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 convert in Java?

List<Integer> x =  new ArrayList<Integer>();
int[] n = (int[])x.toArray(int[x.size()]);

Answer 1

If you are using java-8 there's also another way to do this.

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

What it does is:

• getting a Stream<Integer> from the list
• obtaining an IntStream by mapping each element to itself (identity function), unboxing the int value hold by each Integer object (done automatically since Java 5)
• getting the array of int by calling toArray

You could also explicitly call intValue via a method reference, i.e:

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

It's also worth mentioning that you could get a NullPointerException if you have any null reference in the list. This could be easily avoided by adding a filtering condition to the stream pipeline like this:

                       //.filter(Objects::nonNull) also works
int[] arr = list.stream().filter(i -> i != null).mapToInt(i -> i).toArray();

Example:

List<Integer> list = Arrays.asList(1, 2, 3, 4);
int[] arr = list.stream().mapToInt(i -> i).toArray(); //[1, 2, 3, 4]

list.set(1, null); //[1, null, 3, 4]
arr = list.stream().filter(i -> i != null).mapToInt(i -> i).toArray(); //[1, 3, 4]

Answer 2

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;
}

Answer 3

Google Guava

Google Guava provides a neat way to do this by calling Ints.toArray.

List<Integer> list = ...;
int[] values = Ints.toArray(list);

Answer 4

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.

Answer 5

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 6

Java 8:

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

Answer 7

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)

Answer 8

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 9

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 10

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;
   }
   

Answer 11

Java 8

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

OR 

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

Answer 12

It bewilders me that we encourage one-off custom methods whenever a perfectly good, well used library like Apache Commons has solved the problem already. Though the solution is trivial if not absurd, it is irresponsible to encourage such a behavior due to long term maintenance and accessibility.

Just go with Apache Commons

Answer 13

If you're using Eclipse Collections, you can use the collectInt() method to switch from an object container to a primitive int container.

List<Integer> integers = new ArrayList<>(Arrays.asList(1, 2, 3, 4, 5));
MutableIntList intList =
  ListAdapter.adapt(integers).collectInt(i -> i);
Assert.assertArrayEquals(new int[]{1, 2, 3, 4, 5}, intList.toArray());

If you can convert your ArrayList to a FastList, you can get rid of the adapter.

Assert.assertArrayEquals(
  new int[]{1, 2, 3, 4, 5},
  Lists.mutable.with(1, 2, 3, 4, 5)
    .collectInt(i -> i).toArray());

Note: I am a committer for Eclipse collections.

Answer 14

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 15

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;
    }

Answer 16

This code segment is working for me, try this:

Integer[] arr = x.toArray(new Integer[x.size()]);

Worth to mention ArrayList should be declared like this:

ArrayList<Integer> list = new ArrayList<>();

Answer 17

A very simple one-line solution is:

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

Answer 18

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

    list.add(1);
    list.add(2);

    int[] result = null;
    StringBuffer strBuffer = new StringBuffer();
    for (Object o : list) {
        strBuffer.append(o);
        result = new int[] { Integer.parseInt(strBuffer.toString()) };
        for (Integer i : result) {
            System.out.println(i);
        }
        strBuffer.delete(0, strBuffer.length());
    }

Answer 19

Integer[] arr = (Integer[]) x.toArray(new Integer[x.size()]);

access arr like normal int[].