Iterators - Data Structures and Abstractions with Java - page 393

public ListIterator<T> getIterator()
{

return new IteratorForArrayList();
} // end getIterator

private class IteratorForArrayList implements ListIterator<T>
{

< The description of this implementation begins with Segment 15.43. >
. . .
} // end IteratorForArrayList
} // end ArrayListWithListIterator

The Inner Class
15.43

The data fields and constructor. We begin implementing the inner class IteratorForArrayList by
thinking about how the methods remove and set will throw the exception IllegalStateException .
Both of these methods throw this exception for the same reasons, that is, if either

•

next or previous was not called or

•

remove or add has been called since the last call to next or previous

Figure 15-11 shows calls to remove in various contexts that cause an IllegalStateException .
This aspect of the implementation might be intimidating at first, but it need not be difficult.
When we implemented Iterator 's remove method in Segment 15.28, we tested the boolean data
field wasNextCalled to see whether next had been called. We could do that here and define analogous

FIGURE 15-11

Possible contexts in which the method remove of the iterator
traverse throws an exception when called

Neither next nor p revious has been called
Causes an exception

(a)

traverse.remove();

(b)

traverse.next();

traverse.remove();

Legal

traverse.remove(); Causes an exception

(c)

traverse.previous();

traverse.remove(); Legal

Causes an exception

traverse.remove();

(d)

traverse.next();

traverse.add(...);

traverse.remove(); Causes an exception

(e)

traverse.previous();

traverse.add(...);

traverse.remove();

Causes an exception