Convert an Iterator to a List in Java

Last Updated : 11 Dec, 2018

Given an Iterator, the task is to convert it into List in Java.

Examples:

  Input: Iterator = {1, 2, 3, 4, 5}  Output: {1, 2, 3, 4, 5}    Input: Iterator = {'G', 'e', 'e', 'k', 's'}  Output: {'G', 'e', 'e', 'k', 's'}

Below are the various ways to do so:

Naive Approach:

Get the Iterator.
Create an empty list.
Add each element of the iterator to the list using forEachRemaining() method.
Return the list.

Below is the implementation of the above approach:

   // Java program to get a List 
// from a given Iterator 
  
import java.util.*; 
  
class GFG { 
  
    // Function to get the List 
    public static <T> List<T> 
    getListFromIterator(Iterator<T> iterator) 
    { 
  
        // Create an empty list 
        List<T> list = new ArrayList<>(); 
  
        // Add each element of iterator to the List 
        iterator.forEachRemaining(list::add); 
  
        // Return the List 
        return list; 
    } 
  
    // Driver code 
    public static void main(String[] args) 
    { 
  
        // Get the Iterator 
        Iterator<Integer> 
            iterator = Arrays.asList(1, 2, 3, 4, 5) 
                           .iterator(); 
  
        // Get the List from the Iterator 
        List<Integer> 
            list = getListFromIterator(iterator); 
  
        // Print the list 
        System.out.println(list); 
    } 
} 
 
  

Output:

  [1, 2, 3, 4, 5]

Using Iterable as intermediate:

Get the Iterator.
Convert the iterator to iterable using lambda expression.
Convert the iterable to list using Stream.
Return the list.

Below is the implementation of the above approach:

   // Java program to get a List 
// from a given Iterator 
  
import java.util.*; 
import java.util.stream.Collectors; 
import java.util.stream.StreamSupport; 
  
class GFG { 
  
    // Function to get the List 
    public static <T> List<T> 
    getListFromIterator(Iterator<T> iterator) 
    { 
  
        // Convert iterator to iterable 
        Iterable<T> iterable = () -> iterator; 
  
        // Create a List from the Iterable 
        List<T> list = StreamSupport 
                           .stream(iterable.spliterator(), false) 
                           .collect(Collectors.toList()); 
  
        // Return the List 
        return list; 
    } 
  
    // Driver code 
    public static void main(String[] args) 
    { 
  
        // Get the Iterator 
        Iterator<Integer> 
            iterator = Arrays.asList(1, 2, 3, 4, 5) 
                           .iterator(); 
  
        // Get the List from the Iterator 
        List<Integer> 
            list = getListFromIterator(iterator); 
  
        // Print the list 
        System.out.println(list); 
    } 
} 
 
  