Thread: Why do we make variables of a class private?

There are a few different reasons:

1. It abstracts the design. For example, say you have a rectangle class. How are you going to represent the rectangle? With width/height and the location of the top-left corner? Or by coordinates of the top-left and bottom-right corners? Or the bottom-left and top-right corners? With getter/setters, these functions don't have to directly return the underlying implementation, you can have multiple methods and ways for the user to use your class indepedent of the underlying implementation.

2. You can add checks before a class enters a "bad" state. For example, what if the user tries to set the width of a rectangle to -1? If you give direct access to fields, there's no way to prevent this. However, with a setter you can implement advanced logic and handle situations accordingly.

3. Along with reason 1, what if you decide at a later date you don't like the underlying implementation? If external code has direct access to variables then these change will require them to change every part of their code which uses your class. However, if we provide getter/setters, as long as the contract is the same, you only have to change the getter/setter implementations and everyone can continue to use your code exactly as they did before.

4. You can provide "internally modifiable only" variables by only providing a public getter. For example, take the ArrayList class. It has an internal size field which should only be modified internally. However, it's also very useful for external code to retrieve the size of an ArrayList.