Iterator vs Foreach in Java

In the given tutorial, Iterator verses for each loop is explained neatly.

Iterator :

• Iterator belongs to java.util package, which is an interface and also a cursor.
• Cursors are used to retrieve elements from Collection type of object in Java.
• Iterator is recognized as Universal Java Cursor because supports all types of Collection classes.
• It is unidirectional (forward direction).
• Supporting methods of Iterator are : ref.iterator(), ref.hasNext() and ref.next().
• hasNext() method checks whether the elements are present inside  a Collection object or not. Its return type is boolean.

 

For each loop :

• For each loop is also used for iteration of elements.
• For each loop brings different type of way for iteration of elements from Collection type of object or from an array.
•  For each loop achieves iteration of the elements by executing every element one at a time sequentially.
• It is recommended because it makes code readable.

 

Difference between Iterator & for each loop :

1. While utilizing for each loop, size check is not necessary. But while utilizing iterator, hasNext() has to be used correctly or NoSuchElementException occurs.
2. While utilizing for each loop ConcurrentModificationException can occur, if the given object gets changed or altered. But while utilizing iterator, that will be not a issue.
3. In Performance criteria, both Iterator and for each are same .

 

Program :

Lets see a program where we will use both Iterator & for each loop in a collection.

 

package java;
import java.util;

public class Traversal
{
 public static void main(String[] args)
 {
  ArrayList<Integer> al = new ArrayList<>();

  al.add(11);
  al.add(22);
  al.add(33);
  al.add(44);
  al.add(55);

  System.out.println("Given Arraylist usling for each loop: ");
  //Applying for each loop here
  for(int i: al)
  {
   System.out.print(i+" ");
  }

  //utilizing iterator in here
  Iterator<Integer> it = al.iterator();

  System.out.println("\nGiven Arraylist using iterator: ");
  while(it.hasNext())
  {
   System.out.print(it.next()+" ");
  }
 }
}

Output :

When we will execute the class Traversal, Output is going to be :

 

Given Arraylist using for each loop:
11 22 33 44 55
Given Arraylist using iterator:
11 22 33 44 55

Iterator and for each loop are preferable rather than making use of for loop for traversing the elements without random access.