Java Builder pattern

Question

I recently wrote a builder class and noticed that the standard seems to be as follows

public class PersonBuilder {
    private String firstName;
    private String lastName;

    public PersonBuilder withFirstName(String firstName) {
        this.firstName = firstName;
        return this;
    }

    public PersonBuilder withLastName(String lastName) {
        this.lastName = lastName;
        return this;
    }

    public Person build() {
        return new Person(this);
    }
}

Is there any disadvantage to, instead, doing the following

public class PersonBuilder {
    private Person person;

    public PersonBuilder withFirstName(String firstName) {
        person.setFirstName(firstName);
        return this;
    }

    public PersonBuilder withLastName(String lastName) {
        person.setLastName(lastName);
        return this;
    }

    public Person build() {
        return person;
    }
}

I understand this may be an opinion based question, I was just looking for any answers as to why this may be a bad or better design pattern.

Answer 1

There are several problems with your approach. Some of them are described in previous answers so I'll just mention the others.

The biggest problem with your design, is that you're using a single instance of Person in the builder. This means that if you're using the same builder more than once, you'll be "building" the same instance, while the clients using it are expecting two different instances. No need to mention that this could cause some serious havoc in your application.

The answer you got from @Basilevs mentions that the "built" class will require setters. This is absolutely true, but I'd just like to stress that this is a huge problem, since it means that the classes you "build" can never be immutable ! In other words, you're restricting the implementers of such classes to using synchronization for thread safety if needed, and other problem solving mechanisms that could have been avoided using the common approach.