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

I'd recommend you to look in documentation of generic wildcards especially guidelines for wildcard use

Frankly speaking your method #doesntCompile

static void doesntCompile(Map> map) {}

and call like

doesntCompile(new HashMap>());

Is fundamentally incorrect

Let's add legal implementation:

    static void doesntCompile(Map> map) {
        List list = new ArrayList<>();
        list.add(0.);
        map.put(0, list);
    }

It is really fine, because Double extends Number, so put List is absolutely fine as well as List, right?

However, do you still suppose it's legal to pass here new HashMap>() from your example?

Compiler does not think so, and is doing his (its?) best to avoid such situations.

Try to do the same implementation with method #compile and compiler will obviously does not allow you to put a list of doubles into map.

    static  void compiles(Map> map) {
        List list = new ArrayList<>();
        list.add(10.);
        map.put(10, list); // does not compile
    }

Basically you can put nothing but List that's why it's safe to call that method with new HashMap>() or new HashMap>() or new HashMap>() or new HashMap>().

So in a nutshell, you are trying to cheat with compiler and it fairly defends against such cheating.

NB: answer posted by Maurice Perry is absolutely correct. I'm just not sure it's clear enough, so tried (really hope I managed to) to add more extensive post.