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

In the call:

compiles(new HashMap>());

T is matched to Integer, so the type of the argument is a Map>. It's not the case for the method doesntCompile: the type of the argument stays Map> whatever the actual argument in the call; and that is not assignable from HashMap>.

UPDATE

In the doesntCompile method, nothing prevents you to do something like this:

static void doesntCompile(Map> map) {
    map.put(1, new ArrayList());
}

So obviously, it cannot accept a HashMap> as the argument.