What code is meant to be third party code in the Bean Factories example in the docs?

[mattwelke]

I'm confused by a part of the documentation: https://docs.micronaut.io/latest/guide/#factories

Right now, the code is this:

@Singleton
class CrankShaft {
}

class V8Engine implements Engine {
    private final int cylinders = 8;
    private final CrankShaft crankShaft;

    public V8Engine(CrankShaft crankShaft) {
        this.crankShaft = crankShaft;
    }

    @Override
    public String start() {
        return "Starting V8";
    }
}

@Factory
class EngineFactory {

    @Singleton
    Engine v8Engine(CrankShaft crankShaft) {
        return new V8Engine(crankShaft);
    }
}

Because the class V8Engine is the the concrete type being returned by the EngineFactory class's v8Engine method, wouldn't that mean the V8Engine class is meant to represent third party code one may want to treat as a bean, injecting instances of it into parts of their application? The V8Engine class has no annotations, so that's consistent with my interpretation of the example. But it depends on the CrankShaft class and Engine interface, both of which I'm interpreting as being one's own code. CraftShaft, because it has the @Singleton annotation. Engine, because it wouldn't make sense to use an interface provided by third party code. In my opinion, it would make sense to define your own interfaces and use them to connect different parts of your application together.