OBJECTS AND CLASSES - Introduction to Java Programming: Comprehensive Version - page 338

UML Notation:
underline: static variables or methods

instantiate

circle1: Circle

Memory

After two Circle
Objects were created,
numberOfObjects
is 2.

radius = 1
numberOfObjects = 2

radius 1

Circle

radius: double
numberOfObjects: int

numberOfObjects

2

getNumberOfObjects(): int
getArea(): double

instantiate

circle2: Circle

radius = 5
numberOfObjects = 2

radius

5

F IGURE 9.13

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.

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 9.6:

L ISTING 9.6

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 ;

static variable

7
8 /** Construct a circle with radius 1 */
9 CircleWithStaticMembers() {
10 radius = 1 ;
11

numberOfObjects++;

increase by 1

12 }
13
14 /** Construct a circle with a specified radius */
15 CircleWithStaticMembers( double newRadius) {
16 radius = newRadius;
17 numberOfObjects++; increase by 1

18 }
19
20

/** Return numberOfObjects */

21 static int getNumberOfObjects() {

static method

22 return numberOfObjects;

23 }
24
25

/** Return the area of this circle */

26 double getArea() {

27 return radius * radius * Math.PI;

28 }
29 }

Method getNumberOfObjects() in CircleWithStaticMembers is a static method. All
the methods in the Math class are static. The main method is static, too.