Generics:Issues in Adding SubClass Objects in a Collection

Question

So I've been reading through the Generics tutorial offered by Oracle here: http://docs.oracle.com/javase/tutorial/java/generics/

And I've tried running my own example to make sure I understand how to use Generics. I have the following code:

import java.util.*;

public class Generics {
    class NaturalNumber {
        private int i;

        public NaturalNumber(int i) { this.i = i; }
    }
    class EvenNumber extends NaturalNumber {
        public EvenNumber(int i) { 
            super(i);
        }
    }    
    public static void main(String[] args) {
        Collection<? extends NaturalNumber> c = new ArrayList<>();
        c.add(new EvenNumber(2)); //this line produces a compile time error     
    }
}

My goal is to be able to add any object which is a subtype of NaturalNumber to the Collection c. I'm not sure why this doesn't work and reading through Oracle's tutorial hasn't enlightened me either.

Answer 1

When you have ? extends NaturalNumber, the parameter could be some other subclass of NaturalNumber that is in no way related to EvenNumber. For instance,

Collection<? extends NaturalNumber> c = new ArrayList<OtherNaturalNumber>();

is valid if OtherNaturalNumber extends NaturalNumber.

Consequently, you are not able to add an EvenNumber instance to the list. You can just use this declaration:

Collection<NaturalNumber> c = new ArrayList<>();

which will allow you to add any NaturalNumber instance (including an EvenNumber).

On another note, you probably meant to make those nested classes static (or don't nest them at all).