It is okay to create an interface of interfaces?

Question

Let me first describe the situation

I have a component that uses dependency injection through a service locator. In the first scenario (image 1) the component needs a class that implements Interface0, so it just asks for it at the service locator, something like this (in pseudocode):

Inferface0 i0 = serviceLocator.get(Interface0);

This first scenario is very simple. Now the second scenario, where the component and service locator stays the same, but now the component needs a class that implements Interface1 and Interface2, like image 2 shows.

The question is, there is needed to create a new interface (let's call it Interface4) that implements Interface1 and Interface2 so I can ask for it like following?

Inferface4 i4 = serviceLocator.get(Interface4);

And then, if later the component needs an implementation of Interface2 and Interface3 then a creation of Interface5 would be needed? What is the best approach to handle this interfaces?

Answer 1

The question is, there is needed to create a new interface (let's call it Interface4) that implements Interface1 and Interface2 so I can ask for it like following?

If you want to receive a single object that implements both interfaces, you probably also want to store a single reference to that object. If you declare your reference as Interface1 iA, then you cannot access the Interface2 methods without ugly typecasting tricks and the other way around when you declare it as Interface2 iA.

The only way to declare a reference of a type that can access both Interface1 and Interface2 methods is to create a Interface4 that combines the two interfaces. And your implementation classes would also have to declare they implement Interface4.

And then, if later the component needs an implementation of Interface2 and Interface3 then a creation of Interface5 would be needed? What is the best approach to handle this interfaces?

If you regularly have this problem, you need to critically re-evaluate how you determine which interfaces you need. Maybe you should not have created Interface1, Interface2, and Interface3 at all, but only Interface4 and Interface5.

Answer 2

Well, you could do that. Or:

Interface1 i0 = new Implementation1();

Just do that high up the call stack (say in main maybe) and pass it into something that needs it. That thing can use it as an object that implements Interface1 without knowing or caring that it happens to be Implementation1.

Creating Inferface4 is only really justified if something uses the object that way. At best, creating interfaces just to make a service locator happy is a hack. Interfaces and objects (even singleton objects) were never meant to have an exclusively 1 to 1 relationship. Interfaces are meant to express what the client (using) code needs.

It's also worth noting that interfaces aren't the only kind of key your service-locator/container/map can use. So if your code base doesn't naturally have that 1 to 1 relationship you don't have to force it.