generics - List<interfaceI> vs List<? extends InterfaceI> in java -

arraylist list<interfacei> , list<? extends interfacei> both have objects of classes implementing interfacei. when should used?

suppose foo , bar 2 classes implementing interfacei.

the second 1 (list<? extends interfacei>) doesn't allow adding list (except null), since type list contains unknown: list<foo> or list<bar>: don't know.

so use notation method argument, when want method read elements of list passed argument, , want caller able call method list<interfacei>, list<foo> or list<bar>. using list<interfacei> argument accept lists of type list<interfacei>.

let's take concrete example: want compute maximum double value of list of numbers. such method doesn't need add or set list. iterating on elements, each number , compute maximum. signature be

 public double max(list<number> list); 

but then, won't able do

list<integer> ints = new arraylist<>(); max(ints); 

the way call method do

list<number> ints = new arraylist<>(); max(ints); 

whereas if declare method as

public double max(list<? extends number> list); 

then can do

list<integer> ints = new arraylist<>(); list<long> longs = new arraylist<>(); max(ints); max(longs)