android/guava/src/com/google/common/collect/Maps.java
public static <K, V> Map<K, V> filterKeys(
      Map<K, V> unfiltered, final Predicate<? super K> keyPredicate) {
    checkNotNull(keyPredicate);
    Predicate<Entry<K, ?>> entryPredicate = keyPredicateOnEntries(keyPredicate);
    return (unfiltered instanceof AbstractFilteredMap)
        ? filterFiltered((AbstractFilteredMap<K, V>) unfiltered, entryPredicate)
        : new FilteredKeyMap<K, V>(checkNotNull(unfiltered), keyPredicate, entryPredicate);
  }
public static <K, V> Map<K, V> filterKeys(
      Map<K, V> unfiltered, final Predicate<? super K> keyPredicate) {
    checkNotNull(keyPredicate);
    Predicate<Entry<K, ?>> entryPredicate = keyPredicateOnEntries(keyPredicate);
    return (unfiltered instanceof AbstractFilteredMap)
        ? filterFiltered((AbstractFilteredMap<K, V>) unfiltered, entryPredicate)
        : new FilteredKeyMap<K, V>(checkNotNull(unfiltered), keyPredicate, entryPredicate);
  }
Returns a 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.