Convert an Iterator to a List in Java

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:
  1. Get the Iterator.
  2. Create an empty list.
  3. Add each element of the iterator to the list using forEachRemaining() method.
  4. 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:
  1. Get the Iterator.
  2. Convert the iterator to iterable using lambda expression.
  3. Convert the iterable to list using Stream.
  4. 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);     } }
  Output:

  [1, 2, 3, 4, 5]