Understanding and using value objects
We have observed the main attributes that entity objects should be identified by, which are continuity and identity, and not necessarily their values. This brings us to ask the question, what do we call objects that are indeed defined by their values? These are value objects. They too have their place in the domain model, as they are used to measure and quantify parts of the domain. They do not boast identity keys in the same way that entities do, but their keys are formed through the composition of the values of all their properties, hence the name value objects.
Given that the data they store is so important in defining their identity and uniqueness in our system, it is of the utmost importance that these objects never change once created and are immutable. It is also important to understand the differences between entity models and value objects.
Figure 2.3 shows a comparison between entities and value objects:
