Java – How to convert an ArrayList containing Integers to primitive int array

arraylistarraysjavaprimitive-types

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()]);

Best Answer

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]
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