OBJECTS AND CLASSES - Introduction to Java Programming: Comprehensive Version - page 324

UML Class Diagram Circle Class name

Data fields

radius: double

Circle()
Circle(newRadius: double)
getArea(): double
getPerimeter(): double
setRadius(newRadius: double): void

Constructors and
methods

UML notation
for objects

circle1: Circle circle2: Circle circle3: Circle

radius = 1

radius = 25 radius = 125

F IGURE 9.4

Classes and objects can be represented using UML notation.

The method is denoted as

methodName(parameterName: parameterType): returnType

9.3 Example: Defining Classes and Creating Objects
Classes are definitions for objects and objects are created from classes.

Key
Point

This section gives two examples of defining classes and uses the classes to create objects.
Listing 9.1 is a program that defines the Circle class and uses it to create objects. The pro-
gram constructs three circle objects with radius 1 , 25 , and 125 and displays the radius and
area of each of the three circles. It then changes the radius of the second object to 100 and
displays its new radius and area.

Note
To avoid a naming conflict with several enhanced versions of the Circle class intro-
duced later in the chapter, the Circle class in this example is named SimpleCircle .
For simplicity, we will still refer to the class in the text as Circle .

avoid naming conflicts

L ISTING 9.1

TestSimpleCircle.java
1 public class TestSimpleCircle {
2 /** Main method */
3 public static void main(String[] args) {
4 // Create a circle with radius 1
5 SimpleCircle circle1 = new SimpleCircle();
6 System.out.println( "The area of the circle of radius "
7 + 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 ; // or circle2.setRadius(100)
21 System.out.println( "The area of the circle of radius "
22 + circle2.radius + " is " + circle2.getArea());

main class

main method

create object

create object

create object