Generic method doubt

[Kedar Pethe]

Why doesn't the following code compile?

import java.util.*;
class A{}
class B extends A{}
class chp7_genericwildcardsmethod{
public <T extends A> void go(T t)
{
	List<T> l=new ArrayList<T>();
	l.add(t);
	System.out.println(l);
}
public static void main(String[] args)
{
	List<B> l=new ArrayList<B>();
	l.add(new B());
	new chp7_genericwildcardsmethod().go(l);
}
}

/*
Error-
chp7_genericwildcardsmethod.java:15: <T>go(T) in chp7_genericwildcardsmethod cannot be applied to (java.util.List<B>)
        new chp7_genericwildcardsmethod().go(l);
*/

What is wrong with <T extends A> ??

[James Boswell]

public <T extends A> void go(T t)

Look at your method signature above and then look at what you are attempting to pass to the go method:

List<B> l=new ArrayList<B>();
new chp7_genericwildcardsmethod().go(l);

Notice the type for variable l?

[Gaurangkumar Khalasi]

Try out with

new chp7_genericwildcardsmethod().go(new B());

instead of

List<B> l=new ArrayList<B>();
l.add(new B());
new chp7_genericwildcardsmethod().go(l);

And try to figure out your problem!!!

[Kedar Pethe]

Gaurangkumar Khalasi wrote:Try out with

new chp7_genericwildcardsmethod().go(new B());

instead of

List<B> l=new ArrayList<B>();
l.add(new B());
new chp7_genericwildcardsmethod().go(l);

And try to figure out your problem!!!

Got it! Thanks gaurang!!

[Kedar Pethe]

James Boswell wrote:

public <T extends A> void go(T t)

Look at your method signature above and then look at what you are attempting to pass to the go method:

List<B> l=new ArrayList<B>();
new chp7_genericwildcardsmethod().go(l);

Notice the type for variable l?

Understood.. i should have passed the instance variable of anything that extends A, instead!!

[James Boswell]

No, you are passing a List<T> but the method is expecting T.

[Joe Harry]

James Boswell wrote:No, you are passing a List<T> but the method is expecting T.

And that was exactly what the compiler told you. Have a look at the following page for a much detailed explanation on Generics. Worth the read!

http://www.angelikalanger.com/GenericsFAQ/JavaGenericsFAQ.html