Java Generic List<List<? extends Number>>

Question

How come in java we cannot do:

List<List<? extends Number>> aList = new ArrayList<List<Number>>();

Even though this is OK:

List<? extends Number> aList = new ArrayList<Number>();

Compiler error message is:

Type mismatch: cannot convert from ArrayList<List<Number>> to List<List<? extends Number>>

Answer 1

In Java, if Car is a derived class of Vehicle, then we can treat all Cars as Vehicles; a Car is a Vehicle. However, a List of Cars is not also a List of Vehicles. We say that List<Car> is not covariant with List<Vehicle>.

Java requires you to explicitly tell it when you would like to use covariance and contravariance with wildcards, represented by the ? token. Take a look at where your problem happens:

List<List<? extends Number>> l = new ArrayList<List<Number>>();
//        ----------------                          ------
// 
// "? extends Number" matched by "Number". Success!

The inner List<? extends Number> works because Number does indeed extend Number, so it matches "? extends Number". So far, so good. What's next?

List<List<? extends Number>> l = new ArrayList<List<Number>>();
//   ----------------------                    ------------
// 
// "List<? extends Number>" not matched by "List<Number>". These are
//   different types and covariance is not specified with a wildcard.
//   Failure.

However, the combined inner type parameter List<? extends Number> is not matched by List<Number>; the types must be exactly identical. Another wildcard will tell Java that this combined type should also be covariant:

List<? extends List<? extends Number>> l = new ArrayList<List<Number>>();