[java] How to convert int[] into List<Integer> in Java?

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.

Streams

In Java 8 you can do this

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

Also from guava libraries... com.google.common.primitives.Ints:

List<Integer> Ints.asList(int...)

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

What about this:

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

In Java 8 with stream:

int[] ints = {1, 2, 3};
List<Integer> list = new ArrayList<Integer>();
Collections.addAll(list, Arrays.stream(ints).boxed().toArray(Integer[]::new));

or with Collectors

List<Integer> list =  Arrays.stream(ints).boxed().collect(Collectors.toList());

If you're open to using a third party library, this will work in Eclipse Collections:

int[] a = {1, 2, 3};
List<Integer> integers = IntLists.mutable.with(a).collect(i -> i);
Assert.assertEquals(Lists.mutable.with(1, 2, 3), integers);

Note: I am a committer for Eclipse Collections.

   /* 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);

In Java 8 :

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

Arrays.stream(ints).forEach(list::add);

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);

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."

The best shot:

**
 * Integer modifiable fix length list of an int array or many int's.
 *
 * @author Daniel De Leon.
 */
public class IntegerListWrap extends AbstractList<Integer> {

    int[] data;

    public IntegerListWrap(int... data) {
        this.data = data;
    }

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

    @Override
    public Integer set(int index, Integer element) {
        int r = data[index];
        data[index] = element;
        return r;
    }

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

• Support get and set.
• No memory data duplication.
• No wasting time in loops.

Examples:

int[] intArray = new int[]{1, 2, 3};
List<Integer> integerListWrap = new IntegerListWrap(intArray);
List<Integer> integerListWrap1 = new IntegerListWrap(1, 2, 3);

The smallest piece of code would be:

public List<Integer> myWork(int[] array) {
    return Arrays.asList(ArrayUtils.toObject(array));
}

where ArrayUtils comes from commons-lang :)

Here is another possibility, again with Java 8 Streams:

void intArrayToListOfIntegers(int[] arr, List<Integer> list) {
    IntStream.range(0, arr.length).forEach(i -> list.add(arr[i]));
}

I'll add another answer with a different method; no loop but an anonymous class that will utilize the autoboxing features:

public List<Integer> asList(final int[] is)
{
    return new AbstractList<Integer>() {
            public Integer get(int i) { return is[i]; }
            public int size() { return is.length; }
    };
}

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.

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);

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]

int[] arr = { 1, 2, 3, 4, 5 };

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

see this