How to enforce factory instantiation?

Question

Lets say I want all subclasses of a certain class to be created through a factory. I want to disallow instantiation of subclasses through direct instantiation using "new" .
How can I enforce that? I would like the baseclass to enforce this, so that it should not be possible to define subclasses with a public constructor. If the baseclass has a protected constructor, it seems that subclasses still can have a public one?

Answer 1

One solution to your problem is to only declare an interface for the types that the factory will create and put the implementation classes as inner classes in the factory. That way, noone will ever be able to construct the instance of them except the factory:
public class Factory
    private Factory(){...}
    class ProductImpl_1 implements Product
    class ProductImpl_2 implements Product
    public Product getProduct(String type)
       // return the correct implementation based on the type
public interface Product
}