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OO-Design Part 3: When to Apply Encapsulation

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Part 3 of the series OO-Design

As discussed in the previous post Anemic Domain Models Are Healthy, encapsulation makes code harder to test. We should make important actions easy to do, and testing is important.

Encapsulation and domain modeling are separate concepts. A good domain model is possible with or without encapsulation.

Encapsulation is the design of trust boundaries between programmers. A well-encapsulated API ensures that other programmers cannot put your library or service into an invalid state. For example, a builder-pattern API can be helpful, self-documenting, and prevent invalid states.

Encapsulation makes good sense between different teams

Code ownership boundaries are a good place to apply the rules of encapsulation. Each team of programmers can work against an encapsulated API without having to read the internals of someone else’s codebase.

Encapsulation Costs More

Good encapsulation is often a lot of extra work for a single team in a single codebase. APIs easily devolve into leaky abstractions, so extra care and thought must be spent to offer full utility without allowing invalid states. That extra time and thought should only be spent when necessary.

While appropriate for libraries and API boundaries, encapsulation is often misapplied to internal code with shared ownership. We pretend that we cannot trust ourselves to correctly use our own domain models. Simply putting logic inside a class does not protect it. That is “encapsulation theater”. You and your coworkers all have access to the code, they can do anything they want. “Hiding” logic inside a class instead of putting in a pure function only makes it harder to test!

steve shogren

software developer, manager, author, speaker


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