language agnostic - Main concepts in OOP

Question

I was once asked in an interview 'What are the 3 main concepts of OOP?'. I answered by saying that in my opinion there were 4 which are as follows:

• Inheritance
• Encapsulation
• Abstraction
• Polymorphism

Was I correct?

Answer 1

There are 3 requirements for a language to be object-oriented:

• a language that supports only encapsulation (objects) is not object-oriented, but it is modular
• a language that supports just encapsulation (objects) and message-passing (polymorphism) is not object-oriented, but it is object-based
• a language that supports encapsulation (objects), message-passing (polymorphism), and inheritance (abstraction), is object-oriented

NOTE: Abstraction is a much more general concept; encapsulation et al are kinds of abstraction, just as a subroutine is a kind of abstraction. See Abstraction