Indeed it is a good idea to use composition whenever feasible. But your problems aren't over. Suppose you write a method that can accept anything that implements the Fruit interface. You've got covariance again. Suppose you have a dictionary whose values are of type Apple. You can pass those values into that method. That's contravariance again. And so it goes.

It doesn't matter whether you're defining types by classes, or what interfaces you implement. You will have types of some sort, and as soon as you do, you have covariance and contravariance as concepts again.