Java - How to Use Iterator?

Often, you will want to cycle through the elements in a collection. For example, you might want to display each element. The easiest way to do this is to employ an iterator, which is an object that implements either the Iterator or the ListIterator interface.

Iterator enables you to cycle through a collection, obtaining or removing elements. ListIterator extends Iterator to allow bidirectional traversal of a list, and the modification of elements.

Before you can access a collection through an iterator, you must obtain one. Each of the collection classes provides an iterator( ) method that returns an iterator to the start of the collection. By using this iterator object, you can access each element in the collection, one element at a time.

In general, to use an iterator to cycle through the contents of a collection, follow these steps −

• Obtain an iterator to the start of the collection by calling the collection's iterator( ) method.

• Set up a loop that makes a call to hasNext( ). Have the loop iterate as long as hasNext( ) returns true.

• Within the loop, obtain each element by calling next( ).

For collections that implement List, you can also obtain an iterator by calling ListIterator.

The Methods Declared by Iterator

The Methods Declared by ListIterator

Example 1

Here is an example demonstrating Iterator. It uses an ArrayList object, but the general principles apply to any type of collection.

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

public class IteratorDemo {

   public static void main(String args[]) {
      // Create an array list
      List<String> al = new ArrayList<>();
      
      // add elements to the array list
      al.add("C");
      al.add("A");
      al.add("E");
      al.add("B");
      al.add("D");
      al.add("F");

      // Use iterator to display contents of al
      System.out.print("Original contents of al: ");
      Iterator<String> itr = al.iterator();
      
      while(itr.hasNext()) {
         Object element = itr.next();
         System.out.print(element + " ");
      }
      System.out.println();
   }
}

Output

Original contents of al: C A E B D F

Example 2

Here is an example demonstrating ListIterator. It uses an ArrayList object, but the general principles apply to any type of collection.

Of course, ListIterator is available only to those collections that implement the List interface.

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.ListIterator;
public class IteratorDemo {

   public static void main(String args[]) {
      // Create an array list
      List<String> al = new ArrayList<>();
      
      // add elements to the array list
      al.add("C");
      al.add("A");
      al.add("E");
      al.add("B");
      al.add("D");
      al.add("F");

      // Use iterator to display contents of al
      System.out.print("Original contents of al: ");
      Iterator<String> itr = al.iterator();
      
      while(itr.hasNext()) {
         Object element = itr.next();
         System.out.print(element + " ");
      }
   }
}

Output

Original contents of al: C A E B D F 

Example 3

Here is an example demonstrating ListIterator to modify the list while iterating. It uses an ArrayList object, but the general principles apply to any type of collection.

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.ListIterator;
public class IteratorDemo {

   public static void main(String args[]) {
      // Create an array list
      List<String> al = new ArrayList<>();
      
      // add elements to the array list
      al.add("C");
      al.add("A");
      al.add("E");
      al.add("B");
      al.add("D");
      al.add("F");

      // Use iterator to display contents of al
      System.out.print("Original contents of al: ");
      Iterator<String> itr = al.iterator();
      
      while(itr.hasNext()) {
         Object element = itr.next();
         System.out.print(element + " ");
      }
      System.out.println();
      
      // Modify objects being iterated
      ListIterator<String> litr = al.listIterator();
      
      while(litr.hasNext()) {
         Object element = litr.next();
         litr.set(element + "+");
      }
      System.out.print("Modified contents of al: ");
      itr = al.iterator();
      
      while(itr.hasNext()) {
         Object element = itr.next();
         System.out.print(element + " ");
      }
      System.out.println();

      // Now, display the list backwards
      System.out.print("Modified list backwards: ");
      
      while(litr.hasPrevious()) {
         Object element = litr.previous();
         System.out.print(element + " ");
      }
      System.out.println();
   }
}

Output

Original contents of al: C A E B D F 
Modified contents of al: C+ A+ E+ B+ D+ F+ 
Modified list backwards: F+ D+ B+ E+ A+ C+