List> vs List<? extends Map>

Question

Is there any difference between

List>

and

List>
<         


        

Answer 1

As you mentioned, there could be two below versions of defining a List:

1. List>
2. List

2 is very open. It can hold any object type. This may not be useful in case you want to have a map of a given type. In case someone accidentally puts a different type of map, for example, Map. Your consumer method might break.

In order to ensure that List can hold objects of a given type, Java generics introduced ? extends. So in #1, the List can hold any object which is derived from Map type. Adding any other type of data would throw an exception.