Java Reference
In-Depth Information
UML Notation:
underline: static variables or methods
instantiate
circle1: Circle
Memory
After two
Circle
Objects were created,
numberOfObjects
is 2.
radius = 1
numberOfObjects = 2
1
radius
Circle
radius: double
numberOfObjects: int
numberOfObjects
2
getNumberOfObjects(): int
getArea(): double
instantiate
circle2: Circle
radius = 5
numberOfObjects = 2
radius
5
F
IGURE
8.13
Instance variables belong to the instances and have memory storage independent of one another. Static
variables are shared by all the instances of the same class.
To declare a static variable or define a static method, put the modifier
static
in the vari-
able or method declaration. The static variable
numberOfObjects
and the static method
getNumberOfObjects()
can be declared as follows:
static
int
numberOfObjects;
declare static variable
static
int
getNumberObjects() {
return
numberOfObjects;
define static method
}
Constants in a class are shared by all objects of the class. Thus, constants should be declared
as
final static
. For example, the constant
PI
in the
Math
class is defined as:
declare constant
final static double
PI =
3.14159265358979323846
;
The new circle class, named
CircleWithStaticMembers
, is defined in Listing 8.7:
L
ISTING
8.7
CircleWithStaticMembers.java
1
public class
CircleWithStaticMembers {
2
/** The radius of the circle */
3
double
radius;
4
5
/** The number of objects created */
6
7
8
/** Construct a circle with radius 1 */
9 CircleWithStaticMembers() {
10 radius =
1
;
11
12 }
13
14
/** Construct a circle with a specified radius */
15 CircleWithStaticMembers(
double
newRadius) {
16 radius = newRadius;
17
18 }
19
20
static int
numberOfObjects =
0
;
static variable
numberOfObjects++;
increase by 1
numberOfObjects++;
increase by 1
/** Return numberOfObjects */
21
22
static int
getNumberOfObjects() {
static method
return
numberOfObjects;
23 }
24
