Understanding generic upper bound wildcarts in java

Question

The upper bound wildcard in the method below means we can pass in a list that contains elements of type Object or any List containing elements of type which is subclass Object, I am not understanding why the following is not compiling, because string is subclass of Object:

public static void addSound(List<? extends Object> list) {
list.add("quack"); //does not compile
}

Answer 1

Upper bounded generics are immutable. The extended type can be anything that extends object, it could be a list of Ducks. and then you see why it can't work. (list.add(new Duck()) is not the same as "quack")

Lower bound work though

   public static void addSound(List<? super String> list) {
    list.add("quack"); //does compile
   }