OBJECTS AND CLASSES - Introduction to Java Programming: Comprehensive Version - page 353

43
44

/** Add circle areas */

45 public static double sum(CircleWithPrivateDataFields[] circleArray) {

pass array of objects

46 // Initialize sum

47 double sum = 0 ;

48
49 // Add areas to sum
50 for ( int i = 0 ; i < circleArray.length; i++)
51 sum += circleArray[i].getArea();
52
53 return sum;

54 }
55 }

Radius Area
70.577708 15649.941866
44.152266 6124.291736
24.867853 1942.792644
5.680718 101.380949
36.734246 4239.280350
—————————————————————————————————————————————-
The total area of circles is 28056.687544

The program invokes createCircleArray() (line 8) to create an array of five circle
objects. Several circle classes were introduced in this chapter. This example uses the Cir-
cleWithPrivateDataFields class introduced in Section 9.9, Data Field Encapsulation.
The circle radii are randomly generated using the Math.random() method (line 21).
The createCircleArray method returns an array of CircleWithPrivateDataFields
objects (line 25). The array is passed to the printCircleArray method, which displays the
radius and area of each circle and the total area of the circles.
The sum of the circle areas is computed by invoking the sum method (line 41), which takes
the array of CircleWithPrivateDataFields objects as the argument and returns a dou-
ble value for the total area.

9.27

✓ ✓

What is wrong in the following code?

Check

Point

1 public class Test {
2 public static void main(String[] args) {
3 java.util.Date[] dates = new java.util.Date[ 10 ];
4 System.out.println(dates[ 0 ]);
5 System.out.println(dates[ 0 ].toString());
6 }
7 }

9.12 Immutable Objects and Classes
You can define immutable classes to create immutable objects. The contents of
immutable objects cannot be changed.

Key
Point

Normally, you create an object and allow its contents to be changed later. However, occa-
sionally it is desirable to create an object whose contents cannot be changed once the object
has been created. We call such an object as immutable object and its class as immutable
class . The String class, for example, is immutable. If you deleted the setter method in the
CircleWithPrivateDataFields class in Listing 9.9, the class would be immutable,
because radius is private and cannot be changed without a setter method.

immutable object
immutable class