public class Reverser {
    /**
     * Converts an int array into a string for easy printing.
     *
     * @param arr The array to be printed.
     */
    public static String arrayToString(int[] arr)  {
        String s = "[ ";
        for (int i = 0; i < arr.length; i++) {
            s += arr[i];
            s += " ";
        }
        s += "]";
        return s;
    }

    /**
     * Reverses the order of elements in an int array,
     * in place.
     *
     * @param arr The array to be reversed.
     */
    public static void reverse(int[] arr) {
        // swap elements at i and j;
        // i is an element on the left,
        // j is an element on the right.
        for (int i = 0; i < arr.length / 2; i++) {
            int j = arr.length - i - 1;
            int tmp = arr[i];
            // uncomment to observe swaps
            // System.out.println("Swap i = " + i + " and j = " + j);
            arr[i] = arr[j];
            arr[j] = tmp;
        }
    }
    
    public static void main(String[] args) {
        // an array
        int[] arr = { 1, 2, 3, 4 };

        // print it
        System.out.println(arrayToString(arr));

        // reverse it ("in place")
        reverse(arr);

        // print it again
        System.out.println(arrayToString(arr));
    }
}