OBJECTS AND CLASSES - Introduction to Java Programming: Comprehensive Version - page 345

A private data field cannot be accessed by an object from outside the class that defines
the private field. However, a client often needs to retrieve and modify a data field. To make
a private data field accessible, provide a getter method to return its value. To enable a private
data field to be updated, provide a setter method to set a new value. A getter method is also
referred to as an accessor and a setter to a mutator .
A getter method has the following signature:

getter (or accessor)
setter (or mutator)

public returnType get PropertyName ()

If the returnType is boolean , the getter method should be defined as follows by convention:

boolean accessor

public boolean is PropertyName ()

A setter method has the following signature:

public void set PropertyName ( dataType propertyValue )

Let's create a new circle class with a private data-field radius and its associated accessor and
mutator methods. The class diagram is shown in Figure 9.17. The new circle class, named
CircleWithPrivateDataFields , is defined in Listing 9.8:

Circle
-radius: double
-numberOfObjects: int

The - sign indicates
a private modifier

The radius of this circle (default: 1.0).

The number of circle objects created.

+Circle()

Constructs a default circle object.

+Circle(radius: double) Constructs a circle object with the specified radius.

+getRadius(): double

Returns the radius of this circle.

+setRadius(radius: double): void

Sets a new radius for this circle.

+ getNumberOfObjects(): int

Returns the number of circle objects created.

+getArea(): double

Returns the area of this circle.

F IGURE 9.17

The Circle class encapsulates circle properties and provides getter/setter and other methods.

L ISTING 9.8

CircleWithPrivateDataFields.java
1 public class CircleWithPrivateDataFields {
2 /** The radius of the circle */

3 private double radius = 1 ;

encapsulate radius

4
5 /** The number of objects created */

6 private static int numberOfObjects = 0 ;

encapsulate
numberOfObjects

7
8 /** Construct a circle with radius 1 */
9 public CircleWithPrivateDataFields() {
10 numberOfObjects++;
11 }
12
13 /** Construct a circle with a specified radius */
14 public CircleWithPrivateDataFields( double newRadius) {
15 radius = newRadius;
16 numberOfObjects++;