Do you need an Interface?

Most likely no, and I tell you why

Does this ring a bell to you?

1. You create a controller for your REST API.

2. That controller calls your use case to respond to the request.

3. You create an Interface for that use case, with one method, which receives the DTO, and returns a Domain object (for example).

4. Finally, you create a new class that implements the Interface you just created; implements the use case.

By the book, this appears to be correct. Why?

• You are decoupling your implementation thanks to the Interface.

• Tomorrow, when a new implementation is needed, you just need to create a new class that implements the Interface, and your Controller will not change.

• At the time of testing, you use that Interface and create a Mock implementation for your tests.

But, is this a good idea?

In today’s email, I will share with you when and why you should use and avoid Interfaces.

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Now, let’s back to the topic of today.

✅ Do you really have multiple implementations upfront?

If the answer is yes, you definitely have to add an Interface.

For example, if you are using different services to send data to the outside world (Azure Log Analytics API and AWS Firehose), the Interface allows you to define that abstraction, simplifying the API for the user.

If you just have one single implementation, you are just duplicating code between the Interface and the actual implementation. No point in doing this.

✅ Do you have many things behind a boundary?

Interfaces are an excellent way to define a contract when establishing a boundary.

When there are many things behind a specific boundary, the interface defines a “joint” that is specific to a use case that other components or systems must consume.

In these scenarios, the contract that defines the Interface is fundamental, so that other developers or systems can interact with that complex behavior in a controlled and defined way.

🤔 Do you have one single implementation?

If there is only one concrete implementation, like sending data to AWS Firehose only (from the previous example), the interface is simply a reflection or a “mimicry” of that concrete implementation.

The abstraction is defined purely by the behaviors of the concrete implementation, creating a one-to-one relationship. In these situations, creating an interface changes nothing in terms of coupling, system fragility, or extensibility.

You say:

Well, it’s cheap to create an Interface for the just in case.

I would answer that saying:

What happened last time you did that? did another implementation came up later?

🤔 Do you want to change the behavior for testing?

That’s fair. You plan to implement a class, for testing purposes, that implements that Interface, and you inject it.

But for that, I have for you the Delegation Pattern. With that, you can do exactly the same, and you don’t require an Interface.

🤔 Are you writing code for your internal use?

If you are implementing code for being used by yourself, like an internal component that another internal component will use, and you are creating Interfaces, then you are creating contracts for yourself.

No need to have a contract with no one.

✨ Summary

I don’t want to be the party pooper, or maybe yes, but in today’s email, I just wanted to share with you my conclusions from many years of writing Interfaces all over the place.

I’ve been there, I feel you, you think that an Interface is the way to go; My 2 cents here:

Think twice… most likely you don’t need it.

I have much code written over there that, if I read it today, I would refactor it removing Interfaces.

I would love to hear your tales about coding with or without Interfaces. Reply to this email or comment if you are on Substack.

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