Convert an Iterator to a List in Java

This post will discuss how to convert an iterator to a list in Java.

1. Naive solution

A simple solution is to create an empty list and add each remaining element of the iterator to that list.

import java.util.ArrayList;

import java.util.Arrays;

import java.util.Iterator;

import java.util.List;

class Main

{

// Convert an Iterator to a List in Java 8 and above

public static void main(String[] args)

{

Iterator<Integer> iterator = Arrays.asList(1, 2, 3, 4, 5).iterator();

List<Integer> mutableList = new ArrayList<>();

iterator.forEachRemaining(mutableList::add); // Java 8 and above

System.out.println(mutableList);

}

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Output:

[1, 2, 3, 4, 5]

2. Converting the Iterator to an Iterable

Here, the idea is to convert the Iterator to an Iterable, which can be easily done by using lambda in Java 8 and above. Then we convert the iterable to stream using the StreamSupport.stream() method. Finally, we get a list from the stream using a collector.

import java.util.Arrays;

import java.util.Iterator;

import java.util.List;

import java.util.stream.Collectors;

import java.util.stream.StreamSupport;

class Main

{

// Convert an Iterator to a List in Java 8 and above

public static void main(String[] args)

{

Iterator<Integer> iterator = Arrays.asList(1, 2, 3, 4, 5).iterator();

// Convert Iterator to Iterable

Iterable<Integer> iterable = () -> iterator;

List<Integer> mutableList = StreamSupport

.stream(iterable.spliterator(), false)

.collect(Collectors.toList());

System.out.println(mutableList);

}

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Output:

[1, 2, 3, 4, 5]

3. Using Guava Library

Guava library provides several utility methods to get a mutable or an immutable list from an iterator, as shown below:

import com.google.common.collect.ImmutableList;

import com.google.common.collect.Lists;

import java.util.Arrays;

import java.util.Iterator;

import java.util.List;

class Main

{

// Convert an Iterator to a list in Java

public static void main(String[] args)

{

// 1. Convert Iterator to a mutable list in Java

Iterator<Integer> itr = Arrays.asList(1, 2, 3, 4, 5).iterator();

List<Integer> mutableList = Lists.newArrayList(itr);

System.out.println(mutableList);

// 2. Convert Iterator to an immutable list in Java

Iterator<String> it = Arrays.asList("A", "B", "C").iterator();

List<String> immutableList = ImmutableList.copyOf(it);

System.out.println(immutableList);

}

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Output:

[1, 2, 3, 4, 5]
[A, B, C]

4. Using Apache Commons Collections

Similar to Guava library, Apache Commons Collections also provides static utility methods and decorators for Iterator instances. To serve our purpose, we can use the IteratorUtils.toList() method to get a list based on an iterator.

import org.apache.commons.collections4.IteratorUtils;

import java.util.Arrays;

import java.util.Iterator;

import java.util.List;

class Main

{

// Convert an Iterator to a List in Java

public static void main(String[] args)

{

Iterator<Integer> iterator = Arrays.asList(1, 2, 3, 4, 5).iterator();

List<Integer> mutableList = IteratorUtils.toList(iterator);

System.out.println(mutableList);

}

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Output:

[1, 2, 3, 4, 5]

That’s all about converting an Iterator to a List in Java.

Convert an Iterator to a List in Java

1. Naive solution

2. Converting the Iterator to an Iterable

3. Using Guava Library

4. Using Apache Commons Collections

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