How to convert int[] into List in Java?

Question

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

Of course, I'm interested in any other answer than doing it in a loop, item by item. But if there's no other answer, I'll pick that one as the best to show the fact that this functionality is not part of Java.

Answer 1

   /* Integer[] to List<Integer> */



        Integer[] intArr = { 10, 20, 30, 40, 50, 60, 70, 80, 90, 100 };
        List<Integer> arrList = new ArrayList<>();
        arrList.addAll(Arrays.asList(intArr));
        System.out.println(arrList);


/* Integer[] to Collection<Integer> */


    Integer[] intArr = { 10, 20, 30, 40, 50, 60, 70, 80, 90, 100 };
    Collection<Integer> c = Arrays.asList(intArr);

Answer 2

Streams

In Java 8 you can do this

int[] ints = {1,2,3};
List<Integer> list = Arrays.stream(ints).boxed().collect(Collectors.toList());

Answer 3

give a try to this class:

class PrimitiveWrapper<T> extends AbstractList<T> {

    private final T[] data;

    private PrimitiveWrapper(T[] data) {
        this.data = data; // you can clone this array for preventing aliasing
    }

    public static <T> List<T> ofIntegers(int... data) {
        return new PrimitiveWrapper(toBoxedArray(Integer.class, data));
    }

    public static <T> List<T> ofCharacters(char... data) {
        return new PrimitiveWrapper(toBoxedArray(Character.class, data));
    }

    public static <T> List<T> ofDoubles(double... data) {
        return new PrimitiveWrapper(toBoxedArray(Double.class, data));
    }  

    // ditto for byte, float, boolean, long

    private static <T> T[] toBoxedArray(Class<T> boxClass, Object components) {
        final int length = Array.getLength(components);
        Object res = Array.newInstance(boxClass, length);

        for (int i = 0; i < length; i++) {
            Array.set(res, i, Array.get(components, i));
        }

        return (T[]) res;
    }

    @Override
    public T get(int index) {
        return data[index];
    }

    @Override
    public int size() {
        return data.length;
    }
}

testcase:

List<Integer> ints = PrimitiveWrapper.ofIntegers(10, 20);
List<Double> doubles = PrimitiveWrapper.ofDoubles(10, 20);
// etc

Answer 4

What about this:

int[] a = {1,2,3}; Integer[] b = ArrayUtils.toObject(a); List<Integer> c = Arrays.asList(b);

Answer 5

Arrays.asList will not work as some of the other answers expect.

This code will not create a list of 10 integers. It will print 1, not 10:

int arr[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
List lst = Arrays.asList(arr);
System.out.println(lst.size());

This will create a list of integers:

List<Integer> lst = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);

If you already have the array of ints, there is not quick way to convert, you're better off with the loop.

On the other hand, if your array has Objects, not primitives in it, Arrays.asList will work:

String str[] = { "Homer", "Marge", "Bart", "Lisa", "Maggie" };
List<String> lst = Arrays.asList(str);

Answer 6

In Java 8 :

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