code: public static <K, V> SortedMap<K, V> filterKeys(
        SortedMap<K, V> unfiltered, final Predicate<? super K> keyPredicate) {
      // TODO(lowasser): Return a subclass of Maps.FilteredKeyMap for slightly better
      // performance.
      return filterEntries(unfiltered, Maps.<K>keyPredicateOnEntries(keyPredicate));
    }
docs: Returns a sorted map containing the mappings in {@code unfiltered} whose keys satisfy a
  predicate. The returned map is a live view of {@code unfiltered}; changes to one affect the
  other.
  
  <p>The resulting map's {@code keySet()}, {@code entrySet()}, and {@code values()} views have
  iterators that don't support {@code remove()}, but all other methods are supported by the map
  and its views. When given a key that doesn't satisfy the predicate, the map's {@code put()} and
  {@code putAll()} methods throw an {@link IllegalArgumentException}.
  
  <p>When methods such as {@code removeAll()} and {@code clear()} are called on the filtered map
  or its views, only mappings whose keys satisfy the filter will be removed from the underlying
  map.
  
  <p>The returned map isn't threadsafe or serializable, even if {@code unfiltered} is.
  
  <p>Many of the filtered map's methods, such as {@code size()}, iterate across every key/value
  mapping in the underlying map and determine which satisfy the filter. When a live view is
  <i>not</i> needed, it may be faster to copy the filtered map and use the copy.
  
  <p><b>Warning:</b> {@code keyPredicate} must be <i>consistent with equals</i>, as documented at
  {@link Predicate#apply}. Do not provide a predicate such as {@code
  Predicates.instanceOf(ArrayList.class)}, which is inconsistent with equals.
  
  @since 11.0