Java Generics wildcard extends final class -

why java doesn't throw warning when compiling testgenerics class, considering string class final , cannot extended?

import java.util.*;     public class testgenerics {          public void addstrings(list<? extends string> list) {           // code here         }     } } 

let's had method this:

public list<? extends t> filteroutnulls(list<t> input) { ... 

granted, not best signature in world, still legal. happen if passed list<string> method? according signature, returns list<? extends string>. if java disallowed type, it'd impossible use method list<string> (or @ least, it'd impossible use return value).

secondarily, extends syntax still useful in case, since list<string> , list<? extends string> have different restrictions -- specifically, can't add null literal list<? extends string>. i'll use ? extends signify collection read-only (since ts can pass in null), , ? super signify write-only (since can out ts object). isn't fool-proof (you can still call remove methods, pass in nulls, downcast, etc) serves gentle reminder of how collection meant used.