Repository Pattern Single Interface in Android

Question

After studying a couple of blog posts on the topic, I got confused as there were different implementations by different developers but they have all used single interface for repository.

Assume, we have following interface for repository

interface Repository<T> {
    fun get(): List<T>
    fun save(items: List<T>)
}

For a specific implementation of this repository with T = Users, Repository.get(), downloads some data from sever and cache it.

For another type of object, say Bundles I not only need these methods but some additional methods e.g. querying local database for specific column.

In that case, I can't understand, do I need to create a separate interface or add methods to existing single interface, in which case what should I do with the implementation of new methods in other repositories which do not need these methods? It it must to have same interface for all repositories. Personally, I think, its ok to create multiple interfaces for different repositories as long as I abstract them nicely.

Note that your question is very close to what is covered by the "primarily opinion-based" close vote explanation. I tried to answer your question from a technical standpoint. — CommonsWare
– CommonsWare, Commented May 29, 2019 at 16:25
I have discussed pros and cons of generic repository in this answer in details. — Amit Joshi
– Amit Joshi, Commented May 30, 2019 at 6:17

CommonsWare · Accepted Answer · 2019-05-29 16:25:49Z

1

do I need to create a separate interface or add methods to existing single interface

You can do whatever you like. Android does not know anything about repositories and imposes no restrictions.

In my work, I do not create a generic interface (like the one that you show) in the first place. I cannot think of a project that I worked on where developers created a generic repository interface, either. I am sure that some people do create such an interface, but there is no requirement by the OS for one.

Personally, I think, its ok to create multiple interfaces for different repositories as long as I abstract them nicely.

You are welcome to do that.

edited May 29, 2019 at 16:25

answered May 29, 2019 at 16:23

CommonsWare

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6 Comments

m_allaudin Over a year ago

Gosh! May be the confusion raised from the fact that almost all blog posts created repository for only one use case.

CommonsWare Over a year ago

@mallaudin: Blog posts have their limits. So do book examples. :-)

m_allaudin Over a year ago

Exactly. What I am thinking of, if we really don't want to write same methods in multiple repositories , we can still create a top level interface with generic methods and then create other interfaces for specific repositories. What do you think about this approach?

CommonsWare Over a year ago

@mallaudin: If you find value in doing that, it should be fine. Personally, I am not a fan of creating interfaces for the sake of having interfaces. For a huge, multi-module project, this might be appropriate, to help maintain some amount of isolation between the modules. Or, if there is a pluggable system of repositories, where a consumer might get different implementations, then an interface is appropriate. But "all repositories need interfaces" is overkill, IMHO, for smaller projects where there will only ever be a single implementation of that interface.

CommonsWare Over a year ago

@mallaudin: One exception is if your test code (e.g., mock generator) requires interfaces to work properly. In that case, your interfaces are there to support that particular library.

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