with same sso id but with different values for say name or email or both. Should I change my equals contract to check the equality of all fields? What is your suggestion?

Answer 1

This is (almost) correct for "technical equality", but not for "natural equality". To achieve top technical equality, you should also test the reflexive o == this. It may happen that the object isn’t persisted in DB yet and thus doesn’t have a technical ID yet. E.g.

public class User {

    private Long id;

    @Override
    public boolean equals(Object object) {
        return (object instanceof User) && (id != null) 
             ? id.equals(((User) object).id) 
             : (object == this);
    }

    @Override
    public int hashCode() {
        return (id != null)
             ? (User.class.hashCode() + id.hashCode())
             : super.hashCode();
    }

}

For "natural equality" you should rather compare all non-technical properties. For "real world entities" this is after all more robust (but also more expensive) than technical equality.

public class User {

    private String name;
    private Date birth;
    private int housenumber;
    private long phonenumber;

    @Override
    public boolean equals(Object object) {
        // Basic checks.
        if (object == this) return true;
        if (!(object instanceof User)) return false;

        // Property checks.
        User other = (User) object;
        return Objects.equals(name, other.name)
            && Objects.equals(birth, other.birth)
            && (housenumber == other.housenumber)
            && (phonenumber == other.phonenumber);
    }

    @Override
    public int hashCode() {
        return Objects.hash(name, birth, housenumber, phonenumber);
    }

}

True, that’s lot of code when there are a lot of properties. A bit decent IDE (Eclipse, Netbeans, etc) can just autogenerate equals(), hashCode() (and also toString(), getters and setters) for you. Take benefit of it. In Eclipse, rightclick code and peek the Source (Alt+Shift+S) menu option.

See also:

• JBoss: Equals and HashCode (in view of persistence)
• Hibernate: Persistent Classes – implementing equals() and hashCode()
• Related SO question: Overriding equals and hashCode in Java