Java Reference
In-Depth Information
These properties of wildcards make them not only useful but
essential
for effective use of generic types. Whenever a method takes a paramet-
er that is a parameterized type or returns a parameterized type, that
parameterized type should almost always be expressed with a wildcard
of some form. Typically, parameters use lower-bounded wildcards, while
return values use upper bounded wildcardsthough if there is a constraint
between a parameter type and the return type, it may not be possible
to use a wildcard. Even a type bound that is a parameterized type will
often be expressed with a wildcard. Recall the example of the
SortedCol-
lection<E>
interface, in which we constrained the type parameter
E
such
that
E
extends
Comparable<E>
. This seemed reasonable: to sort the collec-
tion we need the elements to be comparable to each other. In fact, this
constraint is overly tightfor two elements of a class
T
to be comparable
to each other, it is not necessary that
T
extend
Comparable<T>
but only
that
T
extend
Comparable<S>
where
S
is
T
or any supertype of
T
. For ex-
ample, if class
Value
implements
Comparable<Object>
then it can still cor-
rectly compare two
Value
objectsit just happens to be able to do more
than that. Consequently,
SortedCollection
should be declared as follows:
interface SortedCollection<E extends Comparable<? super E>>
{
// ... sorted collection methods ...
}
