Convert int[] to List and Back Using Streams

1. int[] Array → List<Integer> Collection
2. List<Integer> Collection → int[] Array
3. Alternative: Manual Conversion Without Stream API
4. Why List<int> Is Not Allowed
5. Conclusion

Working with arrays and collections is a fundamental part of Java development. But they have different characteristics:

Arrays are fixed-length structures.

Collections (List, Set, etc.) are flexible containers that allow easy adding, removing, and filtering of data.

It's a common task to convert an int[] array to a List<Integer> and back. Below is how it's done using the Stream API.

1. int[] Array → List<Integer> Collection

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

List<Integer> list = Arrays.stream(numbers) // convert to IntStream
                           .boxed()         // box primitives to Integer
                           .collect(Collectors.toList());

System.out.println(list); // [1, 2, 3, 4, 5]

The boxed() method converts the primitive IntStream into a stream of objects Stream<Integer>, which we then collect into a list using collect(Collectors.toList()).

2. List<Integer> Collection → int[] Array

List<Integer> list = Arrays.asList(10, 20, 30, 40);

int[] array = list.stream()
                  .mapToInt(Integer::intValue)
                  .toArray();

System.out.println(Arrays.toString(array)); // [10, 20, 30, 40]

The mapToInt() method converts a stream of Integer objects back to a stream of int primitives, and .toArray() transforms it into an array.

3. Alternative: Manual Conversion Without Stream API

If you're working with older Java versions or prefer avoiding streams:

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

int[] backToArray = new int[list.size()];
for (int i = 0; i < list.size(); i++) {
    backToArray[i] = list.get(i);
}

4. Why List<int> Is Not Allowed

Java collections work only with objects, and int is a primitive. That's why we use the wrapper class Integer. Streams help us with boxing (boxed()) and unboxing (mapToInt()).

5. Conclusion

Use the Stream API if you want concise and readable code. If you need more control — classic loops are still a great alternative.