Sams Teach Yourself C in 21 Days

Working with Java Classes and Methods 739

BD5

Line 1 opens the class definition. By using the extendskeyword with the name

of the class ListOfNumbers, this line indicates that BetterListOfNumberswill

be a subclass of (that is, will inherit from) the class ListOfNumbers. Lines 3 to 8 define

the one new method for this class, called average. Code in this method tests the value of

the property icount. If this is greater than 0 (at least one value has been added to the

list), the average is calculated and returned. If icountis 0 (no values have been added to

the list), 0 is returned. Note that the variables icountanditotalare declared in

ListOfNumbersbut are available in BetterListOfNumbersbecause they were declared

with the protectedkeyword.

The program NumberList, shown in Listing B5.13, demonstrates these two classes.

LISTINGB5.13 NumberList.java.NumberListdemonstrates the classes ListOfNumbersand

BetterListOfNumbers

1: public class NumberList {

2:

3: public static void main(String args[]) {

4:

5: ListOfNumbers MyList = new ListOfNumbers();

6: BetterListOfNumbers MyBetterList = new BetterListOfNumbers();

7:

8: MyList.Add(4);

9: MyList.Add(8);

10: MyList.Add(9.6);

11: MyBetterList.Add(4);

12: MyBetterList.Add(8);

13: MyBetterList.Add(9.6);

14:

15: System.out.println(“From class ListOfNumbers:”);

16: System.out.print(“Total = “);

17: System.out.println(MyList.total());

18: System.out.print(“Count = “);

19: System.out.println(MyList.count());

20: System.out.println(“From class BetterListOfNumbers:”);

21: System.out.print(“Total = “);

22: System.out.println(MyBetterList.total());

23: System.out.print(“Count = “);

24: System.out.println(MyBetterList.count());

25: System.out.print(“Average = “);

26: System.out.println(MyBetterList.average());

27: }

28: }

ANALYSIS

40 448201x-Bonus5 8/13/02 11:23 AM Page 739