Problem in Generics

[Ishan Pandya]

import java.util.*;

public class Organic<E>
{
	void react(E e) {}
	static void main(String []args)
	{
		Organic<? extends Organic> compound= new Aliphatic<Organic>();
		//Organic<? super Aliphatic> compound= new Aliphatic<Organic>();
		compound.react(new Organic());
		compound.react(new Aliphatic());
	}
}
class Aliphatic<F> extends Organic<F>{}
class Hexane<G> extends Aliphatic<G>{}

getting an error like:

javac Organic.java
Organic.java:10: react(capture#591 of ? extends Organic) in Organic<capture#591
of ? extends Organic> cannot be applied to (Organic)
compound.react(new Organic());
^
Organic.java:11: react(capture#105 of ? extends Organic) in Organic<capture#105
of ? extends Organic> cannot be applied to (Aliphatic)
compound.react(new Aliphatic());
^
2 errors

why is it so that <? extends Organic> is not able to take Organic or its subclass Aliphatic??

[Paul Clapham]

The declaration

Organic<? extends Organic> compound

means that the type of the variable will at run time be Organic<X> for some class X which extends Organic. At compile time the value of X is unknown.

For example, X might be Aliphatic at run time, and so the variable would be an Organic<Aliphatic> variable. And therefore the react(E) method would require a parameter which can be assigned to Aliphatic.

Clearly an Organic object can't be assigned to Aliphatic, since that would be down-casting. That explains why line 10 has the error message. As for line 11, well, for some reason you declared Aliphatic to be a generic object. So the Aliphatic object you have in line 11 is an Aliphatic<Object> object, which can't be assigned to Aliphatic<Aliphatic>. I would change line 14 to read

class Aliphatic extends Organic<Aliphatic>{}  

At least that would make line 11's compiler error go away. Whether it's what your design requires, I have no way of telling.

[Ishan Pandya]

Mr. Paul. i really thank for your great reply.
but seriously i am not getting what exactly you said and what does this program say.
Like how can be the generic type can be sometime "<Alpha>" and sometimes "<Object>" at runtime while declaring the Orgainc object in the main method..?? What does the "type erasure" do after the compilation?
can you please give me the detail reason for that..?