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

If you're using Eclipse Collections, you can use the collectInt() method to switch from an object container to a primitive int container.

List<Integer> integers = new ArrayList<>(Arrays.asList(1, 2, 3, 4, 5));
MutableIntList intList =
  ListAdapter.adapt(integers).collectInt(i -> i);
Assert.assertArrayEquals(new int[]{1, 2, 3, 4, 5}, intList.toArray());

If you can convert your ArrayList to a FastList, you can get rid of the adapter.

Assert.assertArrayEquals(
  new int[]{1, 2, 3, 4, 5},
  Lists.mutable.with(1, 2, 3, 4, 5)
    .collectInt(i -> i).toArray());

Note: I am a committer for Eclipse collections.

Answer 2

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]

Answer 3

This code segment is working for me, try this:

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

Worth to mention ArrayList should be declared like this:

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

Answer 4

It bewilders me that we encourage one-off custom methods whenever a perfectly good, well used library like Apache Commons has solved the problem already. Though the solution is trivial if not absurd, it is irresponsible to encourage such a behavior due to long term maintenance and accessibility.

Just go with Apache Commons

Answer 5

Apache Commons has a ArrayUtils class, which has a method toPrimitive() that does exactly this.

import org.apache.commons.lang.ArrayUtils;
...
    List<Integer> list = new ArrayList<Integer>();
    list.add(new Integer(1));
    list.add(new Integer(2));
    int[] intArray = ArrayUtils.toPrimitive(list.toArray(new Integer[0]));

However, as Jon showed, it is pretty easy to do this by yourself instead of using external libraries.

Answer 6

Next lines you can find convertion from int[] -> List -> int[]

   private static int[] convert(int[] arr) {
        List<Integer> myList=new ArrayList<Integer>();
        for(int number:arr){
               myList.add(number);
            }
        }
        int[] myArray=new int[myList.size()];
        for(int i=0;i<myList.size();i++){
           myArray[i]=myList.get(i);
        }
        return myArray;
    }