What are the benefits of the Iterator interface in Java?

Question

I just learned about how the Java Collections Framework implements data structures in linked lists. From what I understand, Iterators are a way of traversing through the items in a data structure such as a list. Why is this interface used? Why are the methods hasNext(), next() and remove() not directly coded to the data structure implementation itself?

From the Java website: link text

public interface Iterator<E>

An iterator over a collection. Iterator takes the place of Enumeration in the Java collections framework. Iterators differ from enumerations in two ways:

• Iterators allow the caller to remove elements from the underlying collection during the iteration with well-defined semantics.
• Method names have been improved.

This interface is a member of the Java Collections Framework.

I tried googling around and can't seem to find a definite answer. Can someone shed some light on why Sun chose to use them? Is it because of better design? Increased security? Good OO practice?

Any help will be greatly appreciated. Thanks.

Answer 1

Why is this interface used?

Because it supports the basic operations that would allow a client programmer to iterate over any kind of collection (note: not necessarily a Collection in the Object sense).

Why are the methods... not directly coded to the data structure implementation itself?

They are, they're just marked Private so you can't reach into them and muck with them. More specifically:

• You can implement or subclass an Iterator such that it does something the standard ones don't do, without having to alter the actual object it iterates over.
• Objects that can be traversed over don't need to have their interfaces cluttered up with traversal methods, in particular any highly specialized methods.
• You can hand out Iterators to however many clients you wish, and each client may traverse in their own time, at their own speed.
• Java Iterators from the java.util package in particular will throw an exception if the storage that backs them is modified while you still have an Iterator out. This exception lets you know that the Iterator may now be returning invalid objects.

For simple programs, none of this probably seems worthwhile. The kind of complexity that makes them useful will come up on you quickly, though.

Answer 2

You ask: "Why are the methods hasNext(), next() and remove() not directly coded to the data structure implementation itself?".

The Java Collections framework chooses to define the Iterator interface as externalized to the collection itself. Normally, since every Java collection implements the Iterable interface, a Java program will call iterator to create its own iterator so that it can be used in a loop. As others have pointed out, Java 5 allows us to direct usage of the iterator, with a for-each loop.

Externalizing the iterator to its collection allows the client to control how one iterates through a collection. One use case that I can think of where this is useful is when one has an an unbounded collection such as all the web pages on the Internet to index.

In the classic GoF book, the contrast between internal and external iterators is spelled out quite clearly.

A fundamental issue is deciding which party conrols the iteration, the iterator or the client that uses the iterator. When the client controls the iteration, the iterator is called an external iterator, and when the iterator controls it, the iterator is an internal iterator. Clients that use an external iterator must advance the traversal and request the next element explicitly from the iterator. In contrast, the client hands an internal iterator an operation to perform, and the iterator applies that operation to every element ....

External iterators are more flexible than internal iterators. It's easy to compare two collections for equality with an external iterator, for example, but it's practically impossible with internal iterators ... But on the other hand, internal iterators are easier to use, because they define the iteration logic for you.

For an example of how internal iterators work, see Ruby's Enumerable API, which has internal iteration methods such as each. In Ruby, the idea is to pass a block of code (i.e. a closure) to an internal iterator so that a collection can take care of its own iteration.

Answer 3

it is important to keep the collection apart from the pointer. the iterator points at a specific place in a collection, and thus is not an integral part of the collection. this way, for an instance, you can use several iterators over the same collection.

the down-side of this seperation is that the iterator is not aware to changes made to the collection it iterates on. so you cannot change the collection's structure and expect the iterator to continue it's work without "complaints".

Answer 4

Using the Iterator interface allows any class that implements its methods to act as iterators. The notion of an interface in Java is to have, in a way, a contractual obligation to provide certain functionalities in a class that implements the interface, to act in a way that is required by the interface. Since the contractual obligations must be met in order to be a valid class, other classes which see the class implements the interface and thus reassured to know that the class will have those certain functionalities.

In this example, rather than implement the methods (hasNext(), next(), remove()) in the LinkedList class itself, the LinkedList class will declare that it implements the Iterator interface, so others know that the LinkedList can be used as an iterator. In turn, the LinkedList class will implement the methods from the Iterator interface (such as hasNext()), so it can function like an iterator.

In other words, implementing an interface is a object-oriented programming notion to let others know that a certain class has what it takes to be what it claims to be.

This notion is enforced by having methods that must be implemented by a class that implements the interface. This makes sure that other classes that want to use the class that implements the Iterator interface that it will indeed have methods that Iterators should have, such as hasNext().

Also, it should be noted that since Java does not have multiple inheritance, the use of interface can be used to emulate that feature. By implementing multiple interfaces, one can have a class that is a subclass to inherit some features, yet also "inherit" the features of another by implementing an interface. One example would be, if I wanted to have a subclass of the LinkedList class called ReversibleLinkedList which could iterate in reverse order, I may create an interface called ReverseIterator and enforce that it provide a previous() method. Since the LinkedList already implements Iterator, the new reversible list would have implemented both the Iterator and ReverseIterator interfaces.

You can read more about interfaces from What is an Interface? from The Java Tutorial from Sun.

Answer 5

Multiple instances of an interator can be used concurrently. Approach them as local cursors for the underlying data.

BTW: favoring interfaces over concrete implementations looses coupling

Look for the iterator design pattern, and here: http://en.wikipedia.org/wiki/Iterator

Answer 6

Because you may be iterating over something that's not a data structure. Let's say I have a networked application that pulls results from a server. I can return an Iterator wrapper around those results and stream them through any standard code that accepts an Iterator object.

Think of it as a key part of a good MVC design. The data has to get from the Model (i.e. data structure) to the View somehow. Using an Iterator as a go-between ensures that the implementation of the Model is never exposed. You could be keeping a LinkedList in memory, pulling information out of a decryption algorithm, or wrapping JDBC calls. It simply doesn't matter to the view, because the view only cares about the Iterator interface.

Answer 7

An interesting paper discussing the pro's and con's of using iterators:

http://www.sei.cmu.edu/pacc/CBSE5/Sridhar-cbse5-final.pdf

Answer 8

I think it is just good OO practice. You can have code that deals with all kinds of iterators, and even gives you the opportunity to create your own data structures or just generic classes that implement the iterator interface. You don't have to worry about what kind of implementation is behind it.

Answer 9

Just M2C, if you weren't aware: you can avoid directly using the iterator interface in situations where the for-each loop will suffice.

Answer 10

Ultimately, because Iterator captures a control abstraction that is applicable to a large number of data structures. If you're up on your category theory fu, you can have your mind blown by this paper: The Essence of the Iterator Pattern.

Answer 11

The java.util.Iterator interface is used in the Java Collections Framework to allow modification of the collection while still iterating through it. If you just want to cleanly iterate over an entire collection, use a for-each instead, but a upside of Iterators is the functionality that you get: a optional remove() operation, and even better for the List Iterator interface, which offers add() and set() operations too. Both of these interfaces allow you to iterate over a collection and changing it structurally at the same time. Trying to modify a collection while iterating through it with a for-each would throw a ConcurrentModificationException, usually because the collection is unexpectedly modified!

Take a look at the ArrayList class

It has 2 private classes inside it (inner classes) called Itr and ListItr

They implement Iterator and the ListIterator interfaces respectively

public class ArrayList..... { //enclosing class

  private class Itr implements Iterator<E> {

        public E next() {
            return ArrayList.this.get(index++); //rough, not exact
        }

        //we have to use ArrayList.this.get() so the compiler will
        //know that we are referring to the methods in the 
        //enclosing ArrayList class

        public void remove() {
            ArrayList.this.remove(prevIndex);
        }

        //checks for...co mod of the list
        final void checkForComodification() {  //ListItr gets this method as well
             if (ArrayList.this.modCount != expectedModCount) { 
                 throw new ConcurrentModificationException();
             }
        }
  }

  private class ListItr extends Itr implements ListIterator<E> {
         //methods inherted....
        public void add(E e) {
            ArrayList.this.add(cursor, e);
        }

        public void set(E e) {
            ArrayList.this.set(cursor, e);
        }
  }

}

When you call the methods iterator() and listIterator(), they return a new instance of the private class Itr or ListItr, and since these inner classes are "within" the enclosing ArrayList class, they can freely modify the ArrayList without triggering a ConcurrentModificationException, unless you change the list at the same time (conccurently) through set() add() or remove() methods of the ArrayList class.

Answer 12

Well it seems like the first bullet point allows for multi-threaded (or single threaded if you screw up) applications to not need to lock the collection for concurrency violations. In .NET for example you cannot enumerate and modify a collection (or list or any IEnumerable) at the same time without locking or inheriting from IEnumerable and overriding methods (we get exceptions).

Answer 13

Iterator simply adds a common way of going over a collection of items. One of the nice features is the i.remove() in which you can remove elements from the list that you are iterating over. If you just tried to remove items from a list normally it would have weird effects or throw and exception.

The interface is like a contract for all things that implement it. You are basically saying.. anything that implements an iterator is guaranteed to have these methods that behave the same way. You can also use it to pass around iterator types if that is all you care about dealing with in your code. (you might not care what type of list it is.. you just want to pass an Iterator) You could put all these methods independently in the collections but you are not guaranteeing that they behave the same or that they even have the same name and signatures.

Answer 14

Iterators are one of the many design patterns available in java. Design patterns can be thought of as convenient building blocks, styles, usage of your code/structure.

To read more about the Iterator design pattern check out the this website that talks about Iterator as well as many other design patterns. Here is a snippet from the site on Iterator: http://www.patterndepot.com/put/8/Behavioral.html

The Iterator is one of the simplest and most frequently used of the design patterns. The Iterator pattern allows you to move through a list or collection of data using a standard interface without having to know the details of the internal representations of that data. In addition you can also define special iterators that perform some special processing and return only specified elements of the data collection.

Answer 15

Iterators can be used against any sort of collection. They allow you to define an algorithm against a collection of items regardless of the underlying implementation. This means you can process a List, Set, String, File, Array, etc.

Ten years from now you can change your List implementation to a better implementation and the algorithm will still run seamlessly against it.

Answer 16

Iterator is useful when you are dealing with Collections in Java.

Use For-Each loop(Java1.5) for iterating over a collection or array or list.