Question

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How to convert an ArrayList containing Integers to primitive int array?

I tried the code below, but it does not work. Do I have to make a loop copy the ArrayList into an array ?

int[]intArray = (int[]) integerArrayList.toArray();

Answer 1

If you want primitive type array, you can use Apache Commons ArrayUtils#toPrimitive method to get primitive array from wrapper array: –

public static int[] toPrimitive(Integer[] array)

You can use it like this: –

int[] arr = ArrayUtils.toPrimitive((Integer[])integerArrayList.toArray());

Or, you can use the 1-arg version of toArray method, that takes an array and returns the array of that type only. That way, you won’t have to the typecasting.

List<Integer> myList = new ArrayList<Integer>();
Integer[] wrapperArr = myList.toArray(new Integer[myList.size()]);

// If you want a `primitive` type array
int[] arr = ArrayUtils.toPrimitive(wrapperArr);

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However, if you are Ok with the wrapper type array (Which certainly will not trouble you), then you don’t need to do that last step.

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Also, you can also create a primitive array, without having to create that intermediate Wrapper type array. For that you would have to write it through loop: –

int[] arr = new int[myList.size()];

for(int i = 0; i < myList.size(); i++) {
    if (myList.get(i) != null) {
        arr[i] = myList.get(i);
    }
}