How to reverse an array

Short answer

Swap the first and last elements, moving towards the middle:

void main() {
    int[] numbers = {1, 2, 3, 4, 5};
    for (int i = 0, j = numbers.length - 1; i < j; i++, j--) {
        int tmp = numbers[i];
        numbers[i] = numbers[j];
        numbers[j] = tmp;
    }
    IO.println(Arrays.toString(numbers));
}

Output:

[5, 4, 3, 2, 1]

An array of objects

Arrays.asList() gives a view of the array, so Collections.reverse() reverses the array itself:

void main() {
    String[] letters = {"a", "b", "c"};
    Collections.reverse(Arrays.asList(letters));
 
    IO.println(Arrays.toString(letters));
}

Output:

[c, b, a]

A new array

If you need the original array unchanged, create a reversed copy with a stream:

void main() {
    int[] numbers = {1, 2, 3};
    int[] reversed = IntStream.range(0, numbers.length)
            .map(i -> numbers[numbers.length - 1 - i])
            .toArray();
 
    IO.println(Arrays.toString(numbers));
    IO.println(Arrays.toString(reversed));
}

Output:

[1, 2, 3]
[3, 2, 1]

Common mistake

Running the loop to the end of the array instead of the middle. Each pair is swapped twice, and the array returns to the original order:

void main() {
    int[] numbers = {1, 2, 3, 4};
    for (int i = 0; i < numbers.length; i++) {
        int j = numbers.length - 1 - i;
        int tmp = numbers[i];
        numbers[i] = numbers[j];
        numbers[j] = tmp;
    }
    IO.println(Arrays.toString(numbers));
}

Output:

[1, 2, 3, 4]

Thanks for reading!

Short answer

An array of objects

A new array

Common mistake

Functions used in this recipe

Thanks for reading!