Question

Although a static class has only one instance and can’t be instantiated, a class with a private constructor can’t be instantiated (as the constructor can’t be seen), so every time you call this class, this is the same one instance?

Factory classes always follow the last convention (instance class with private constructor). Why is this?

Thanks

Answer 1

There’s nothing stopping the class with the private constructor from having a public static method which returns instances of the class:

public class NoPublicConstructor
{
    private NoPublicConstructor()
    {
    }

    public static NoPublicConstructor NewInstance()
    {
        return new NoPublicConstructor();
    }
}

As you can see, the static method does not return the same one instance.

edit: One of the reasons factory classes do this is to be able to separate responsibility in future versions: while your code always calls the factory creation method, the author may move all the “guts” out of that class into a different one and your code won’t need to know the difference. Calling that class’ (public) constructor ties it to an extent to the original class implementation.