Is it proper for equals() to depend only on an ID?

Question

Let's suppose I have class User:

public class User {   private Long id;   private String name;   private Integer age;   private BigDecimal account;   // other fields, getters and setters } 

Is it proper to override the equals method as follows?

@Override public boolean equals(Object ob) {    if (ob == null) {        return false;    }    if (this == ob) {        return true;    }    if (ob instanceof User) {        User other = (User) ob;        return this.id.equals(other.getId());    }    return false; } 

It turns out that the uniqueness of an object is determined only by its ID. But in my application, id is always unique. It's provided at the database. Is my equals implementation competent enough to account for this? Or is this not best practice?

Of course I understand that in this case the hashCode implementation should be the following:

@Override public int hashCode() {    return id.intValue(); } 

Answer 1

Whether you should do this depends on the semantics of your class. That is, what does it mean to say that two objects of your class are equivalent?

The most important distinction is between objects with value semantics and objects with entity semantics. Entity objects are not equivalent even if they have equivalent attributes (colour, length, etc.). In many cases, including when the object has been read from a database table that has a primary key, entity objects will have an ID field that is unique. Comparing only the ID field in that case is the right thing to do.