Java OOP Public vs Private vs Protected

Question

I understand what public, private, and protected do. I know that you are supposed to use them to comply with the concept of Object Oriented Programming, and I know how to implement them in a program using multiple classes.

My question is: Why do we do this? Why shouldn't I have one class modifying the global variables of another class directly? And even if you shouldn't why are the protected, private, and public modifiers even necessary? It's as if programmers don't trust themselves not to do it, even though they are the ones writing the program.

Thanks in advance.

Answer 1

You're right, it's because we can't trust ourselves. Mutable state is a major factor in complexity of computer programs, it's too easy to build something that seems ok at first and later grows out of control as the system gets bigger. Restricting access helps to reduce the opportunities for objects' states to change in unpredictable ways. The idea is for objects to communicate with each other through well-defined channels, as opposed to tweaking each others' data directly. That way we have some hope of testing the individual objects and having some confidence in how they'll behave as part of a larger system.