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Iterator & ArrayList
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In the discussion of Vector we mentioned the Enumeration class, which provides for stepping through the elements of a list such as a Vector. However, Iterator, introduced with Java 1.2, is now preferred to Enumeration.

Iterator differs from Enumeration in 2 ways:

  • Elements can be safely removed from Vector using the remove (int index) without causing the Iterator to fail..
  • The hasMore () and next () methods in Iterator are short and direct as opposed to the corresponding long and boring methods nextElement () and hasMoreElements () in Enumeration.

All the old container classes were retro-fitted with the Java 1.2 release to support the Iterator interface and the new container classes added in JDK 1.2 with the Collections Framework support only the Iterator interface.

The syntax for iterating over the elements in a Vector is as follows

  Vector vec = new Vector;
  // Populate it... Then later, iterate over its elements
  Iterator it = vec.iterator ();
  while (it.hasNext ()) {
    Object o = it.next ();

We use an java.lang.Object reference for the retrieved element but could cast it to a specific type if we knew what type of object is in the list.

The following snippet illustrates another popular iteration style, this time using an ArrayList, which is preferred over Vector in a thread-safe situation:

  ArrayList a_list = new ArrayList ();
  for (Iterator it = a_list.iterator (); it.hasNext (); ) {
    Object o = it.next ();

Again, we retrieve Object types from the Iterator. All these container objects, Vector, ArrayList, and all the others, accept input of any kind of object. They can do this because the add() method receives Object as the parameter type, and since Object is the superclass of all other object types, any kind of object can be added.

The containers, however, don't know what kinds of objects are being stored in them (see, however, the generics feature of J2SE in the next section).

Therefore, when retrieving an object from one of the containers, it can only be returned as a Object type. In most cases, you need to cast the retrieved Object to the specific object type you desire. You should know what kind of objects you store into a container, so you can do the cast correctly.

If, however, you cast a returned Object to a type that it does not represent (say, you cast an Integer to a String), the program will fail due to a runtime ClassCastException.

The next snippet illustrates the ability of the Iterator to remove elements. First we make a list of integers from 0 to 19 and then remove all the odd integers.

  // Build an ArrayList and populate it with integers from 0 to 19.
  ArrayList al = new ArrayList ();
  for (int i=0; i < 20; i++) al.add (i);

  // Iterate through the ArrayList, removing all the odd integers.
  int count = 0;
  for (Iterator it = al.iterator (); it.hasNext (); ) {
    it.next ();
    if (count%2 == 0) it.remove ();

  // Print out the remaining elements with another Iterator.
  for (Iterator it = al.iterator (); it.hasNext (); ) {
    System.out.println ("element is " + it.next ());

The first loop simply loads the integers from 0 to 19 into the ArrayList. The next loop uses an iterator to retrieve each element and remove the ones when count%2 is zero. The final loop uses another iterator to print out the contents of the modified list. Note that we used the Iterator.remove() method, not ArrayList.remove().

Attempting to remove an element directly from the ArrayList generates a runtime ConcurrentModificationException.

Note also that we used the autoboxing feature (see Chapter 3) to add primitive int types in the first loop (autoboxed to Integer type) without having to explicitly convert them to integer wrapper types.


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Latest update: Nov. 18, 2004

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