How to convert int[] into List in Java?

Question

How do I convert int[] into List in Java?

Of course, I\'m interested in any other answer than doing it in a loop, item by it

Answer 1

In Java 8 :

int[] arr = {1,2,3};
IntStream.of(arr).boxed().collect(Collectors.toList());

Answer 2

If you are using java 8, we can use the stream API to convert it into a list.

List<Integer> list = Arrays.stream(arr)     // IntStream 
                                .boxed()        // Stream<Integer>
                                .collect(Collectors.toList());

You can also use the IntStream to convert as well.

List<Integer> list = IntStream.of(arr) // return Intstream
                                    .boxed()        // Stream<Integer>
                                    .collect(Collectors.toList());

There are other external library like guava and apache commons also available convert it.

cheers.

Answer 3

Here is a generic way to convert array to ArrayList

<T> ArrayList<T> toArrayList(Object o, Class<T> type){
    ArrayList<T> objects = new ArrayList<>();
    for (int i = 0; i < Array.getLength(o); i++) {
        //noinspection unchecked
        objects.add((T) Array.get(o, i));
    }
    return objects;
}

Usage

ArrayList<Integer> list = toArrayList(new int[]{1,2,3}, Integer.class);

Answer 4

There is no shortcut for converting from int[] to List<Integer> as Arrays.asList does not deal with boxing and will just create a List<int[]> which is not what you want. You have to make a utility method.

int[] ints = {1, 2, 3};
List<Integer> intList = new ArrayList<Integer>(ints.length);
for (int i : ints)
{
    intList.add(i);
}

Answer 5

It's also worth checking out this bug report, which was closed with reason "Not a defect" and the following text:

"Autoboxing of entire arrays is not specified behavior, for good reason. It can be prohibitively expensive for large arrays."

Answer 6

Here is a solution:

int[] array = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

Integer[] iArray = Arrays.stream(array).boxed().toArray(Integer[]::new);
System.out.println(Arrays.toString(iArray));

List<Integer> list = new ArrayList<>();
Collections.addAll(list, iArray);
System.out.println(list);

Output:

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]