What is the difference between <? extends Base> and ?

Question

In this example:

import java.util.*;

public class Example {
    static void doesntCompile(Map> map) {}
    static &l         


        

Answer 1

Simplied example of demonstration. Same example can be visualize like below.

static void demo(List> lst) {} // doesn't work
static void demo(List> lst) {} // works
demo(new ArrayList()); // works
demo(new ArrayList()); // works for subtype too

public static class Pair {}
public static class SubPair extends Pair {}

List> is a multi-level wildcards type whereas List is a standard wildcard type .

Valid concrete instantiations of the wild card type List include Number and any subtypes of Number whereas in case of List> which is a type argument of type argument and itself has a concrete instantiation of the generic type.

Generics are invariant so Pair wild card type can only accept Pair>. Inner type ? extends Number is already covariant. You have to make the enclosing type as covariant to allow covariance.