They are supposed to ... but finding correct abstractions is hard. After a year or two abstractions usually are not correct anymore. That is why "favor composit

They are supposed to ... but finding correct abstractions is hard. After a year or two abstractions usually are not correct anymore. That is why "favor composition instead of inheritance" is important because hierarchy of inheritance is fragile abstraction and if you compose things it is easier to swap one thing with other if they have the same interface.