Covariance and Contravariance in Java

extends
The wildcard declaration of  List extends Number> foo3  means that any of these are legal assignments:

List extends Number> foo3 = new ArrayList<Number>();  // Number "extends" Number (in this context)
List extends Number> foo3 = new ArrayList<Integer>(); // Integer extends Number
List extends Number> foo3 = new ArrayList<Double>();  // Double extends Number

Reading - Given the above possible assignments, what type of object are you guarenteed to read from  List foo3 :
You can read a  Number  because any of the lists that could be assigned to  foo3  contain a  Number  or a subclass of  Number .
You can't read an  Integer  because  foo3  could be pointing at a  List .
You can't read a  Double  because  foo3  could be pointing at a  List .

Writing - Given the above possible assignments, what type of object could you add to  List foo3  that would be legal for all the above possible  ArrayList  assignments:
You can't add an  Integer  because  foo3  could be pointing at a  List .
You can't add a  Double  because  foo3  could be pointing at a  List .
You can't add a  Number  because  foo3  could be pointing at a  List .

You can't add any object to  List extends T>  because you can't guarantee what kind of  List  it is really pointing to, so you can't guarantee that the object is allowed in that  List . The only "guarantee" is that you can only read from it and you'll get a  T  or subclass of  T . super
Now consider  List super T > .
The wildcard declaration of  List super Integer> foo3  means that any of these are legal assignments:

List super Integer> foo3 = new ArrayList<Integer>();  // Integer is a "superclass" of Integer (in this context)
List super Integer> foo3 = new ArrayList<Number>();   // Number is a superclass of Integer
List super Integer> foo3 = new ArrayList<Object>();   // Object is a superclass of Integer

Reading - Given the above possible assignments, what type of object are you guaranteed to receive when you read from  List foo3 :
You aren't guaranteed an  Integer  because  foo3  could be pointing at a  List  or  List