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

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;
}