Question

Ok, the question might not be crystal clear. Let me give some details:

Let’s say I have an Shoe (CShoe) object factory class called CFactory.
CFactory is a singleton class that creates and stores all instanciated shoes using a simple hashmap. It is then accessed through static methods to use the created objects.

Is there a way to force CShoe’s constructor so that it can only called by the factory? (in other words, ensure that the creation of shoes can only be done by the shoes factory singleton class and not by other classes)

Answer 1

You could make Shoe an inner class of ShoeFactory:

public class ShoeFactory {

    public static class Shoe {
        private String name;

        private Shoe() {
        }

        private Shoe(String name) {
            this.name = name;
        }
    }

    public static Shoe createShoe(String shoeName) {
        return new Shoe(shoeName);
    }
}

I think this pretty much covers all the cases except …. reflection:

public class SmellyShoe {

    public static void main(String[] args) {
        try {
            java.lang.reflect.Constructor c = Shoe.class.getDeclaredConstructors()[0];
            c.setAccessible(true);
            Shoe smelly = (Shoe)c.newInstance(null);
            // grr
        } catch (InstantiationException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (IllegalArgumentException e) {
            e.printStackTrace();
        } catch (InvocationTargetException e) {
            e.printStackTrace();
        }
    }
}