Object-oriented – what is the difference between Object and Value in Python

There is no hard and fast rule which methods an object must have.

Something is an object if you can talk about it as an entity with a name that is familiar to domain experts in either your problem domain or the solution domain. In essence, if you have a related set of properties and behaviors that you can refer to by a single name, then that set of properties and behaviors is an object.

The behaviors that I refer to fall into two broad categories:

• Requests to take an action. The classic example of Dog::bark() falls in this category.
• Requests for information. Most getter functions fall in this category.

There is nothing wrong with an object that has mostly (or only) getters. Some objects in some domains are just model an entity that must exist but that does not have any active behavior. Your User class might be one of those.

A class X with mostly getters becomes a code smell when

• each getter also has a setter to change the property, and
• either class X has invariants (invalid combinations of property values) that can be broken if you use the setters 'wrongly', or
• class Y is performing operations on the properties of X that would logically fall within the responsibility of X.

An example of the first class with a code smell would be a class like this:

class Square {
    int x1, x2, y1, y2;
public:
    void SetTopLeft(int x, int y);
    void SetBottomRight(int x, int y);
};

Allowing to change the top-left and bottom-right coordinates independently makes it very easy to break the invariant that all sides of a square must have the same length and all corners are square.

An example of the second form of the code smell is a Dog class where the client moves a Tail object to make the dog wag its tail.

Internal state mainly makes sense as a term to differentiate from externally visible (and possibly directly modifiable) state.

Otherwise state is state. Don't worry about saying internal unless you need to differentiate from external. If you just say "state", it implies all state information unless you make it more specific through context.

Linux has a lot of internal state that it does not share with users. But you can see some external representation of state in tools like top or ps. Trying for a useful analogy.

Yes private members (variables, properties depends on the language) are part of state, as long as the object exists. They are internal, of course.

There's a whole other way to use the term external state, to represent the state of the environment in which the object or system exists. Some people use it this way. Other people just mean object state that is public.