What is object oriented programming?

(written by lawrence krubner, however indented passages are often quotes). You can contact lawrence at: lawrence@krubner.com


Here is an a la carte menu of features or properties that are related to these terms; I have heard OO defined to be many different subsets of this list.

Encapsulation – the ability to syntactically hide the implementation of a type. E.g. in C or Pascal you always know whether something is a struct or an array, but in CLU and Java you can hide the difference.

Protection – the inability of the client of a type to detect its implementation. This guarantees that a behavior-preserving change to an implementation will not break its clients, and also makes sure that things like passwords don’t leak out.

Ad hoc polymorphism – functions and data structures with parameters that can take on values of many different types.

Parametric polymorphism – functions and data structures that parameterize over arbitrary values (e.g. list of anything). ML and Lisp both have this. Java doesn’t quite because of its non-Object types.

Everything is an object – all values are objects. True in Smalltalk (?) but not in Java (because of int and friends).

All you can do is send a message (AYCDISAM) = Actors model – there is no direct manipulation of objects, only communication with (or invocation of) them. The presence of fields in Java violates this.

Specification inheritance = subtyping – there are distinct types known to the language with the property that a value of one type is as good as a value of another for the purposes of type correctness. (E.g. Java interface inheritance.)

Implementation inheritance/reuse – having written one pile of code, a similar pile (e.g. a superset) can be generated in a controlled manner, i.e. the code doesn’t have to be copied and edited. A limited and peculiar kind of abstraction. (E.g. Java class inheritance.)

Sum-of-product-of-function pattern – objects are (in effect) restricted to be functions that take as first argument a distinguished method key argument that is drawn from a finite set of simple names.