Radix sort in Java

   /** frequency of the byte */
   public static int[] freq = new int[256];

   /** number of positions */
   public static final int KEYLEN = 4;

   /** Get the value of the position i. */
   public static int getValue (int key, int i) {
      return (key >>> (8*i))&0xff;
   } // getValue

   /** Sort non-negative keys by position i in a stable manner. */
   public static int[] countSort (int[] keys, int i) {
      if (keys == null) return null;
      int[] res = new int [keys.length];
      for (int k=0; k < freq.length; k++) {
         freq [k] = 0;
      }
      for (int j=0; j < keys.length; j++) {
         freq [getValue (keys[j], i)]++;
      }
      for (int k=1; k < freq.length; k++) {
         freq [k] = freq[k-1] + freq[k];
      }
      for (int j=keys.length-1; j >= 0; j--) {
         int ind = --freq [getValue (keys[j], i)];
         res[ind] = keys[j];
      }
      return res;
   } // countSort

   /** Radix sort for non-negative integers. */
   public static void radixSort (int[] keys) {
      if (keys == null) return;
      int[] res = keys;
      for (int p=0; p < KEYLEN; p++) {
         res = countSort (res, p);
      }
      for (int i=0; i < keys.length; i++) {
         keys[i] = res[i];
      }
   } // radixSort