Covariance and Contravariance in Java

extends

The wildcard declaration of List 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

1. 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.

2. 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 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 .

The wildcard declaration of List 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

1. 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