Java Reference
In-Depth Information
Behind the scenes, the compiler converts
enum Coin { PENNY, ICKEL,
DIME, QUARTER }
into a class declaration that is similar to
Listing 3-66
.
Thefollowingrulesshowyouhowtointerpret
Enum<E extends Enum<E>>
in
the context of
Coin extends Enum<Coin>
:
• Anysubclass of
Enum
mustsupplyanactual typeargumentto
Enum
.Forex-
ample,
Coin
's header specifies
Enum<Coin>
.
• Theactualtypeargumentmustbeasubclassof
Enum
.Forexample,
Coin
isa
subclass of
Enum
.
• A subclass of
Enum
(such as
Coin
) must follow the idiom that it supplies its
own name (
Coin
) as an actual type argument.
Thethirdruleallows
Enum
todeclaremethods—
compareTo()
,
getDeclarin-
gClass()
, and
valueOf()
—whose parameter and/or return types are specified in
termsofthesubclass(
Coin
),andnotintermsof
Enum
.Therationalefordoingthisis
to avoid having to specify casts. For example, you do not need to cast
valueOf()
's
return value to
Coin
in
Coin penny = Enum.valueOf(Coin.class,
"PENNY");
.
Note
You cannot compile
Listing 3-66
because the compiler will not compile any
class that extends
Enum
. It will also complain about
super(name, ordinal);
.
EXERCISES
The following exercises are designed to test your understanding of nested types,
packages, static imports, exceptions, assertions, annotations, generics, and enums:
1. A 2D graphics package supports two-dimensional drawing and transform-
ations (rotation, scaling, translation, and so on). These transformations re-
quire a 3-by-3 matrix (a table). Declare a
G2D
class that encloses a
private Matrix
nonstaticmemberclass.Inadditiontodeclaringa
Mat-
rix(int rows, int cols)
constructor,
Matrix
declares a
void
dump()
methodthatoutputsthematrixvaluestostandardoutputinatab-
ularformat.Instantiate
Matrix
within
G2D
'snoargumentconstructor,and
initialize the
Matrix
instance to the
identity matrix
(a matrix where all
entriesare0exceptforthoseontheupper-lefttolower-rightdiagonal,which
are1.Theninvokethisinstance's
dump()
methodfromtheconstructor.In-
clude a
main()
method to test
G2D
.
