Java Reference
In-Depth Information
The unknown type sometimes confuses developers—provoking questions like,
“Why wouldn't you just use
Object
instead of the unknown type?” However, as
we've seen, the need to have subtyping relationships between generic types essen‐
tially requires us to have a notion of the unknown type.
Bounded wildcards
In fact, Java's wildcard types extend beyond just the unknown type, with the concept
of
bounded wildcards
, also called
type parameter constraints
. This is the ability to
restrict the types that can be used as the value of a type parameter.
They are used to describe the inheritance hierarchy of a mostly unknown type—
effectively making statements like, for example, “I don't know anything about this
type, except that it must implement
List
.” This would be written as
? extends
List
in the type parameter. This provides a useful lifeline to the programmer—
instead of being restricted to the totally unknown type, she knows that at least the
capabilities of the type bound are available.
The
extends
keyword is always used, regardless of whether
the constraining type is a class or interface type.
This is an example of a concept called
type variance
, which is the general theory of
how inheritance between container types relates to the inheritance of their payload
types.
Type covariance
This means that the container types have the same relationship to each other as
the payload types do. This is expressed using the
extends
keyword.
Type contravariance
This means that the container types have the inverse relationship to each other
as the payload types. This is expressed using the
super
keyword.
These principles tend to appear when discussing container types that act as produc‐
ers or consumers of types. For example, if
Cat
extends
Pet
, then
List<Cat>
is a sub‐
type of
List<? extends Pet>
. The
List
is acting as a
producer
of
Cat
objects and
the appropriate keyword is
extends
.
For a container type that is acting purely as a
consumer
of instances of a type, we
would use the
super
keyword.
