Convert Integer List to an int array in Java

This post will discuss how to convert a list of Integer to a primitive integer array in Java.

1. Using Java 8

We can use the Stream provided by Java 8 to convert a list of Integer to a primitive integer array in Java. We start by converting given List<Integer> to Stream<Integer> using List.stream() method. Now all we need to do is convert Stream<Integer> to int[]. Please refer to this post for a detailed explanation of it.

import java.util.Arrays;

import java.util.List;

class Main

{

// Program to convert a list of Integer to a primitive integer array in Java

public static void main(String[] args)

{

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

int[] primitive = list.stream()

.mapToInt(Integer::intValue)

.toArray();

System.out.println(Arrays.toString(primitive));

}

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Output:

[1, 2, 3, 4, 5]

How to handle null values in the list?

IntStream.toArray() throws a NullPointerException if any null values are present in the list. There are many options to handle null values:

1. Filter out the null values before mapping:

int[] primitive = list.stream()

.filter(Objects::nonNull)

.mapToInt(Integer::intValue)

.toArray();

2. Map the null values to a default value:

int[] primitive = list.stream()

.map(i -> (i == null ? 0 : i))

.mapToInt(Integer::intValue)

.toArray();

3. Handle null inside the lambda expression itself:

int[] primitive = list.stream()

.mapToInt(i -> (i == null ? 0 : i))

.toArray();

2. Using Apache Commons Lang

Apache Commons Lang’s ArrayUtils class provides a toPrimitive() method that can convert an Integer array to primitive ints. We need to convert a list of integers to an Integer array first. We can use List.toArray() for easy conversion.

import org.apache.commons.lang3.ArrayUtils;

import java.util.Arrays;

import java.util.List;

class Main

{

// Program to convert a list of Integer to a primitive integer array in Java

public static void main(String[] args)

{

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

int[] primitive = ArrayUtils.toPrimitive(list.toArray(new Integer[0]));

System.out.println(Arrays.toString(primitive));

}

toPrimitive() throws a NullPointerException if any null values are present in the list. We can filter null values before passing them to the function, as shown below:

1 2	Integer[] boxed = ints.stream().filter(Objects::nonNull).toArray(Integer[]::new); int[] primitive = ArrayUtils.toPrimitive(boxed);

3. Using Guava Library

Guava’s Ints.toArray() can also convert a list of Integer to a primitive integer array.

import com.google.common.primitives.Ints;

import java.util.Arrays;

import java.util.List;

class Main

{

// Program to convert a list of Integer to a primitive integer array using Guava

public static void main(String[] args)

{

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

int[] primitive = Ints.toArray(ints);

System.out.println(Arrays.toString(primitive));

}

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Output:

[1, 2, 3, 4, 5]

Ints.toArray() throws a NullPointerException if any null values are present in the list. We can filter null values before passing it to Ints.toArray() as seen in previous sections.

int[] primitive = Ints.toArray(ints.stream()

.filter(Objects::nonNull)

.collect(Collectors.toList()));

That’s all about converting Integer List to an array of int in Java.

Convert Integer List to an int array in Java

1. Using Java 8

How to handle null values in the list?

2. Using Apache Commons Lang

3. Using Guava Library

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