​Yes, but my point was still about what the public API is. Starting the car with the key should be easier than starting it without the key. Surely you should be able to start the engine without the key if you look under the hood and figure it out. And maybe you'd need a tool for that to be kept in your garage for debugging, but not as a button attached to the outside of the car or on the dashboard :-).

The problem becomes even worse if you instead make Car inherit from its parts (including Engine), because the relationship of a car and and engine is not well described by inheritance. A car is not a kind of engine, nor should it pretend to be an implementation of an engine. A car *comprises* (or contains) an engine.  And who knows, maybe it has two engines!

-- Koos