https://codeahoy.com/java/How-To-Convery-ArrayList-To-Array/

Method 1-

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;
}

Method 2-

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

Method 3- using Steam
int[] arr = list.stream().mapToInt(i -> i).toArray();

(from stackoverflow)

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 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();