Java Reference
In-Depth Information
Unfortunately,
NumericFns
will not compile as written because both methods will gen-
erate compile-time errors. First, examine the
reciprocal( )
method, which attempts to re-
turn the reciprocal of
num
. To do this, it must divide 1 by the value of
num
. The value
of
num
is obtained by calling
doubleValue( )
, which obtains the
double
version of the
numeric object stored in
num
. Because all numeric classes, such as
Integer
and
Double
,
are subclasses of
Number
, and
Number
defines the
doubleValue( )
method, this method
is available to all numeric wrapper classes. The trouble is that the compiler has no way
to know that you are intending to create
NumericFns
objects using only numeric types.
Thus, when you try to compile
NumericFns
, an error is reported that indicates that the
doubleValue( )
method is unknown. The same type of error occurs twice in
fraction( )
,
which needs to call both
doubleValue( )
and
intValue( )
. Both calls result in error mes-
sages stating that these methods are unknown. To solve this problem, you need some way
to tell the compiler that you intend to pass only numeric types to
T
. Furthermore, you need
some way to
ensure
that
only
numeric types are actually passed.
To handle such situations, Java provides
bounded types
. When specifying a type para-
meter, you can create an upper bound that declares the superclass from which all type ar-
guments must be derived. This is accomplished through the use of an
extends
clause when
specifying the type parameter, as shown here:
<
T
extends
superclass
>
This specifies that
T
can be replaced only by
superclass
, or subclasses of
superclass
. Thus,
superclass
defines an inclusive, upper limit.
You can use an upper bound to fix the
NumericFns
class shown earlier by specifying
Number
as an upper bound, as shown here:
