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 worksint[] 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]

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;}

Google Guava

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

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