How to use Dependency Injection without breaking encapsulation?

Question

How can i perform dependency injection without breaking encapsulation?

Using a Dependency Injection example from Wikipedia:

public Car {
    public f         


        

Answer 1

I don't think a car is a particularly good example of the real world usefulness of dependency injection.

I think in the case of your last code example, the purpose of the Car class is not clear. Is is a class that holds data/state? Is it a service to calculate things like speed? Or is it a mix, allowing you to construct its state and then call services on it to make calculations based on that state?

The way I see it, the Car class itself would likely be a stateful object, whose purpose is to hold the details of its composition, and the service to calculate speed (which could be injected, if desired) would be a separate class (with a method like "getSpeed(ICar car)"). Many developers who use DI tend to separate stateful and service objects--although there are cases where an object will have both state and service, the majority tend to be separated. In addition, the vast majority of DI usage tends to be on the service side.

The next question would be: how should the car class be composed? Is the intent that every specific car is just an instance of a Car class, or is there a separate class for each make and model that inherit from CarBase or ICar? If it's the former, then there must be some means of setting/injecting these values into the car--there is no way around this, even if you'd never heard of dependency inversion. If it's the latter, then these values are simply part of the car, and I would see no reason to ever want to make them settable/injectable. It comes down to whether things like Engine and Tires are specific to the implementation (hard dependencies) or if they are composable (loosely coupled dependencies).

I understand the car is just an example, but in the real world you are going to be the one who knows whether inverting dependencies on your classes violates encapsulation. If it does, the question you should be asking is "why?" and not "how?" (which is what you are doing, of course).