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 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.

Accepted Answer

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 c = new ArrayList(); ``` 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 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).

Generics:Issues in Adding SubClass Objects in a Collection

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