Java Reference
In-Depth Information
own use. The advantage of enumerations is that
final
variables don't have to be
pressed into service for a job for which they are not ideally suited.
Two Important Restrictions
There are two restrictions that apply to enumerations. First, an enumeration can't inherit
another class. Second, an
enum
cannot be a superclass. This means that an
enum
can't be
extended. Otherwise,
enum
acts much like any other class type. The key is to remember
that each of the enumeration constants is an object of the class in which it is defined.
Enumerations Inherit Enum
Although you can't inherit a superclass when declaring an
enum
, all enumerations auto-
matically inherit one:
java.lang.Enum
. This class defines several methods that are avail-
able for use by all enumerations. Most often, you won't need to use these methods, but
there are two that you may occasionally employ:
ordinal( )
and
compareTo( )
.
The
ordinal( )
method obtains a value that indicates an enumeration constant's position
in the list of constants. This is called its
ordinal value
. The
ordinal( )
method is shown
here:
final int ordinal( )
It returns the ordinal value of the invoking constant. Ordinal values begin at zero. Thus,
in the
Transport
enumeration,
CAR
has an ordinal value of zero,
TRUCK
has an ordinal
value of 1,
AIRPLANE
has an ordinal value of 2, and so on.
You can compare the ordinal value of two constants of the same enumeration by using
the
compareTo( )
method. It has this general form:
final int compareTo(
enum-type e
)
Here,
enum-type
is the type of the enumeration and
e
is the constant being compared to the
invoking constant. Remember, both the invoking constant and
e
must be of the same enu-
meration. If the invoking constant has an ordinal value less than
e
's, then
compareTo( )
returns a negative value. If the two ordinal values are the same, then zero is returned. If the
invoking constant has an ordinal value greater than
e
's, then a positive value is returned.
The following program demonstrates
ordinal( )
and
compareTo( )
:
