Friday, July 27, 2012

What are differences between Enumeration and Iterator

What are differences between Enumeration and Iterator  This question is from early ages of interview , I have not seen this question on recent interviews but it was common during 2006-2007 , now days questions like implementation of HashMap, ConcurrentHashMap etc has take its place, nonetheless its very useful

to know fundamental difference between Iterator and EnumerationSome time its also asked as  Iterator vs Enumeration or Enumeration vs Iterator which is same. important point to note is that both Iterator and Enumeration provides way to traverse or navigate through entire collection in java

Between Enumeration and Iterator, Enumeration is older and its there from JDK1.0 while iterator was introduced later. Iterator can be used with Java arraylist,  java hashmap keyset  and with any other collection classes.

difference between iterator and enumeration in javaAnother similarity between Iterator and Enumeration in Java is that  functionality of Enumeration interface is duplicated by the Iterator interface.

Only major difference between Enumeration and iterator is Iterator has a remove() method while Enumeration doesn't. Enumeration acts as Read-only interface, because it has the methods only to traverse and fetch the objects, where as by using Iterator we can manipulate the objects like adding and removing the objects from collection e.g. Arraylist.

Also Iterator is more secure and safe as compared to Enumeration because it  does not allow other thread to modify the collection object while some thread is iterating over it and throws ConcurrentModificationException. This is by far most important fact for me for deciding between Iterator vs Enumeration in Java.

In Summary both Enumeration and Iterator will give successive elements, but Iterator is new and improved version where method names are shorter, and has new method called remove. Here is a short comparison:



So Enumeration is used when ever we want to make Collection objects as Read-only.

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