Algorithm Implement of Sort - Mergesort

/***********************************************************************
  * Compilation: javac Merge.java
  * Execution: java Merge
  * An bare-bone NlogN implementation of mergesort.
**************************************************************************/
public class Merge {

    public static void sort(Comparable[] a) {
        sort(a, 0, a.length);
    }

    public static void sort(Comparable[] a, int lo, int hi) {
        int N = hi - lo; // size of subarray
        if (N <= 1) return; // 0 or 1 sub-array, so we're done

        // recursively sort left and right halves
        int mid = lo + N/2;
        sort (a, lo, mid);
        sort (a, mid, hi);

        // merge two sorted subarrays
        int i = lo, j = mid;
        Comparable[] aux = new Comparable[N];
        for (int k = 0; k < N; k++) {
            if (i == mid) aux[k] = a[j++];
            else if (j == hi) aux[k] = a[i++];
            else if (a[j].compareTo(a[i]) < 0) aux[k] = a[j++];
            else aux[k] = a[i++];
        }

        // copy back to array a[]
        for (int k = 0; k < N; k++)
            a[lo+k] = aux[k];

    }

    /**
     * Check if array is sorted
     * */
    public static boolean isSorted(Comparable[] a) {
        for (int i = 1; i < a.length; i++)
            if (a[i].compareTo(a[i-1]) < 0) return false;
        return true;
    }

    public static void show(Comparable[] a) {
        for (int i = 0; i < a.length; i++)
            System.out.println(a[i]);
    }

    public static void main(String[] args) {
     int N = Integer.parseInt(args[0]);

     // generate N random real number between 0 and 1
     long start = System.currentTimeMillis();
     Double[] a = new Double[N];
     for (int i = 0; i < N; i++)
         a[i] = Math.random();
     long stop = System.currentTimeMillis();
     double elapsed = (stop - start) / 1000.0;
     System.out.println("Generating input: " + elapsed + " seconds");

     // sort them
     start = System.currentTimeMillis();
     sort(a);
     stop = System.currentTimeMillis();
     elapsed = (stop - start) / 1000.0;
     System.out.println("Mergesort: " + elapsed + " seconds");
     System.out.println("The array sorted is " + isSorted(a));

     if (N < 100) show(a);
    }
}