why l.add() method is not working in this context?

[sandeep sree]

class p1
{
public static void main(String a[])
{
List<? extends String> l=new ArrayList<String>();
l.add("abc");

}
};

[Christophe Verré]

"sandeep",
Welcome to the ranch. You may not be aware of the ranch Naming Policy. Please read it carefully and change your name accordingly (you need to set both first and last names). Thank you.

[Christophe Verré]

The parameter of add() is <? extends String> which means an unknown subtype of String is in there. String is not a great example. You should look at a simplier example like this :

class A {}
class B extends A{}
class C extends A{}
 
public class Bob {
public static void main(String[] args) {
List<? extends A> l=new ArrayList<B>();
doSomething(l);
}
 
public static void doSomething(List<? extends A> l) {
l.add(new B());
}
}

So, l is a List<? extends A>. But what can you really put in there ? In the doSomething method, you don't know what kind of list it really is. Could you call l.add(new C()) ? C extends A. But we declared the list as containing B objects. With this type of wildcards, you can get objects from a collection, but not add any.