public static long calculateHash(Method method) 
{
      Map methodHashes = (Map)hashMap.get(method.getDeclaringClass());
      if (methodHashes == null)
      {
         methodHashes = getInterfaceHashes(method.getDeclaringClass());
         // Copy and add
         WeakHashMap newHashMap = new WeakHashMap();
         newHashMap.putAll(hashMap);
         newHashMap.put(method.getDeclaringClass(), methodHashes);
         hashMap = newHashMap;
      }
      return ((Long)methodHashes.get(method.toString())).longValue();
}

 public static Map getInterfaceHashes(Class intf) 
{
   
      // Create method hashes
      Method[] methods = intf.getMethods();
      HashMap map = new HashMap();
      for (int i = 0; i <  methods.length; i++)
      {
         Method method = methods[i];
         Class[] parameterTypes = method.getParameterTypes();
         String methodDesc = method.getName()+"(";
         for(int j = 0; j <  parameterTypes.length; j++)
         {
            methodDesc += getTypeString(parameterTypes[j]);
         }
         methodDesc += ")"+getTypeString(method.getReturnType());
         try
         {
            long hash = 0;
            ByteArrayOutputStream bytearrayoutputstream = new ByteArrayOutputStream(512);
            MessageDigest messagedigest = MessageDigest.getInstance("SHA");
            DataOutputStream dataoutputstream = new DataOutputStream(new DigestOutputStream(bytearrayoutputstream, messagedigest));
            dataoutputstream.writeUTF(methodDesc);
            dataoutputstream.flush();
            byte abyte0[] = messagedigest.digest();
            for(int j = 0; j <  Math.min(8, abyte0.length); j++)
               hash += (long)(abyte0[j] & 0xff) < <  j * 8;
            map.put(method.toString(), new Long(hash));
         }
         catch (Exception e)
         {
            e.printStackTrace();
         }
      }
      return map;
}

Calculate method hashes. This algo is taken from RMI.

 static String getTypeString(Class cl) 
{
      if (cl == Byte.TYPE)
      {
         return "B";
      } else if (cl == Character.TYPE)
      {
         return "C";
      } else if (cl == Double.TYPE)
      {
         return "D";
      } else if (cl == Float.TYPE)
      {
         return "F";
      } else if (cl == Integer.TYPE)
      {
         return "I";
      } else if (cl == Long.TYPE)
      {
         return "J";
      } else if (cl == Short.TYPE)
      {
         return "S";
      } else if (cl == Boolean.TYPE)
      {
         return "Z";
      } else if (cl == Void.TYPE)
      {
         return "V";
      } else if (cl.isArray())
      {
         return "["+getTypeString(cl.getComponentType());
      } else
      {
         return "L"+cl.getName().replace('.",'/")+";";
      }
}