Java Generic List>

Question

How come in java we cannot do:

List> aList = new ArrayList>();

Even though thi

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 is not covariant with List.

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

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

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

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

List> l = new ArrayList>();