code: public static byte[] join(byte[] a, byte[] b) {
          byte[] result = new byte[a.length + b.length];
          System.arraycopy(a, 0, result, 0, a.length);
          System.arraycopy(b, 0, result, a.length, b.length);
          return result;
      }
docs: Joins two byte arrays into one.
  
  @param a
  first byte array to join, not null.
  @param b
  second byte array to join, not null.
  
  @return Joined byte array.