Are there any rules for OOP?

Question

Recently I heard that there are 9 rules for OOP(Java). I know only four as Abstraction, Polymorphism, Inheritance and Encapsulation. Are there any more rules for OOP?

Answer 1

These are concepts, not rules. There are no rules really, just decisions to make, some designs are better than others, some much better than others :-)

There are plenty of guidelines though :-) Some are language specific (C++ is riddled with them) others are OO specific. Too many to list though :-)

Off the top of my head, important ones are:

• Loose coupling, high cohesion
• Write testable classes, which you test
• Use inheritence sparingly and only where it makes sense (prefer composition)
• Try stick to the open/close principle.
• (most important) KISS

Plenty to expand upon and add :-)

EDIT: I should add, the rules which you listed are not unique to OO