After extract method or extract class introducing a value object is one of the most powerful refactorings available. A value object encapsulates a value or concept within your domain. While the term is more formally known from Domain Driven Design, DDD is not a pre-requresite for use. Introducing a value object can be applied to any code base.
Some excellent examples of value objects would include
PhoneNumber. These could all be identified as integers, strings or decimal numbers, but doing so would lead to a series of downsides.
Making use of primitive data types to express concepts within an application is a code smell known as primitive obsession. Replacing primitives with value objects is the solution to this smell.
- Duplication will be thrown throughout the codebase. Both in the form of simple guard clauses, or core domain logic.
- More tests are required. This ties into the duplication above.
- Your domain lends itself towards an anaemic model, full of utility classes that operate upon state.
The implementation of
PersonalDetails would be straight forward to begin with.
Over time areas of logic can slowly migrate and move towards the class. In most IDE's, simply wrapping a primitive type as the first step can be carried out in a few keystrokes.
The constructor performs basic validation on a technical level. Once complete we can carry out any domain logic. Likewise the behaviour attached to this object (hidden for beravity) would include various domain specific logic. For example, when changing surnames any leading or trailing whitespace is removed.
One recommendation would be to expose the underlying primitive. In this example
ToString has been overridden to return the string value that is being used. This should be a read only operation idealy, and enables the object to play nicely with third parties. Such use cases for this would be serialization, or writing the value to a persistent store.
Equality (and hashcode in this case) should also be implemented. This is because the nature of value objects allows them to be equal to other instances that share the same value, despite being different references in memory. The beauty of this is that value objects can be used as needed, no need for injection or other patterns.
- Removes duplication. Only the object in question will be the source of truth.
- Less tests need to be written. As the duplication has been removed, only one test per behaviour is required. Rather than duplicating checks for validation or formatting this can be contained to the object. As the rest of the system deals with our value object, we don't have to worry about dealing with an invalid representation.
- In statically typed languages you can lean on the compiler. It's impossible to supply anything other than PersonalDetails when we ask for an instance. Even for dynamic languages, the stack trace presented upon error would be far more useful than had a primitive type been provided.
- The surface area of mis-configuring arguments is smaller also. Previously we would accept two strings that are order dependant. Now this configuration has been reduced to a few areas.
- Using the example above, we can now rely on class pre-conditions to simplify our expectations when working with this type. Given any instance of
PersonalDetailswe can be sure that the forename and surname are never null or empty, and that each personal details instance will have a forename of at least one character long. A simple string can never guarantee such conditions.
- Making value objects public generally makes sense. This provides an excellent seam for testing and integration.
- The introduction of a value object plays nicely with my three basic steps to code quality.