Prentice.Hall.Introduction.to.Java.Programming,.Brief.Version.9th.(2014).[sharethefiles.com]

8.7 Static Variables, Constants, and Methods 313
UML Notation:
underline: static variables or methods
Circle
radius: double
numberOfObjects: int
getNumberOfObjects(): int
getArea(): double
instantiate
instantiate
circle1: Circle
radius = 1
numberOfObjects = 2
circle2: Circle
Memory
1
2
radius
numberOfObjects
After two Circle
Objects were created,
numberOfObjects
is 2.
radius = 5
numberOfObjects = 2
5
radius
FIGURE 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 variable
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;
}
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:
define static method
declare constant
final static double PI = 3.14159265358979323846;
The new circle class, named CircleWithStaticMembers, is defined in Listing 8.7:
LISTING 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 static int numberOfObjects = 0;
7
8 /** Construct a circle with radius 1 */
9 CircleWithStaticMembers() {
10 radius = 1;
11 numberOfObjects++;
12 }
13
14 /** Construct a circle with a specified radius */
15 CircleWithStaticMembers(double newRadius) {
16 radius = newRadius;
17 numberOfObjects++;
18 }
19
20 /** Return numberOfObjects */
21 static int getNumberOfObjects() {
22 return numberOfObjects;
23 }
24
static variable
increase by 1
increase by 1
static method