Retrieving Elements from Collection in Java (For-each, Iterator, ListIterator & EnumerationIterator)

Prerequisite: Collection in Java
Following are the 4 ways to retrieve any elements from a collection object:

For-each

For each loop is meant for traversing items in a collection.

// Iterating over collection 'c' using for-each 
   for (Element e: c)
       System.out.println(e);

We read the ‘:’ used in the for-each loop as “in”. So loop reads as “for each element e in elements”, here elements are the collection that stores Element type items.

Note: In Java 8 using lambda expressions we can simply replace the for-each loop with

elements.forEach (e -> System.out.println(e) );

Using Cursors

The cursor is an interface and it is used to retrieve data from collection objects,one by one. The cursor has 3 types, which are given below:

Output :

In Forward direction:
10 20 30 

In backward direction:
30 20 10 

1. Iterator Interface: Iterator is an interface provided by the collection framework to traverse a collection and for sequential access of items in the collection.

      
      // Iterating over collection 'c' using terator
      for (Iterator i = c.iterator(); i.hasNext(); ) 
          System.out.println(i.next());
   

   It has 3 methods:

   • boolean hasNext(): This method returns true if the iterator has more elements.
   • elements next(): This method returns the next elements in the iterator.
   • void remove(): This method removes from the collection the last elements returned by the iterator.
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   // Java program to demonstrate working of iterators import java.util.*; public class IteratorDemo {     public static void main(String args[])     {         //create a Hashset to store strings         HashSet<String> hs = new HashSet<String>() ;            // store some string elements         hs.add("India");         hs.add ("America");         hs.add("Japan");            // Add an Iterator to hs.         Iterator it = hs.iterator();            // Display element by element using Iterator         while (it.hasNext())             System.out.println(it.next());     } }

   Output:

   America
   Japan
   India

   Refer Iterator vs For-each for differences between iterator and for-each.

2. ListIterator Interface: It is an interface that contains methods to retrieve the elements from a collection object, both in forward and reverse directions. This iterator is for list-based collections.
   It has the following important methods:
   • booleanhasNext(): This returns true if the ListIterator has more elements when traversing the list in the forward direction.
   • booleanhasPrevious(): This returns true if the ListIterator has more elements when traversing the list in the reverse direction.
   • element next(): This returns the next element in the list.
   • element previous(): This returns the previous element in the list.
   • void remove(): This removes from the list the last elements that were returned by the next() or previous() methods.
   • int nextIndex() Returns the index of the element that would be returned by a subsequent call to next(). (Returns list size if the list iterator is at the end of the list.)
   • int previous index() Returns the index of the element that would be returned by a subsequent call to previous(). (Returns -1 if the list iterator is at the beginning of the list.)

   Source: ListIterator Oracle Docs

   How is Iterator different from ListIterator?

   Iterator can retrieve the elements only in the forward direction. But ListIterator can retrieve the elements in forward and reverse direction also. So ListIterator is preferred to Iterator.

   Using ListItarator, we can get iterator’s current position

   Since ListIterator can access elements in both directions and supports additional operators, ListIterator cannot be applied on Set (e.g., HashSet and TreeSet. See this). However, we can use LisIterator with vector and list (e.g. ArrayList ).

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   // Java program to demonstrate working of ListIterator import java. util.* ;    class Test {     public static void main(String args[])     {         // take a vector to store Integer objects         Vector<Integer> v = new Vector<Integer>();            // Adding Elements to Vector         v.add(10);         v.add(20);         v.add(30);            // Creating a ListIterator         ListIterator lit = v.listIterator();         System.out.println("In Forward direction:");            while (lit.hasNext())             System.out.print(lit.next()+" ") ;            System.out.print("\n\nIn backward direction:\n") ;         while (lit.hasPrevious())             System.out.print(lit.previous()+" ");     } }

3. EnumerationIterator Interface: The interface is useful to retrieve one by one the element. This iterator is based on data from Enumeration and has methods:
   • booleanhasMoreElements(): This method tests if the Enumeration has any more elements or not.
   • element nextElement(): This returns the next element that is available in elements that are available in Enumeration

   What is the difference between Iterator and Enumeration?

   Iterator has an option to remove elements from the collection which is not available in Enumeration. Also, Iterator has methods whose names are easy to follow and Enumeration methods are difficult to remember.

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   import java.util.Vector; import java.util.Enumeration; public class Test {     public static void main(String args[])     {         Vector dayNames = new Vector();         dayNames.add("Sunday");         dayNames.add("Monday");         dayNames.add("Tuesday");         dayNames.add("Wednesday");         dayNames.add("Thursday");         dayNames.add("Friday");         dayNames.add("Saturday");            // Creating enumeration          Enumeration days = dayNames.elements();            // Retrieving elements of enumeration         while (days.hasMoreElements())             System.out.println(days.nextElement());     } }

   Output:

   Sunday
   Monday
   Tuesday
   Wednesday
   Thursday
   Friday
   Saturday

Related Articles:
Iterators in Java
Iterator vs Foreach In Java

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