What methods should go in my DDD factory class?

Question

I am struggling to understand what my factory class should do in my DDD project. Yes a factory should be used for creating objects, but what exactly should it be doing. Consid

Answer 1

In the builder you can have any logic you need to inforce the invariants on your entites, a little example using Java as development language...

I have a User entity that has a username, a password and an email, all attributes required so I have:

public class User {
    private String username;
    private String password;
    private String email:

    /**
     * @throws IllegalArgumentException if the username is null, the password is null or the
     * email is null.
     */
    public User(final String theUsername, final String thePassword, final String theEmail) {
        Validate.notNull(theUsername);
        Validate.notNull(thePassword);
        Validate.notNull(theEmail);

        this.username = theUsername;
        this.password = thePassword;
        this.email = theEmail;
    }

    // Getters / Setters / equal / hashCode / toString
}

and then I have the UserBuilder:

public class UserBuilder {
    private String username;
    private String password;
    private String email;

    public UserBuilder withUsername(final String theUsername) {
        Validate.notNull(theUsername);

        this.username = theUsername;

        return this;
    }

    public UserBuilder withPassword(final String thePassword) {
        Validate.notNull(thePassword);

        this.password = thePassword;

        return this;
    }

    public UserBuilder withEmail(final String theEmail) {
        Validate.notNull(theEmail);

        this.email = theEmail;

        return this;
    }

    public User build() {
        User user = new User(this.username, this.password, this.email);

        return user;
    }
}

And you can use the builder like this:

UserBuilder builder = new UserBuilder();

try {
    User user = builder.withUsername("pmviva").withPassword("My Nifty Password").withEmail("pmviva@somehost.com").build();
} catch (IllegalArgument exception) {
    // Tried to create the user with invalid arguments
}

The factory's solely purpose is th create valid instances of objects. In order not to duplicate creation and hydration code you can have your repositories to query a rowset from the database and delegate the creation of the object to a builder passing the rowset's data.

Hope this helps

Thanks Pablo