Examples of cases where using private variable/methods in oop languages is useful/necessary

Question

Whenever I try to research this question the answer usually presented is along the lines of "so the outside world does not have direct access to the local variables." What's missing here for me is the context to which this applies. I get that, for instance, in the code below

function Person(firstName, lastName) {
  var _firstName = firstName,
  _lastName = lastName;

  this.firstName = function(value) {
  if (!arguments.length) return _firstName;
  _firstName = value;
}};

the variables are considered private because they can only be accessed from calling the functions. What is the significance of this? Why does it matter? And most achingly mysterious is what is a "real life" situation in which the common explanation of "so the outside world does not have direct access to the local variables..." would apply/make sense to someone who has not seen a situation where it matters.

Thanks SO.

Answer 1

It's all to ease development.

Only public properties or methods of objects are accessible to "outside code", i.e. any code that uses an object, accesses its properties or calls its methods can only use the public interfaces of said object. When code uses the public interface in any way, you get code coupling. When you declare a method foo on your object/class, and somewhere in your code you have obj.foo(), that line of code is now coupled to the existence of the foo method. If you want to remove or change the foo method, you (probably) also have to change every line of code that calls it.

Protected and private properties and methods are pieces of code which are explicitly only usable by the class/object itself. "Outside code" cannot couple to it, neither purposefully nor accidentally. When you need to refactor that code, you can be sure the changes are isolated to within the class itself and not worry about possibly breaking tons of other code.

Protected and private properties also don't run the risk of being modified by outside code in an incorrect way. A private property can only be modified by the class/object itself, so the class/object itself is always in full control of the value of that property. This ensures a consistent state of the object.

Therefore, a class/object should keep its public interface to a minimum and keep everything that's not intended or necessary for "public consumption" private. This allows you to focus on your class design in terms of what needs to be public for an object to be useful, and keep the rest of the implementation flexible.

Answer 2

In an OO language, classes should expose behaviour and hide implementation. That's called encapsulation, makes reutilization easier, and is one of the bases of OO.

Generally speaking, variables are part of implementation and not of behaviour. That's why, normally, you don't declare them public.

An example from "real life" is any API you have to interact with (i.e. Amazon REST API). Your work as a developer is easier, if you only have to learn behaviour details (public methods), than if you had to learn the usage of every internal variable of the site. Also, if client code only interacts with the behaviour (again, public methods) of a system, the system may change (i.e. optimize) it's implementation without affecting it's users.