Implementing a Collection Class - Building Java Programs: A Back to Basics Approach - page 913

This method also has an important precondition to consider. A client is supposed
to call next before calling remove . One possibility is that the client will call remove
before making any call on next . If that happens, it will be obvious from the fact that
position will be zero. Another possibility is that the client will call remove twice in
a row without calling next in between. That is not a legal operation either. You won't
know just from looking at the value of position whether the client has violated this
precondition.
In this case, you need an extra bit of state for the object. You need to keep track of
whether it is currently legal to remove a value, so this is a good time to add a field. It
will be of type boolean and you can call it removeOK . You can use this field to throw
an exception if the precondition is violated. And once a call on remove has been per-
formed, you have to remember that it is no longer legal to remove a value until next
is called again:
public void remove() {
if (!removeOK) {
throw new IllegalStateException();
}
list.remove(position - 1);
position--;
removeOK = false;
}
Notice that you throw an IllegalStateException because a call on the method
is not appropriate if it is not okay to remove a value. You can then add code to the
constructor to initialize this field to false , and you can reset it to true whenever
next is called.
Here is the final class definition:
1 // Objects of this class can be used to iterate over an
2 // ArrayIntList and remove values from the list.
3
4 import java.util.*;
5
6 public class ArrayIntListIterator {
7 private ArrayIntList list; // list to iterate over
8 private int position; // current list position
9 private boolean removeOK; // okay to remove now?
10
11 // post: constructs an iterator for the given list
12 public ArrayIntListIterator(ArrayIntList list) {
13 this .list = list;
14 position = 0;
15 removeOK = false;