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; }