Why do we declare Private variables in Java?

Question

I'm confused because all I keep hearing is that private variables in Java are supposed to protect the code or the variable. But if anybody has access to the code, then it makes no difference if it is private, they can still change it. So how is it considered protected when anybody who has access to the code can change it?

Answer 1

When programmers talk about accessing a variable, they mean accessing its value when the program runs. Protecting the code from changes is another matter entirely and requires human processes rather than syntax of a programming language. Making a variable private "protects" its value when the code runs. At this level, we are not concerned with protecting it from other programmers changing the code itself. The point of so-called "data hiding" is to keep internal data hidden from other classes which use the class. Those other classes should only access behavior by calling methods on the class, not by changing values of variables directly.

General programming principles such as "data hiding" are followed to help us as programmers write correct code. If any class can change a variable's value, then it is difficult to ensure that the value is valid. Say for example, you have a variable which counts the number of widgets a factory manufactures. By making the variable a private data member, you can more easily ensure that the value is never negative. On the other hand, if the variable is public, another class could change it to a negative value which can cause other parts of the code to crash.