Convert int[] into ArrayList [duplicate]

Question

What is best way recopying int[] array elements into ArrayList?

I need to do this fast so what would be the fastest way?

Answer 1

ArrayList is not the best storage for primitives such as int. ArrayList stores objects not primitives. There are libraries out there for Lists with primitives. If you really want to store all primitive int as anInteger object you will have to convert them one by one using Integer.valueOf(...) (so you don't create extra Integer instances when not needed).

While you can do this:

List<Integer> list = Arrays.asList(1, 2, 3);

You cannot do this:

int[] i = { 1, 2, 3 };
List<Integer> list = Arrays.asList(i); // this will get you List<int[]>

as an array is actually an object as well an treated as such in var-args when using primitives such as int. Object arrays works though:

List<Object> list = Arrays.asList(new Object[2]);

So something along the lines of:

int[] array = { 1, 2, 3 };
ArrayList<Integer> list = new ArrayList<Integer>(array.length);
for (int i = 0; i < array.length; i++)
  list.add(Integer.valueOf(array[i]));

Answer 2

Use Arrays#asList(T... a) to create "a fixed-size list backed by the specified array. (Changes to the returned list "write through" to the array.)"

Integer[] intArray = {1, 2, 3, 42}; // cannot use int[] here
List<Integer> intList = Arrays.asList(intArray);

Alternatively, to decouple the two data structures:

List<Integer> intList = new ArrayList<Integer>(intArray.length);

for (int i=0; i<intArray.length; i++)
{
    intList.add(intArray[i]);
}

Or even more tersely:

List<Integer> intList = new ArrayList<Integer>(Arrays.asList(intArray));