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

Answer 1

Arrays.setAll()

    List<Integer> x = new ArrayList<>(Arrays.asList(7, 9, 13));
    int[] n = new int[x.size()];
    Arrays.setAll(n, x::get);

    System.out.println("Array of primitive ints: " + Arrays.toString(n));

Output:

Array of primitive ints: [7, 9, 13]

The same works for an array of long or double, but not for arrays of boolean, char, byte, short or float. If you’ve got a really huge list, there’s even a parallelSetAll method that you may use instead.

To me this is good and elgant enough that I wouldn’t want to get an external library nor use streams for it.

Documentation link: Arrays.setAll(int[], IntUnaryOperator)

Answer 2

A very simple one-line solution is:

Integer[] i = arrlist.stream().toArray(Integer[]::new);

Answer 3

Arrays.setAll() will work for most scenarios:

1. Integer List to primitive int array:

   public static int[] convert(final List<Integer> list)
   {
      final int[] out = new int[list.size()];
      Arrays.setAll(out, list::get);
      return out;
   }
   

2. Integer List (made of Strings) to primitive int array:

   public static int[] convert(final List<String> list)
   {
      final int[] out = new int[list.size()];
      Arrays.setAll(out, i -> Integer.parseInt(list.get(i)));
      return out;
   }
   

3. Integer array to primitive int array:

   public static int[] convert(final Integer[] array)
   {
      final int[] out = new int[array.length];
      Arrays.setAll(out, i -> array[i]);
      return out;
   }
   

4. Primitive int array to Integer array:

   public static Integer[] convert(final int[] array)
   {
      final Integer[] out = new Integer[array.length];
      Arrays.setAll(out, i -> array[i]);
      return out;
   }
   

Answer 4

   List<Integer> list = new ArrayList<Integer>();

    list.add(1);
    list.add(2);

    int[] result = null;
    StringBuffer strBuffer = new StringBuffer();
    for (Object o : list) {
        strBuffer.append(o);
        result = new int[] { Integer.parseInt(strBuffer.toString()) };
        for (Integer i : result) {
            System.out.println(i);
        }
        strBuffer.delete(0, strBuffer.length());
    }

Answer 5

Google Guava

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

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

Answer 6

Integer[] arr = (Integer[]) x.toArray(new Integer[x.size()]);

access arr like normal int[].