Iterators - Data Structures and Abstractions with Java - page 382

< Segment 15.25 begins a description of the following inner class. >
private class IteratorForArrayList implements Iterator<T>
{

private int nextIndex;
private boolean wasNextCalled; // needed by remove

private IteratorForArrayList()
{

nextIndex = 0;
wasNextCalled = false ;
} // end default constructor

< Implementations of the methods in the interface Iterator go here;
you can see them in Segments 15.26 through 15.28. >
. . .
} // end IteratorForArrayList
} // end ArrayListWithIterator

15.25

The inner class IteratorForArrayList . Just as you can use your finger to keep track of your
place on this page, our iterator implementation uses an index to keep track of the iterator's position
within the array of list entries. This index, which we call nextIndex , is a data field of the private
inner class IteratorForArrayList . It will be the index of the next entry in the iteration. The con-
structor initializes nextIndex to zero, as Listing 15.5 shows.
Just as you saw earlier in Segments 15.12 and 15.15, providing an iterator with a remove oper-
ation requires an additional data field that the remove method can use to see whether next was
called. Again, we name this data field wasNextCalled , but here it is defined within the inner class.
The constructor initializes this field to false.

15.26

The method hasNext . The iterator has a next entry to retrieve if nextIndex is less than the length
of the list. Thus, hasNext has the following straightforward implementation:

public boolean hasNext()
{

return nextIndex < numberOfEntries;
} // end hasNext
Notice that hasNext returns false when the list is empty, that is, when numberOfEntries is zero.

15.27

The method next . The implementation of the method next has the same general form as the ver-
sion given in Segment 15.14 for the separate class iterator. If hasNext returns true, next returns the
next entry in the iteration. Here, the next entry is list[nextIndex] . The method also advances the
iteration by incrementing nextIndex and sets the flag wasNextCalled to true. On the other hand, if
hasNext returns false, next throws an exception.

public T next()
{

if (hasNext())
{

wasNextCalled = true ;
T nextEntry = list[nextIndex];