Java Reference
In-Depth Information
Recall that the method
compareTo
is required to be a member of any class that
implements the
Comparable
interface. The
Comparable
interface is a standard Java
interface that was discussed in Chapter 13 . Recall that the
Comparable
interface has
only the following method heading that must be implemented:
compareTo
public int
compareTo(Object other);
When defining a class that implements the
Comparable
interface, the programmer is
expected to define
compareTo
so that it returns
a negative number if the calling object “comes before” the parameter
other
,
a zero if the calling object “equals” the parameter
other
,
and a positive number if the calling object “comes after” the parameter
other
.
This all works fine, except for one problem: This makes sense only if the type
plugged in for the type parameter
T
satisfies the
Comparable
interface, but Java allows
you to plug in any type for the type parameter
T
.
You can have Java enforce this restriction on the possible types that can be plugged
in for
T
. To ensure that only classes that implement the
Comparable
interface are
plugged in for
T
, begin the class definition as follows:
public class
Pair<T
extends
Comparable>
The part
extends
Comparable
is called a
bound
on the type parameter
T
. If you
attempt to plug in a type for
T
that does not implement the
Comparable
interface, you
will get a compiler error message. Note that you use the keyword
extends
, not the
keyword
implements
as you would naturally expect.
Note that the bound
extends
Comparable
is not just an optional little nicety. If
you omit it, you will get an error message from the compiler saying it does not know
about the method
compareTo
.
This version of the generic class
Pair
with the method
max
is summarized in
Display 14.10 . On the accompanying website, this version of
Pair
is in a subdirectory
named
Bounded Pair
.
A bound on a type may be a class name (rather than an interface name) in which
case only descendent classes of the bounding class may be plugged in for the type
parameters. For example, the following says that only descendent classes of the class
Employee
may be plugged in for
T
, where
Employee
is some class:
extends
bound
public class
SameGenericClass<T
extends
Employee>
