this wasn’t an iOS or even a mobile coding question, but more of a CS fundamental that a developer a few years removed from grad school would likely have trouble recalling (i.e. me). The four principles that I quickly googled up (and hopefully didn’t sound like I had just googled up) were: encapsulation (hiding data implementation by restricting access only to public methods of this object), abstraction (one class should not know inner details of another in order to use it, just knowing the interfaces should e good enough), inheritance (derived classes can reuse the code of existing super classes), polymorphism (one name many forms ; static means method overloading and dynamic means method overriding).