Java Generics putting on Map<String, ? extends List<String>>

Question

Is there a way to make the following implementation in a type safe manner?

public void myMethod( Map<String, ? extends List<String>> map )
{
   map.put("foo", Collections.singletonList("bar");
}

The above implementation doesn't work. It requires a Map<String, ? super List<String>> to compile the method map.put() correctly. But myMethod won't accept any subtype of List this way. So, I have to use Map<String, ? extends List<String>> instead. How can I solve this problem in a type safe manner?

Solution 1

public void myMethod( Map<String, List<String>> map ) {
    map.put("foo", Collections.singletonList("bar") );
}

You can't put a List (the return type of Collections.singletonList() into a Map of ? extends List since the actual type could be any implementation of List. For example, it is not safe to put a generic List into a Map<String,LinkedList> since the List might not be a LinkedList. However, we can easily put a LinkedList into a Map of <String,List>.

I think you were over thinking your generics. You do not have to use ? extends for a non-generic class. For example, List<Object> will hold any Object, the ? extends is not needed to add an object. A List<List<Foo>> will only take List<Foo> objects and not List<FooSubclass> objects [Generic classes don't inherit based on their parameters]. This is where ? extends comes into play. To add both List<Foo> and List<FooSubclass> to a List, the type must be List<List<? extends Foo>>.

Solution 2

Wouldn't Map<String, List<String> work? Is there some particular reason you have to have a wildcard there at all?

Comments

[palantus]

+1 for a good explanation (now that your generics are visible :P).

[KitsuneYMG]

Oops. I keep thinking that since SOF filters text anyway that it fixes the angled bracket stuff. Oh well...