It is not just functions that can be parameterized types themselves can be parameterized as well. Such types are sometimes referred to as container types because of the close association with collections and the fact that they contain one or more type parameters.
To declare a parameterized type, we again use the angle bracket syntax, this time on the right-hand side of the type name. For example, to declare a Sequence of an element T, we would write the following:
class Sequence<T>
Again, we can declare more than one type parameter:
class Dictionary<K, V>
Note
The most commonly used parameterized types are collections, and these are covered in more detail in Chapter 10, Collections.
When a type has been declared with a type parameter, we must "fill in" that type when we instantiate it by replacing the parameters with concrete or proper types. So, to create an instance of our Sequence class for Boolean, we would write the following:
val seq...