Difference between Java Enumeration and Iterator

Looking at the Java API Specification for the Iterator interface, there is an explanation of the differences between Enumeration:

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.

The bottom line is, both Enumeration and Iterator will give successive elements, but Iterator improved the method names by shortening away the verbiage, and it has an additional remove method. Here is a side-by-side comparison:

  Enumeration                     Iterator
  ----------------                ----------------
  hasMoreElements()               hasNext()
  nextElement()                   next()
  N/A                             remove()

As also mentioned in the Java API Specifications, for newer programs, Iterator should be preferred over Enumeration, as "Iterator takes the place of Enumeration in the Java collections framework." (From the Iterator specifications.)

"Officially", they are supposed to be similar with the iterator interface supporting extra operations (e.g., removal). Generally, the tendency is to use iterators.

Here is from the enumeration interface javadocs:

NOTE: The functionality of this interface is duplicated by the Iterator interface. In addition, Iterator adds an optional remove operation, and has shorter method names. New implementations should consider using Iterator in preference to Enumeration.

1) The main difference between Iterator and Enumeration is removal of the element while traversing the collection. Iterator can remove the element during traversal of collection as it has remove() method. Enumeration does not have remove() method.

2) Enumeration is fail-safe in nature. It does not throw ConcurrentModificationException if Collection is modified during the traversal. Iterator is fail-fast in nature. It throws ConcurrentModificationException if a Collection is modified while iterating other than its own remove() method.

3) Enumeration is a legacy interface which is used for traversing Vector, Hashtable. Iterator is not a legacy interface. Iterator can be used for the traversal of HashMap, LinkedList, ArrayList, HashSet, TreeMap, TreeSet .

If you're writing your own collection class, and you're extending any of the existing classes or implementing any of the Collections framework interfaces, you basically have no choice but to use Iterator.

If for some reason (that I can't think of) you're creating a custom collection class that does not relate to java.util.Collection or java.util.Map in any way, you should still implement Iterable so people can use your class in for loops.

The main different is Enumeration doesn't expose remove() method. Moreover, Iterator don't allow a simultaneously navigation and modification on an underlying object. They have a control to see if there are concurrent modifications or so, and hence takes more processing. So Enumeration's performance is virtually 50% faster than Iterator. If we need only navigation ignoring such a synchronization, just use Enumeration.