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

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;
}

Apache Commons has a ArrayUtils class, which has a method toPrimitive() that does exactly this.

import org.apache.commons.lang.ArrayUtils;
...
    List<Integer> list = new ArrayList<Integer>();
    list.add(new Integer(1));
    list.add(new Integer(2));
    int[] intArray = ArrayUtils.toPrimitive(list.toArray(new Integer[0]));

However, as Jon showed, it is pretty easy to do this by yourself instead of using external libraries.

Java 8:

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

Arrays.setAll()

    List<Integer> x = new ArrayList<>(Arrays.asList(7, 9, 13));
    int[] n = new int[x.size()];
    Arrays.setAll(n, x::get);

    System.out.println("Array of primitive ints: " + Arrays.toString(n));

Output:

Array of primitive ints: [7, 9, 13]

The same works for an array of long or double, but not for arrays of boolean, char, byte, short or float. If you’ve got a really huge list, there’s even a parallelSetAll method that you may use instead.

To me this is good and elgant enough that I wouldn’t want to get an external library nor use streams for it.

Documentation link: Arrays.setAll(int[], IntUnaryOperator)

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

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));
    }
}

Arrays.setAll() will work for most scenarios:

1. Integer List to primitive int array:

   public static int[] convert(final List<Integer> list)
   {
      final int[] out = new int[list.size()];
      Arrays.setAll(out, list::get);
      return out;
   }
   

2. Integer List (made of Strings) to primitive int array:

   public static int[] convert(final List<String> list)
   {
      final int[] out = new int[list.size()];
      Arrays.setAll(out, i -> Integer.parseInt(list.get(i)));
      return out;
   }
   

3. Integer array to primitive int array:

   public static int[] convert(final Integer[] array)
   {
      final int[] out = new int[array.length];
      Arrays.setAll(out, i -> array[i]);
      return out;
   }
   

4. Primitive int array to Integer array:

   public static Integer[] convert(final int[] array)
   {
      final Integer[] out = new Integer[array.length];
      Arrays.setAll(out, i -> array[i]);
      return out;
   }
   

Next lines you can find convertion from int[] -> List -> int[]

   private static int[] convert(int[] arr) {
        List<Integer> myList=new ArrayList<Integer>();
        for(int number:arr){
               myList.add(number);
            }
        }
        int[] myArray=new int[myList.size()];
        for(int i=0;i<myList.size();i++){
           myArray[i]=myList.get(i);
        }
        return myArray;
    }

Java 8

int[] array = list.stream().mapToInt(i->i).toArray();

OR 

int[] array = list.stream().mapToInt(Integer::intValue).toArray();

A very simple one-line solution is:

Integer[] i = arrlist.stream().toArray(Integer[]::new);