Value Object

A small simple object, like money or a date range, whose equality isn’t based on identity.

• Overview
• How It Works
• When to Use It

Overview

Value Object is similar to the primitive types present in many languages that aren’t purely object oriented.

How It Works

• We like to think that Value Objects are small objects, such as a money object or a date, while reference objects are large, such as an order or a customer.

• The key difference lies in how they deal with equality. A reference object uses identity as the basis for equality (maybe the identity within the programming system, such as the built-in identity of OO programming languages, or maybe some kind of ID number, such as the primary key in a relational database). A Value Object bases its notion of equality on field values within the class. Thus, two date objects may be the same if their day, month, and year values are the same.

• They are often passed around by value instead of by reference.

• It’s a good idea to make them immutable, to avoid aliasing bugs (when two objects share the same value object and one of the owners changes the values in it).

• Value Objects shouldn’t be persisted as complete records. Instead use Embedded Value or Serialized LOB.

When to Use It

Treat something as a Value Object when you’re basing equality on something other than an identity.

It’s worth considering this for any small object that’s easy to create.