A Bag Implementation That Links Data - Data Structures and Abstractions with Java - page 67

Node currentNode = firstNode;
while ((index < numberOfEntries) && (currentNode != null ))
{

result[index] = currentNode.data;
index++;
currentNode = currentNode.next;
} // end while
return result;
} // end toArray

Programming Tip
If ref is a reference to a node in a chain, be sure that ref is not null before you use it to access
ref.data or ref.next . Otherwise, if ref is null , a NullPointerException will occur.

Question 4 In the previous definition of toArray , the while statement uses the boolean
expression (index < numberOfEntries) && (currentNode != null) to control the loop. Is it
necessary to test the values of both index and currentNode ? Explain your answer.

Testing the Core Methods
3.15

Earlier, we realized that the add method is fundamental to our class, so it is one of the core methods
that we implement and test first. The method toArray lets us see whether add works correctly, so it
too is in our core group. But what about the methods that are not in our core group? Because
LinkedBag implements the interface BagInterface , it must define every method in the interface.
As the previous chapter described, we write stubs for methods that are declared in the interface but
are not a part of our core group. Since the methods getCurrentSize , isFull , and isEmpty have
simple definitions, we will write them instead of stubs in this first draft of the class LinkedBag .

Note: The method isFull should always return false. As noted in Segment 3.13, the only
time a bag whose implementation is linked could appear full is when the system cannot pro-
vide memory to the add method for a new node. In that case, an OutOfMemoryError occurs,
which is fatal. A client would not have the opportunity to call isFull .

A test program for LinkedBag 1 could be just like the one for ArrayBag , as given in Listing 2-2
of the previous chapter, except for its name, the class used to create a bag, and one other significant
distinction: Although an instance of ArrayBag can become full, an instance of LinkedBag will not.
Listing 3-3 outlines such a test program. Notice that the private static methods here are exactly the
same as those given in Listing 2-2 of the previous chapter. This is possible because the methods use
BagInterface as the data type of a bag.

1. Note that this version of the class LinkedBag is available online at the topic's website and is named LinkedBag1 .