Difference between iterator and enumerator

This is one of the good interview questions. In this post, I am going to explain some difference between iterator and enumerator, how to create custom iterator etc.,

One important difference is, Enumeration is introduced in Jdk 1.0, all the legacy classes like HashTable, Vector, StringTokenizer implements Enumeration interface. Where as Iterator interface is introduced in Jdk 1.2, it provides all the functionality provided by Enumeration, in addition to this, by using iterator you can remove an element while traversing.

Enumeration interface provide following methods.

Method	Description
boolean hasMoreElements()	Return true if this enumeration object contains at least one more element to provide; false otherwise.
E nextElement()	Return next element of this enumeration

Example

import java.util.Enumeration;
import java.util.Vector;

public class EnumerationDemo {
 public static void main(String args[]) {
  Vector<Integer> vector = new Vector<>();

  vector.add(100);
  vector.add(35);
  vector.add(54);
  vector.add(321);
  vector.add(432);
  vector.add(5);

  Enumeration<Integer> enumerator = vector.elements();

  while (enumerator.hasMoreElements()) {
   System.out.println(enumerator.nextElement());
  }
 }
}

Output

100
35
54
321
432
5

Iterator interface provides following methods.

Method	Description
boolean hasNext()	Return true, if the iterator has more elements.
E next()	Return the next element in the iteration.
default void remove()	Remove the last element returned by this iterator.
default void forEachRemaining(Consumer<? super E> action)	Performs the given action for each remaining element until all elements have been processed or the action throws an exception.

                                                                 

import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;

public class IteratorDemo {
 public static void main(String args[]) {
  List<Integer> primes = new LinkedList<>();

  primes.add(2);
  primes.add(3);
  primes.add(5);
  primes.add(7);
  primes.add(11);

  Iterator<Integer> iter = primes.iterator();

  while (iter.hasNext()) {
   System.out.println(iter.next());
  }
 }
}

Output

2
3
5
7
11

Implementing Custom Iterator

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.NoSuchElementException;

public class CustomList<E> {
 private List<E> tempList = new ArrayList<>();
 private int size = -1;

 public void add(E ele) {
  size++;
  tempList.add(ele);
 }

 public Iterator<E> iterator() {
  return new Itr();
 }

 private class Itr implements Iterator<E> {
  private int cursor = -1;

  @Override
  public boolean hasNext() {
   return cursor < size;
  }

  @Override
  public E next() {
   if (cursor >= size) {
    throw new NoSuchElementException();
   }
   cursor++;
   return tempList.get(cursor);
  }

 }

}

Output

10
20
30
40
50
60

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