Java Reference
In-Depth Information
13
/** A method for displaying an object */
displayObject(Object object)
14
public static void
{
15
if
(
object
instanceof
CircleFromSimpleGeometricObject
) {
16 System.out.println(
"The circle area is "
+
17 ((CircleFromSimpleGeometricObject)object).getArea());
18 System.out.println(
"The circle diameter is "
+
19 ((CircleFromSimpleGeometricObject)object).getDiameter());
20 }
21
else if
(
22
polymorphic call
object
instanceof
RectangleFromSimpleGeometricObject
) {
23 System.out.println(
"The rectangle area is "
+
24 ((RectangleFromSimpleGeometricObject)object).getArea());
25 }
26 }
27 }
polymorphic call
The circle area is 3.141592653589793
The circle diameter is 2.0
The rectangle area is 1.0
The
displayObject(Object object)
method is an example of generic programming. It
can be invoked by passing any instance of
Object
.
The program uses implicit casting to assign a
Circle
object to
object1
and a
Rectangle
object to
object2
(lines 5-6), then invokes the
displayObject
method to dis-
play the information on these objects (lines 9-10).
In the
displayObject
method (lines 14-26), explicit casting is used to cast the object to
Circle
if the object is an instance of
Circle
, and the methods
getArea
and
getDiameter
are used to display the area and diameter of the circle.
Casting can be done only when the source object is an instance of the target class. The pro-
gram uses the
instanceof
operator to ensure that the source object is an instance of the tar-
get class before performing a casting (line 15).
Explicit casting to
Circle
(lines 17, 19) and to
Rectangle
(line 24) is necessary because
the
getArea
and
getDiameter
methods are not available in the
Object
class.
Caution
The object member access operator (
.
) precedes the casting operator. Use parentheses
to ensure that casting is done before the
.
operator, as in
precedes casting
((Circle)object).getArea());
Casting a primitive type value is different from casting an object reference. Casting a primi-
tive type value returns a new value. For example:
int
age =
45
;
byte
newAge = (
int
)age;
// A new value is assigned to newAge
However, casting an object reference does not create a new object. For example:
Object o =
new
Circle();
Circle c = (Circle)o;
// No new object is created
Now reference variables
o
and
c
point to the same object.
