Convert an Iterator to a List in Java

An Iterator is used for cycling through a Collection of objects. Whereas a List is an Interface which is used for storing an ordered collection of elements. Both classes are part of the Java Collections Framework.

Converting an Iterator to a List in Java 

Sometimes we need to convert an Iterator to a collection. In this article, we will learn how to convert an Iterator to a List in Java using multiple ways. Those are:

• Using a while loop
• Using Iterator.forEachRemaining()
• Using Stream API

Let's understand each of them in detail.

Using a while Loop

In this method, we will use a while loop to iterate over the Iterator and add each element to a new List. We will use the method hasNext() to check if there are more elements in the Iterator. If there are more elements, we will use the method next() to get the next element and add it to the List.

The syntax of the hasNext() and next() methods is as follows -

public boolean hasNext()
public E next() throws NoSuchElementException

Example

Following is the code to convert an Iterator to a List using a while loop -

import java.util.*;
import java.io.*;

public class IteratorToList {
   public static void main(String[] args) {
      // Create an Iterator object
      Iterator<String> iterator = Arrays.asList("A", "B", "C", "D", "E").iterator();
      // Create a List object
      List<String> list = new ArrayList<>();
      // Iterate over the Iterator and add each element to the List
      while(iterator.hasNext()) {
         list.add(iterator.next());
      }
      // Print the List
      System.out.println("List: " + list);
   }
}

Output

The output of the above code will be:

List: [A, B, C, D, E]

Using the Iterator.forEachRemaining() Method

In this method, we will use the Iterator.forEachRemaining() method to convert an Iterator to a List. The forEachRemaining() method is a default method in the Iterator interface. It performs the given action for each remaining element in the iteration until all elements have been processed or the action throws an exception.

The syntax of the forEachRemaining() method is as follows:

public default void forEachRemaining(Consumer<? super E> action)

Example

Following is the code to convert an Iterator to a List using the forEachRemaining() method:

import java.util.*;
import java.io.*;
import java.util.function.Consumer;

public class IteratorToList {
   public static void main(String[] args) {
      // Create an Iterator object
      Iterator<String> iterator = Arrays.asList("A", "B", "C", "D", "E").iterator();
      // Create a List object
      List<String> list = new ArrayList<>();
      // Use forEachRemaining() method to add each element to the List
      iterator.forEachRemaining(new Consumer<String>() {
         public void accept(String element) {
            list.add(element);
         }
      });
      // Print the List
      System.out.println("List: " + list);
   }
}

Output

The output of the above code will be:

List: [A, B, C, D, E]

Using the Stream API

We will use the Stream API to convert an Iterator to a List. The Stream API is a new abstraction introduced in Java 8. It allows us to process sequences of elements in a functional style.

We will use the StreamSupport.stream() method to convert the Iterator to a Stream. Then we will use the collect() method to convert the Stream to a List. The syntax of the StreamSupport.stream() and collect() methods is as follows:

public static <T> Stream<T> stream(Iterator<T> iterator, boolean parallel)
public final <R,A> R collect(Collector<? super T, A, R> collector)

Example

Following is the code to convert an Iterator to a List using the Stream API -

import java.util.*;
import java.io.*;
import java.util.stream.*;

public class IteratorToList {
   public static void main(String[] args) {
      // Create an Iterator object
      Iterator<String> iterator = Arrays.asList("A", "B", "C", "D", "E").iterator();
      // Convert the Iterator to a List using Stream API
      List<String> list = StreamSupport.stream(((Iterable<String>) () -> iterator).spliterator(), false)
         .collect(Collectors.toList());
      // Print the List
      System.out.println("List: " + list);
   }
}

Output

The output of the above code will be -

List: [A, B, C, D, E]