Collections, Maps and Iterators - Absolute Java - page 933

PITFALL: (continued)

for the standard predefined collection classes, such as ArrayList<T> and HashSet<T> ,
return references. So, you can modify the elements in the collection by using mutator
methods on i.next() . This is illustrated in Display 16.17. The comments we made
about i.next() also apply to i.previous() .
The fact that next and previous return references to elements in the collection is
not necessarily bad news. It means you must be careful, but it also means you can cycle
through all the elements in the collection and perform some processing that might
modify the elements. For example, if the elements in the collection are records of some
sort, you can use mutator methods to update the records.
If you read the APIs for the Iterator<T> and ListIterator<T> interfaces, they
say that a ListIterator<T> can change the collection, but presumably, a plain old
Iterator<T> cannot. These API comments do not refer to whether or not a reference
is returned by i.next() . They simply refer to the fact that the ListIterator<T>
interface has a set method, whereas the Iterator<T> interface does not. Do not
confuse this with the point discussed in the previous paragraph. ■

Display 16.17 An Iterator Returns a Reference (part 1 of 2)

The class Date is defined in Display 4.13,
but you can easily guess all you need to
know about Date for this example.

1 import java.util.ArrayList;
2 import java.util.Iterator;

3 public class IteratorReferenceDemo
4 {
5 public static void main(String[] args)
6 {
7 ArrayList<Date> birthdays = new ArrayList<Date>();

8 birthdays.add( new Date(1, 1, 1990));
9 birthdays.add( new Date(2, 2, 1990));
10 birthdays.add( new Date(3, 3, 1990));

11 System.out.println("The list contains:");

12 Iterator<Date> i = birthdays.iterator();
13 while (i.hasNext())
14 System.out.println(i.next());

15 i = birthdays.iterator();
16 Date d = null ; //To keep the compiler happy.
17 System.out.println("Changing the references.");
18 while (i.hasNext())
19 {
20 d = i.next();
21 d.setDate(4, 1, 1990);
22 }

(continued)