Immutable class with Builder inside

package com.vertigo.soundcloud9.stuff;

import java.util.BitSet;
import java.util.Date;

interface WithBuild<T> {
    T build();
}

interface WithCopy<B> {
    B copy();
}

/** A {@code User} is an example of an immutable class */
public class User implements WithCopy<User.Builder> {
    private final String username;
    private final String password;
    private final String firstname;
    private final String lastname;
    private final String email;
    private final Date creationDate;

    public User(Builder builder) {
        this.username = builder.username;
        this.password = builder.password;
        this.creationDate = builder.creationDate;
        this.firstname = builder.firstname;
        this.lastname = builder.lastname;
        this.email = builder.email;
    }

    public User(String username,
                String password,
                String firstname,
                String lastname,
                String email,
                Date creationDate) {
        this.username = username;
        this.password = password;
        this.creationDate = creationDate;
        this.firstname = firstname;
        this.lastname = lastname;
        this.email = email;
    }

    public String getUsername() {
        return username;
    }

    public String getPassword() {
        return password;
    }

    public String getFirstname() {
        return firstname;
    }

    public String getLastname() {
        return lastname;
    }

    public String getEmail() {
        return email;
    }

    public Date getCreationDate() {
        return new Date(creationDate.getTime());
    }

    @Override
    public Builder copy() {
        return new Builder()
            .username(username)
            .password(password)
            .creationDate(creationDate)
            .firstname(firstname)
            .lastname(lastname)
            .email(email);
    }

    public static Builder builder() {
        return new Builder();
    }

    @Override
    public String toString() {
        //noinspection StringBufferReplaceableByString
        final StringBuilder sb = new StringBuilder("User(");
        sb.append("username='").append(username).append('\'');
        sb.append(",password='").append(password).append('\'');
        sb.append(",firstname='").append(firstname).append('\'');
        sb.append(",lastname='").append(lastname).append('\'');
        sb.append(",email='").append(email).append('\'');
        sb.append(",creationDate=").append(creationDate);
        sb.append(')');
        return sb.toString();
    }

    public static class Builder implements WithBuild<User> {
        private final BitSet bs = new BitSet(16);
        private String username;
        private String password;
        private String firstname;
        private String lastname;
        private String email;
        private Date creationDate;

        private Builder() {
        }

        public Builder username(String username) {
            this.bs.set(0);
            this.username = username;
            return this;
        }

        public Builder password(String password) {
            this.bs.set(1);
            this.password = password;
            return this;
        }

        public Builder firstname(String firstname) {
            this.bs.set(2);
            this.firstname = firstname;
            return this;
        }

        public Builder lastname(String lastname) {
            this.bs.set(3);
            this.lastname = lastname;
            return this;
        }

        public Builder email(String email) {
            this.bs.set(4);
            this.email = email;
            return this;
        }

        public Builder creationDate(Date creationDate) {
            this.bs.set(5);
            this.creationDate = creationDate;
            return this;
        }

        @Override
        public User build() {
            final BitSet all = new BitSet(bs.size());
            all.set(0, 6);
            if (bs.equals(all))
                return new User(
                    username,
                    password,
                    firstname,
                    lastname,
                    email,
                    creationDate
                );
            else {
                all.andNot(bs);
                throw new RuntimeException("Initialization incomplete: " + all);
            }
        }
    }

    public static void main(String[] args) {
        try {
            User user = User.builder()
                .email("vp@vmg.io")
                .firstname("Vasya")
                .lastname("Pupkin")
                .build();
            System.out.println("user = " + user);
        } catch (Exception e) {
            System.out.println("e = " + e);
        }
        User user1 = User.builder()
            .username("ii")
            .password("secure1")
            .email("ii@vmg.io")
            .firstname("Ivan")
            .lastname("Ivanov")
            .creationDate(new Date())
            .build();

        User user2 = user1.copy()
            .username("pp")
            .email("pp@vmg.io")
            .firstname("Petr")
            .lastname("Petrov")
            .build();
        System.out.println("user1 = " + user1);
        System.out.println("user2 = " + user2);

    }
}