<? super E> and <? extends E> for List

Question

Having the following simple class structure:

class A {
}

class B extends A {
}

class C extends B {
}

I\'m creating an ArrayList to keep o

Answer 1

List allows you to use Lists of any supertype of B, i.e. list5 = new ArrayList<A>(); or list5 = new ArrayList<Object>();

You can safely add B (and subtypes) to every list that use supertypes of B, but you can not add any supertype of B. Imagine this:

public void myAdd(List lst) {
  lst.add(new Object()) //this is a supertype of B (compile time error)
}
...
ArrayList list = new ArrayList();
myAdd(list); //tries to add Object to a list of type A