Iterator and Enumeration - Differences Explained

In this article, I will be comparing the Iterator and Enumeration interfaces. I will be explaining the similarities and differences between Iterator and Enumeration.

Definitions

Both Iterator and Enumeration are interfaces defined in the Java Collection framework. You can use both to loop through the elements of a Collection. Let us first understand how they work

 

Iterator

Iterator is an interface defined in the Java Collection framework. You can use it to loop through the elements of a collection. The following code demonstrates this:

List<Integer> numbers = Arrays.asList(2,4,6,8,10);  Iterator<Integer> itr = numbers.iterator();  while (itr.hasNext()) {    System.out.print(itr.next()+" ");  }

This code uses an Iterator to loop through the elements in a List and print them. Iterator.hasNext()method returns true if there are more elements in the collection. The iterator.next()method returns the next element in the collection. So this code prints the following output:

2 4 6 8 10

Enumeration

Enumeration is also an interface in the java.util.package. Just like Iterator, you can use it to loop through values. However, Enumeration  is a legacy interface you can only use it for legacy classes like Vector and HashTable, The following code demonstrates this:

Vector vector = new Vector();vector.add(2);vector.add(4);Enumeration e = vector.elements();while (e.hasMoreElements()) {  System.out.println(e.nextElement());}

This code uses an Enumeration to loop through the elements in a Vector and print them.Enumeration.hasMoreElements()method returns true if there are more elements in the collection. The enumeration.nextElementmethod returns the next element in the collection. So this code prints the following output:

2 4

Differences

Type of Collection

As seen earlier, you can use Iterator to loop through any Collection. However you can use an Enumeration only for legacy collections like Vector and HashTable.

Methods

The methods in the Iterator and Eumeration interfaces differ slightly. While Iterator has methods hasNextand next, Enumeration has the methods hasMoreElementsand nextElement

Direction of iteration

You can use an Iterator to navigate in forward as well as backwards direction. Iterator has a sub-interface ListIterator. This has methods hasPreviousand previous.

The following code demonstrates this:

List<Integer> numbers = Arrays.asList(2,4,6,8,10);ListIterator<Integer> itr = numbers.listIterator();System.out.println("Navigating forward............");while (itr.hasNext()) {  System.out.print(itr.next()+" ");}System.out.println();System.out.println("Navigating backward............");while (itr.hasPrevious()) {  System.out.print(itr.previous()+" ");}

So this code uses the hasNextand next methods to navigate forwards and the hasPreviousand previous methods to navigate backwards.

However, you can only use an Enumeration to navigate in the forward direction.

Removal of elements

Iterator can modify the Collection on which it operates. It has a removemethod which removes an element from the Collection. The following code demonstrates this:

List<Integer> numbers = new ArrayList<Integer>();numbers.add(2);numbers.add(4);numbers.add(6);Iterator<Integer> itr = numbers.iterator();while (itr.hasNext()) {  int num = itr.next();  if(num == 4) {    itr.remove();  }}System.out.println("List with element removed:");for(int i = 0; i < numbers.size();i++) {  System.out.print(numbers.get(i)+" ");}

So this code checks if the input list has the element 6 and if so, uses the iterator.removemethod to remove it. It then uses a separate for loop to print the elements in the list. So this code will print the following output:

List with element removed:2 6

Enumeration on the other hand cannot modify the input collection, so you cannot remove an element from the Collection using Enumeration.

Summary

The following table summarizes the differences between Iterator and Enumeration:

Edit
IteratorEnumeration
Can be used on all collection classesCan be used only on legacy collection classes like Vector and HashMap
Uses methods hasNext() and next()Uses methods hasMoreElements() and nextElement()
Can be used to navigate in both forward and backward directionCan only be used to navigate in forward direction
Can also be used to remove the elements in the collectionCannot be used to remove elements, can only be used to retrieve elements

Edit

Conclusion

So to summarize, you can use both Iterator and Enumeration to loop through the elements in a Collection. While you an use Iterator on any Collection, you can use Enumeration only for legacy collections. Iterator also supports features like navigating in both directions and modifying the input Collection, which are not supported by Enumeration.

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