Functional Testing – Why is Black Box Testing Called Functional?

OO, according to Alan Kay is all about message passing, and that's it. You will see that qualities such as polymorphism and encapsulation are actually derived from message passing.

Now that stance is in fact very extreme, because it basically means that only Smalltalk and languages alike would qualify.

I think you can define OO as building your system on entities, that fully encapsulate their state, and that are rendered exchangeable due to their inherent polymorphous qualities. One could thus argue a purely OO language ensures these two core qualities are always met. What renders OO languages "impure" would be mechanisms that allow the creation of constructs that do not meet these criteria, such as the possibilities to:

• declare public fields
• declare variables that can only hold instances of a specific class and its subclasses (i.e. variables should be typed against interfaces, which is what the biological objects communicate through in Kay's anology), which is rendered even narrower if the class in question is final, as that leaves even less room for polymorphism
• declare primitives

Then again, IMHO language purity is more a weakness than a strength. OO is not a silver bullet. No single paradigm is.

When you are doing unit testing, you do not care about the actual implementation of the solution, only that it gives the expected results. The difference being the level of unit that you are testing. If you are testing a class, you are not making sure that internal methods are called but that the correct response is returned. How the unit gets the response is not what you are testing for.

The different levels of testing cover the interaction and logic of the pieces of your implementation but are agnostic of what they actually are.

UPDATE Generally, developers do fine grain unit testing. However, even a tester can do lower level unit testing. Focusing on specific pieces of an operation, for example sending data to the server themselves rather than through the web interface (removing the web interface from the unit). Or for testing the web interface, spoofing the response provided so that you are able to test its behavior in isolation.

Knowing the implementation would allow you to determine the lower level 'units', however the tests themselves do not know anything about the actual details of the solution.