Java Reference
In-Depth Information
11.4. Wildcard Capture
Wildcards represent an unknown type, but whenever a variable that has
a wildcard type is used, the compiler must treat it as having some spe-
cific type so that it can check for correct usage. This specific (but still un-
known) type is referred to as the
capture
of the wildcard. The place you
will most commonly come across the capture of a wildcard is in the error
messages the compiler produces when you use a parameterized type the
wrong way. For example, recall the incorrect attempt to add a
String
ob-
ject to a queue accessed through an unbounded wildcard reference:
SingleLinkQueue<?> strings =
new SingleLinkQueue<String>();
strings.add("Hello"); // INVALID: won't compile
The error message this produced from the compiler we used was:
add(capture of ?) in SingleLinkQueue<capture of ?> cannot be
applied to (java.lang.String)
This is telling us that when the wildcard reference
strings
is used, the
type of queue is
SingleLinkQueue<captureof ?>
, so the type of the para-
meter to
add
is also "
captureof ?
". Because
String
is not compatible with
"
captureof ?
" the call is not allowed. Even if the wildcard is bounded, as
in the earlier example with the queue of numbers ("
?extends Number
"
)
, the
type of the parameter to
add
is "
captureof ?extends Number
" which is still
not compatible with
Integer
, even though
Integer
is a subtype of
Number
.
If a wildcard is always represented by its capture, it would seem that
once you have a wildcard type you can only use it wherever a wildcard
type is expected. Indeed this is a basic rule that ensures the integrity of
the type system.
