Method retainAll

guava/src/com/google/common/collect/Maps.java:4215–4234

(Collection<?> c)

    }

    @Override
    public boolean retainAll(Collection<?> c) {
      try {
        return super.retainAll(checkNotNull(c));
      } catch (UnsupportedOperationException e) {
        // if the iterators don't support remove
        Set<@Nullable Object> keys = Sets.newHashSetWithExpectedSize(c.size());
        for (Object o : c) {
          /*
           * `o instanceof Entry` is guaranteed by `contains`, but we check it here to satisfy our
           * nullness checker.
           */
          if (contains(o) && o instanceof Entry) {
            Entry<?, ?> entry = (Entry<?, ?>) o;
            keys.add(entry.getKey());
          }
        }
        return map().keySet().retainAll(keys);
      }
    }
  }

  @GwtIncompatible // NavigableMap