Quick Sort Implementation

Some small notes:

1. Instead of commenting here:

   void quicksort (int[] a, int lo, int hi) {
       //  lo is the lower index, hi is the upper index
       //  of the region of array a that is to be sorted
       int i=lo, j=hi, h;
   

   rename the variables:

   void quicksort (final int[] a, final int lowerIndex, final int upperIndex)
   

   It's easier to read.

   (Check Clean Code by Robert C. Martin, page 53-54 also.)

2. Try to minimize the scope of local variables. It's not necessary to declare them at the beginning of the method.

   (Effective Java, Second Edition, Item 45: Minimize the scope of local variables. (Google for "minimize the scope of local variables", it's on Google Books too.))

3. This:

   h=a[i]; 
   a[i]=a[j]; 
   a[j]=h;
   

   can be extracted out to a swap method:

   public void swap(final int[] arr, final int pos1, final int pos2) {
       final int temp = arr[pos1];
       arr[pos1] = arr[pos2];
       arr[pos2] = temp;
   }
   

4. Maybe you should provide an easier to use helper method too for the clients:

   public void quicksort(final int[] data) {
       if (data == null) {
           throw new NullPointerException("data cannot be null");
       }
       if (data.length == 0) {
           return;
       }
       quicksort(data, 0, data.length - 1);
   }
   

   Note the input validation.

You shorten the code a little bit, but it might hurt readability:

    a[i]=a[j]; 
    a[j]=h;
    i++; 
    j--;

    //can be written as:

    a[i++]=a[j]; 
    a[j--]=h;