A Bag Implementation That Links Data - Data Structures and Abstractions with Java - page 68

LISTING 3-3

A sample program that tests some methods in the class LinkedBag

/** A test of the methods add, toArray, isFull, isEmpty, and
getCurrentSize, as defined in the first draft of the class LinkedBag.
@author Frank M. Carrano

*/
public class LinkedBagDemo1
{

public static void main(String[] args)
{

System.out.println("Creating an empty bag.");
BagInterface<String> aBag = new LinkedBag<String>();
testIsEmpty(aBag, true );
displayBag(aBag);

String[] contentsOfBag = {"A", "D", "B", "A", "C", "A", "D"};
testAdd(aBag, contentsOfBag);
testIsEmpty(aBag, false );
testIsFull(aBag, false );
} // end main

< The static methods testAdd , testIsFull , and displayBag from Listing 2-2 are here. >
< The static method testIsEmpty is analogous to testIsFull and is here. >
} // end LinkedBagDemo1

The Method getFrequencyOf
3.16

To count the number of times a given entry appears in a bag, we must traverse the chain of nodes and
look at the entry in each one. The traversal is much like the one we used in the method toArray . Thus,
if currentNode will reference the node that we want to examine, we set it initially to firstNode —the
first node in the chain—and then use the statement

currentNode = currentNode.next;
to advance it to the next node. Using this technique, we can write a loop like the following one:
int counter = 0;
Node currentNode = firstNode;
while ((counter < numberOfEntries) && (currentNode != null ))
{

VideoNote

Completing the class
LinkedBag

. . .
counter++;
currentNode = currentNode.next;
} // end while
Although the method toArray uses the variable index , since it deals with an array, we use the
variable counter here, as we do not have an array. You should note that counter is counting nodes
for loop control; it is not counting how many times a given entry occurs in a bag. Moreover, we
could omit counter entirely, but we retain it as a check on our logic.