An entity that represents a customer

Customer.java

public class Customer implements Serializable {

    private Long id;

    private String firstName;

    private String lastName;

    private String email;

    public Long getId() {
      return id;
    }

    public String getFirstName() {
      return firstName;
    }

    public String getLastName() {
      return lastName;
    }

    public String getEmail() {
      return email;
    }

    @Override
    public int hashCode() {
      if (id == null) {
        return new Object().hashCode();
      } else {
        return id.hashCode();
      }
    }

    @Override
    public boolean equals(final Object object) {
      if (this == object) {
        return true;
      }
      if (object == null
          || !(object.getClass().isAssignableFrom(getClass()) && getClass().isAssignableFrom(object.getClass()))) {
        return false;
      }
      Customer customer = (Customer)object;
      return getId() != null && getId().equals(customer.getId());
    }

    @Override
    public String toString() {
      return getClass().getSimpleName() + "#" + id;
    }

    /**
     * Creates a new instance of builder {@link CustomerBuilder} for entity {@link Customer}.
     *
     * @return new builder instance
     */
    public static CustomerBuilder newBuilder() {
      return new CustomerBuilder();
    }

    public static class CustomerBuilder {

      private String firstName;

      private String lastName;

      private String email;

      private CustomerBuilder() {
      }

      /**
       * Sets firstName to builder.
       *
       * @param firstName the given firstName.
       * @return builder instance
       */
      public CustomerBuilder setFirstName(final String firstName) {
        this.firstName = firstName;
        return this;
      }

      /**
       * Sets lastName to builder.
       *
       * @param lastName the given lastName.
       * @return builder instance
       */
      public CustomerBuilder setLastName(final String lastName) {
        this.lastName = lastName;
        return this;
      }

      /**
       * Sets email to builder.
       *
       * @param email the given email.
       * @return builder instance
       */
      public CustomerBuilder setEmail(final String email) {
        this.email = email;
        return this;
      }

      public Customer build() {
        Customer customer = new Customer();
        customer.firstName = firstName;
        customer.lastName = lastName;
        customer.email = email;
        return customer;
      }
    }
}