Java Reference
In-Depth Information
youcanonlypassanargumentof List<Object> type,whichlimitstheusefulnessof
this method.
However, generics offer a solution: the wildcard argument ( ? ), which stands for
any type. By changing outputList() 's parameter type from List<Object> to
List<?> , you can call outputList() with a List of String , a List of Em-
ployee , and so on.
Generic Methods
Suppose you need a method to copy a List of any kind of object to another List .
Although you might consider coding a void copyList(List<Object> src,
List<Object> dest) method,thismethodwouldhavelimitedusefulnessbecause
itcouldonlycopylistswhoseelementtypeis Object .Youcouldn'tcopya List<Em-
ployee> , for example.
If you want to pass source and destination lists whose elements are of arbitrary
type (but their element types agree), you need to specify the wildcard character as
a placeholder for that type. For example, you might consider writing the following
copyList() classmethodthatacceptscollectionsofarbitrary-typedobjectsasitsar-
static void copyList(List<?> src, List<?> dest)
for (int i = 0; i < src.size(); i++)
puts the following error message when it encounters dest.add(src.get(i)); : error: no suitable method found for
method List.add(int,CAP#1) is not applicable
(actual and formal argument lists differ in length)
method List.add(CAP#1) is not applicable
(actual argument Object cannot be converted to CAP#1
by method invocation conversion)
where CAP#1 is a fresh type-variable:
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