Java Reference
In-Depth Information
The main class contains the
main
method (line 3) that creates three objects. As in creating an
array, the
new
operator is used to create an object from the constructor.
new SimpleCircle()
creates an object with radius
1
(line 5),
new SimpleCircle(25)
creates an object with radius
25
(line 10), and
new SimpleCircle(125)
creates an object with radius
125
(line 15).
These three objects (referenced by
circle1
,
circle2
, and
circle3
) have different data
but the same methods. Therefore, you can compute their respective areas by using the
getArea()
method. The data fields can be accessed via the reference of the object using
circle1.radius
,
circle2.radius
, and
circle3.radius
, respectively. The object can
invoke its method via the reference of the object using
circle1.getArea()
,
circle2.getArea()
, and
circle3.getArea()
, respectively.
These three objects are independent. The radius of
circle2
is changed to
100
in line 20.
The object's new radius and area are displayed in lines 21-22.
There are many ways to write Java programs. For instance, you can combine the two
classes in the example into one, as shown in Listing 8.2.
L
ISTING
8.2
SimpleCircle.java
1
public class
SimpleCircle
{
2
/** Main method */
3
4
public static void
main(String[] args) {
main
method
// Create a circle with radius 1
5
6 System.out.println(
"The area of the circle of radius "
7 +
SimpleCircle circle1 =
new
SimpleCircle();
circle1.radius
+
" is "
+
circle1.getArea()
);
8
9
// Create a circle with radius 25
10 SimpleCircle circle2 =
new
SimpleCircle(
25
);
11 System.out.println(
"The area of the circle of radius "
12 + circle2.radius +
" is "
+ circle2.getArea());
13
14
// Create a circle with radius 125
15 SimpleCircle circle3 =
new
SimpleCircle(
125
);
16 System.out.println(
"The area of the circle of radius "
17 + circle3.radius +
" is "
+ circle3.getArea());
18
19
// Modify circle radius
20 circle2.radius =
100
;
21 System.out.println(
"The area of the circle of radius "
22 + circle2.radius +
" is "
+ circle2.getArea());
23 }
24
25
double
radius;
data field
26
27
/** Construct a circle with radius 1 */
28
SimpleCircle()
{
no-arg constructor
29 radius =
1
;
30 }
31
32
/** Construct a circle with a specified radius */
33
SimpleCircle(
double
newRadius)
{
second constructor
34 radius = newRadius;
35 }
36
37
/** Return the area of this circle */
38
double
getArea()
{
method
39
return
radius * radius * Math.PI;
40 }
41
