code: @SuppressWarnings("unchecked")
    public static <E, N extends Number> Counter<E> fromMap(final Map<E, N> map) {
      if (map.isEmpty()) {
        throw new IllegalArgumentException("Map must have at least one element" + " to infer numeric type; add an element first or use e.g." + " fromMap(map, Integer.class)");
      }
      return fromMap(map, (Class) map.values().iterator().next().getClass());
    }
docs: Returns a counter view of the given map. Infers the numeric type of the
  values from the first element in map.values().