Convert Set of Integer to array of int in Java

This post will discuss how to convert a set 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 Set<Integer> to a primitive integer array. Following are the complete steps with an explanation:

Convert given Set<Integer> to Stream<Integer> using Set.stream() method.
Convert Stream<Integer> to IntStream.

We can’t directly convert Stream<Integer> to int[] as Stream.toArray() returns an Object[] and Stream.toArray(IntFunction<A[]>) returns a generic A[]. Fortunately, we have IntStream that can handle primitive int, whose toArray() method returns int[].

Now the problem reduces to how to convert Stream<Integer> to IntStream. There are many ways to map Integer to their primitive form:

mapToInt(Integer::intValue)    // method reference
mapToInt(i -> i.intValue())    // Explicit unboxing with a lambda expression
mapToInt(i -> i)                               // Implicit auto-unboxing with a lambda expression

import java.util.Arrays;

import java.util.Set;

import java.util.HashSet;

class Main

{

// Program to convert a `Set<Integer>` to a primitive integer array in Java

public static void main(String[] args)

{

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

int[] primitive = ints.stream()

.mapToInt(Integer::intValue)

.toArray();

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

}

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Output:

[1, 2, 3, 4, 5]

IntStream.toArray() throws a NullPointerException if any null values are present in the set. We can easily filter out the null values before mapping:

int[] primitive = set.stream()

.filter(Objects::nonNull)

.mapToInt(Integer::intValue)

.toArray();

2. Using Guava Library

Guava’s Ints.toArray() can also convert a Set<Integer> to a primitive integer array.

import com.google.common.primitives.Ints;

import java.util.Arrays;

import java.util.Set;

import java.util.HashSet;

class Main

{

// Program to convert a `Set<Integer>` to a primitive integer array using Guava

public static void main(String[] args)

{

Set<Integer> ints = new HashSet<>(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 set. We can filter null values before passing it to Ints.toArray() as seen before.

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

.filter(Objects::nonNull)

.collect(Collectors.toSet()));

3. Using Apache Commons Lang

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

import org.apache.commons.lang3.ArrayUtils;

import java.util.Arrays;

import java.util.Set;

import java.util.HashSet;

class Main

{

// Program to convert a `Set<Integer>` to a primitive integer array in Java

public static void main(String[] args)

{

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

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

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

}

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toPrimitive() throws a NullPointerException if any null values are present in the set. 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);

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

Convert Set of Integer to array of int in Java

1. Using Java 8

2. Using Guava Library

3. Using Apache Commons Lang

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