Iterators - Data Structures and Abstractions with Java - page 374

that tracks where we are in the iteration. This field is simply the integer position of the entry in the list
that the method next last returned. It is convenient to initialize this field to zero.

LISTING 15-2

An outline of the class SeparateIterator

import java.util.Iterator;
import java.util.NoSuchElementException;
public class SeparateIterator<T> implements Iterator<T>
{

private ListInterface<T> list;
private int nextPosition; // position of entry last returned by next()
private boolean wasNextCalled; // needed by remove

public SeparateIterator(ListInterface<T> aList)
{

list = aList;
nextPosition = 0;
wasNextCalled = false ;
} // end constructor

< Implementations of the methods hasNext , next , and remove go here >
. . .
} // end SeparateIterator

Providing an iterator with a remove operation is optional; however, we shall do so here
because the previous examples used one. This desire complicates our class somewhat, because the
client must call the method next before each call to remove . This requirement isn't simply a pre-
condition. The remove method must throw an exception if it isn't met. Therefore, we need an addi-
tional data field—a boolean flag—that enables remove to check whether next was called. We name
this data field wasNextCalled . The constructor initializes this field to false.

15.13

The method hasNext . The class SeparateIterator has no special access to the private data fields
of the class that implements the list. It is a client of the list and so can process the list only by using
the list's ADT operations. Figure 15-5 shows a separate class iterator with a reference to a list but
with no knowledge of the list's implementation. The implementations of the iterator methods will
use methods specified in ListInterface . The resulting implementations are rather straightforward
but take longer to execute, in general, than the implementation of an inner class iterator. For exam-
ple, the method hasNext calls the list's getLength method:

public boolean hasNext()
{

return nextPosition < list.getLength();
} // end hasNext

Question 5 What does the method hasNext return when the list is empty? Why?