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But we find that this code won't compile. The problem is that, as
defined,
processValues
will only accept an instance of a class that imple-
ments
Lookup<Object>
, and
IntegerLookup
does not do that. Even though
Integer
is a subtype of
Object
,
Lookup<Integer>
is
not
a subtype of
Look-
up<Object>
and therefore an instance of the former can not be used
where an instance of the latter is required. Is there a way to declare that
we want to deal with an instance that implements
Lookup
of anything?
Yeswe use what is called a
wildcard
to specify the type parameter:
void processValues(String[] names, Lookup<?> table) {
for (int i = 0; i < names.length; i++) {
Object value = table.find(names[i]);
if (value != null)
processValue(names[i], value);
}
}
The wildcard is written as
?
and is read as "of unspecified type", or just
as "of some type":
table
is an instance of a class that implements a
lookup of some type. We don't know what type, and in this example we
don't care. The only thing we know about the type we can lookup is that
it must at least be an
Object
, so we can invoke
table.find
and store the
return value as an
Object
. Now we can use
IntegerLookup
with
processVal-
ues
without a problem.
Suppose in our application we knew that we would always be looking up
Integer
or
Long
or
Double
, etc.all the subclasses of the
Number
class. We
know, from the previous example, that we can't define
processValues
in
terms of
Lookup<Number>
, but there is a way to limit the types that the
wildcard can represent:
void processValues(String[] names,
Lookup<? extends Number> table) {
for (int i = 0; i < names.length; i++) {
Number value = table.find(names[i]);
if (value != null)
