NavigableMap<K, V> synchronizedNavigableMap(NavigableMap<K, V> navigableMap) {
    return Synchronized.navigableMap(navigableMap);
  }

  /**
   * {@code AbstractMap} extension that makes it easy to cache customized keySet, values, and
   * entrySet views.
   */
  @GwtCompatible
  abstract static class ViewCachingAbstractMap<
          K extends @Nullable Object, V extends @Nullable Object>

         * It's very unlikely that any loaded Futures class will see getChecked called with more
         * than a handful of exceptions. But it seems prudent to set a cap on how many we'll cache.
         * This avoids out-of-control memory consumption, and it keeps the cache from growing so
         * large that doing the lookup is noticeably slower than redoing the work would be.
         *

@GwtCompatible
@ElementTypesAreNonnullByDefault
public final class HttpHeaders {
  private HttpHeaders() {}

  // HTTP Request and Response header fields

  /** The HTTP {@code Cache-Control} header field name. */
  public static final String CACHE_CONTROL = "Cache-Control";
  /** The HTTP {@code Content-Length} header field name. */
  public static final String CONTENT_LENGTH = "Content-Length";

 * @author Jon Noack
 */
@GwtIncompatible // not worth using in GWT for now
@ElementTypesAreNonnullByDefault
class CompactHashSet<E extends @Nullable Object> extends AbstractSet<E> implements Serializable {
  // TODO(user): cache all field accesses in local vars

  /** Creates an empty {@code CompactHashSet} instance. */
  public static <E extends @Nullable Object> CompactHashSet<E> create() {
    return new CompactHashSet<>();
  }

  /**