Java Reference
In-Depth Information
«interface»
Edible
Animal
+howToEat(): String
+sound(): String
Fruit
Chicken
Tiger
Orange
Apple
F
IGURE
15.4
Edible
is a supertype for
Chicken
and
Fruit
.
Animal
is a supertype for
Chicken
and
Tiger
.
Fruit
is a supertype for
Orange
and
Apple
.
signature and return type. The
Chicken
class implements the
howToEat
method (lines
22-24).
Chicken
also extends
Animal
to implement the
sound
method (lines 27-29).
The
Fruit
class implements
Edible
. Since it does not implement the
howToEat
method,
Fruit
must be denoted as
abstract
(line 39). The concrete subclasses of
Fruit
must
implement the
howToEat
method. The
Apple
and
Orange
classes implement the
howToEat
method (lines 45, 52).
The
main
method creates an array with three objects for
Tiger
,
Chicken
, and
Apple
(line 3), and invokes the
howToEat
method if the element is edible (line 6) and the
sound
method if the element is an animal (line 9).
In essence, the
Edible
interface defines common behavior for edible objects. All edible
objects have the
howToEat
method.
common behavior
Note
Since all data fields are
public static final
and all methods are
public
abstract
in an interface, Java allows these modifiers to be omitted. Therefore the fol-
lowing interface definitions are equivalent:
omitting modifiers
public interface
T {
final int
K =
1
;
public interface
T {
int
K =
1
;
public static
Equivalent
void
p();
void
p();
public abstract
}
}
Tip
A constant defined in an interface can be accessed using the syntax
InterfaceName.CONSTANT_NAME
(e.g.,
T.K
). It is a good practice to define com-
mon constants that are shared by many classes in an interface. For example, the con-
stants
LEFT
,
CENTER
,
RIGHT
,
LEADING
,
TRAILING
,
TOP
, and
BOTTOM
used in
AbstractButton
are also used in many other Swing components. These constants are
centrally defined in the
javax.swing.SwingConstants
interface. All Swing GUI
components implement
SwingConstants
. You can reference the constants through
SwingConstants
or a GUI component. For example,
SwingConstants.CENTER
is
the same as
JButton.CENTER
.
accessing constants
SwingConstants
✓
✓
Check
Point
15.13
Suppose
A
is an interface. Can you create an instance using
new A()
?
