Table of contents

• Given problem
• Using Arrays utitlity class
• Using Collections class
• Using the traditional for loop
• Wrapping up

Given problem

When we work on some projects, or practice coding in LeetCode, sometimes we need to do conversion from int[] to List<Integer>. This is a tedious task.

Normally, we will think about Arrays.asList() method to convert int[] to List<Integer>. But this doesn’t solve our issue because asList() method will return List<int[]>.

public class Arrays {
    ...

    public static <T> asList(T... a) {
        return new ArrayList<>(a);
    }

    ...
}

So how do we convert int[] to List<Integer> with elegant solutions?

Using Arrays utitlity class

1. Below is the solution that we use stream() method of Arrays class.

    int[] rawData = {1, 3, 5};
    List<Integer> data = Arrays.stream(rawData)     // IntStream
                            .boxed()             // Stream<Integer>
                            .collect(Collectors.toList());
   

   The definition of Arrays.stream() method is:

    public class Arrays {
        ...
   
        public static IntStream stream(int[] array) {
            return stream(array, 0, array.length);
        }
   
        ...
    }
   

2. Using forEach() method to add elements.

    int[] rawData = {1, 3, 5};
   
    List<Integer> data = new ArrayList<Integer>();
    Arrays.stream(rawData).forEach(data::add);
   

3. To reduce this version, we can use IntStream.of() method.

    int[] rawData = {1, 3, 5};
    List<Integer> data = IntStream.of(rawData)      // IntStream
                                .boxed()             // Stream<Integer>
                                .collect(Collectors.toList());
   

   Then, we have the definition of IntStream.of() method in Java.

    public interface IntStream extends BaseStream<Integer, IntStream> {
        ...
   
        public static IntStream of(int... values) {
            return Arrays.stream(values);
        }
   
        ...
    }
   

4. Using the improved version of Arrays.stream() in Java 16+.

    List<Integer> data = Arrays.stream(rawData)
                               .boxed()
                               .toList();
   

Using Collections class

int[] rawData = {1, 3, 5};
List<Integer> data = new ArrayList<Integer>();
Collections.addAll(data, Arrays.stream(rawData).boxed().toArray(Integer::new));

Then, below is the definition of Collections.addAll() method.

public class Collections {

    ...

    public static <T> boolean addAll(Collection<? super T> c, T... elements) {
        boolean result = false;
        for (T element : elements)
            result |= c.add(element);
        return result;
    }

    ...
}

Because the addAll() method needs the second arguments as the array of Integer. So we will convert the int[] to Integer[], then pass it as the argument.

Using the traditional for loop

int[] rawData = {1, 3, 5};
List<Integer> data = new ArrayList<Integer>();

for (int num : rawData) {
    data.add(num);
}

Wrapping up