How to convert an ArrayList containing Integers to primitive int array?

Question

I\'m trying to convert an ArrayList containing Integer objects to primitive int[] with the following piece of code, but it is throwing compile time error. Is it possible to

Answer 1

Java 8:

int[] intArr = Arrays.stream(integerList).mapToInt(i->i).toArray();

Answer 2

You can simply copy it to an array:

int[] arr = new int[list.size()];
for(int i = 0; i < list.size(); i++) {
    arr[i] = list.get(i);
}

Not too fancy; but, hey, it works...

Answer 3

using Dollar should be quite simple:

List<Integer> list = $(5).toList(); // the list 0, 1, 2, 3, 4  
int[] array = $($(list).toArray()).toIntArray();

I'm planning to improve the DSL in order to remove the intermediate toArray() call

Answer 4

I believe iterating using the List's iterator is a better idea, as list.get(i) can have poor performance depending on the List implementation:

private int[] buildIntArray(List<Integer> integers) {
    int[] ints = new int[integers.size()];
    int i = 0;
    for (Integer n : integers) {
        ints[i++] = n;
    }
    return ints;
}

Answer 5

This works nice for me :)

Found at https://www.techiedelight.com/convert-list-integer-array-int/

import java.util.Arrays;
import java.util.List;

class ListUtil
{
    // Program to convert list of integer to array of int 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));
    }
}

Answer 6

You can convert, but I don't think there's anything built in to do it automatically:

public static int[] convertIntegers(List<Integer> integers)
{
    int[] ret = new int[integers.size()];
    for (int i=0; i < ret.length; i++)
    {
        ret[i] = integers.get(i).intValue();
    }
    return ret;
}

(Note that this will throw a NullPointerException if either integers or any element within it is null.)

EDIT: As per comments, you may want to use the list iterator to avoid nasty costs with lists such as LinkedList:

public static int[] convertIntegers(List<Integer> integers)
{
    int[] ret = new int[integers.size()];
    Iterator<Integer> iterator = integers.iterator();
    for (int i = 0; i < ret.length; i++)
    {
        ret[i] = iterator.next().intValue();
    }
    return ret;
}