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 think you're breaking encapsulation with your Car constructor. Specifically you're dictating that an Engine must be injected to the Car instead of some type of interface used to determine your speed (IVelocity in the below example.)

With an interface, the Car is able to get it's current speed independent of what's determining that speed. For example:

public Interface IVelocity {
   public float getSpeed();
}

public class Car {
   private m_velocityObject;
   public constructor(IVelocity velocityObject) { 
       m_velocityObject = velocityObject; 
   }
   public float getSpeed() { return m_velocityObject.getSpeed(); }
}

public class Engine : IVelocity {
   private float m_rpm;
   private float m_currentGearRatio;
   public float getSpeed( return m_rpm * m_currentGearRatio; );
}

public class GPS : IVelocity {
    private float m_foo;
    private float m_bar;
    public float getSpeed( return m_foo * m_bar; ); 
}

An Engine or GPS can then have multiple interfaces based upon the type of work that it does. The interface is key to DI, without it DI does break encapsulation.