When to use private members - when to pass as function/method parameter

Question

I often found myself when writing a new class that inside a class I pass a parameter inside of many private methods. On the other side I also create sometime private members and just use them in one method.

So my question is "After which rules do you create a private member and when you don't and pass the variable from private method to private method"?

Can you give me some simple tips or hints, so that the design will get better?

Answer 1

If a parameter is part of the 'state' of the class, then use a private member. If on the other hand the parameter is just a temporary object, just pass it as a parameter.

Answer 2

I always pass variables into fuctions. You never know when you will need to use that function in a different context, and the parameters will change.

Answer 3

Creating private data just to avoid having to pass a round data as function arguments seems like global data in disguise to me. Data should be part of the class only if it is part of the class' state (i.e. it needs to be kept between invocations of the class' member function).

Answer 4

Classes are supposed to be self contained, self sustained objects. If the data being used is part of the state of the class, you should use an internal variable. If the data is only going to be used temporarily for a calculation or something, pass it in as a parameter.

Additionally, you should be using private class members, using accessors and mutators to access and modify them. Don't allow external code to get it's hands on the internals of your class.

Answer 5

Do what the Python community does.

Tip 1: Don't waste time on privacy.

Tip 2: Privacy is often more easily handled by delegation to a Strategy plug-in (or "injection") than by complex private attributes and methods.

Tip 3: Privacy makes unit testing more complex than it needs to be. It's easier to simply make things public.

Tip 4: Only use private when your code will be managed by lawyers who need to protect intellectual property by concealing an algorithm. This does happen, and when it does, it will be clear that you must declare things as private to prevent disclosure.

Answer 6

I agree with "If a parameter is part of the 'state' of the class, then use a private member", but that immediately brings a new question: when a parameter is consider part of the state of the class?

Just to compliment what has been said, I would add:

A parameter is part of the state of the class if after the call to the method we need to keep the value.

Example 1 The parameter is not part of the state of the class.

The class Car doesn't have any reason to remember what key was used to start the car.

Class Car{
    Lock lock;        

    [...]

    public boolean startCar (Key keyUsedToStart){
        return (canStartCarWithThisKey (keyUsedToStart));
    }

    private boolean canStartCarWithThisKey (Key keyUsedToStart){
        return (lock.canStartCarWithThisKey(keyUsedToStart));
    }

    [...]
}

Example 2 the parameter is part of the state of the class.

The class Car needs to know how much fuel it has.

Class Car{
    Fuel fuel;        

    [...]

    public void putSomeFuel (Fuel fuelToAdd){
        this.fuel.add(fuelToAdd);
    }

    [...]
}

Answer 7

Should this entity has the same lifetime as the object?

Of course, it's not a strict crterion which gives right answer in all cases, but it works good quite often and it's very simple...