The Iterator Pattern

The Iterator Pattern
An Introduction with Examples in Java

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Motivation

Computers vs. Calculators:
- Computers can perform the same operation many times
Programming Languages:
- Harness this power using loops
A Problem:
- Traditional looping requires an understanding of the structure of the "aggregate"

Looping and Aggregates

Looping Over an Array

javaexamples/iterator/ArrayExample.java

(Fragment: 1)

       String    city;

       for (int i=0; i < cities.length; i++)
       {
          city = cities[i];
          System.out.println(city);
       }

Looping and Aggregates (cont.)

Looping Over an ArrayList

javaexamples/iterator/ArrayListExample.java

(Fragment: 1)

       String    city;

       for (int i=0; i < cities.size(); i++)
       {
          city = cities.get(i);
          System.out.println(city);
       }

Looping and Aggregates (cont.)

Looping Over a Linked Structure

javaexamples/iterator/LinkedListExample.java

(Fragment: 1)

       Node<String>      current;
       String            city;

       current = first;
       while (current != null)
       {
          city = current.value;
          System.out.println(city);
          current = current.next;
       }

Some Observations

Applications often "loop" over the same aggregate object in many classes and methods
Changing from one aggregate to another is, as a result, very inconvenient
The Iterator design pattern enables us to access the elements of an aggregate object while hiding its internal structure

Important Operations

Reset its "pointer" (or cursor) to the first element.
Determine if there are any more elements in the sequence.
Move its "pointer" to the next element.
Retrieve the "current" element.

The Iterator Pattern

In UML:

Examples in Java

The Aggregates:
- Classes that realize the Iterable interface (including classes that implement the List interface like ArrayList and classes that realize the Set interface like HashSet )
The Iterator:
- Classes that realize the Iterator interface

Examples in Java (cont.)

Note that the Map interface does not extend Iterable or Collection, but one can get a Set or Collection object from a Map.

Benefits of the Iterator

What code must change to use a HashSet instead of an ArrayList?

    Iterable<String>    cities;

    cities = new ArrayList<String>();

    // Important code

    // An example loop
    Iterator<String>    i = cities.iterator();
    while (i.hasNext())
    {
       String city = i.next();
       System.out.println(city);
    }

Benefits of the Iterator (cont.)

The Principle Benefit:
- You can easily change the type of aggregate being used without having to change any loops (and, if you declare variables properly, without having to change any of the declarations, only the instantiations)
Other Benefits:
- Several objects can be "looping" over the elements in the aggregate at the same time
- A "filtered" list (e.g., names starting with the letter "A") is handled in exactly the same way that the "unfiltered" version is

The "New" Loop

The Idea:
- Include a control structure that takes advantage of the Iterator Pattern (i.e., create a "for each" loop for any Object that implements the Iterable interface)
Syntax:
- for (Class element : Iterable) statement

Example:

    Iterable<String>    cities;

    cities = new ArrayList<String>();

    // Important code

    // An example loop
    for (String city: cities)
    {
       System.out.println(city);
    }