*/
  @J2ktIncompatible // serialization
  static class SerializedForm<K, V> implements Serializable {
    // This object retains references to collections returned by keySet() and value(). This saves
    // bytes when the both the map and its keySet or value collection are written to the same
    // instance of ObjectOutputStream.

   * listeners for future n may complete before some for future n-1.) However, it is possible, if
   * one input completes with result X and another later with result Y, for Y to come before X in
   * the output future list. (Such races are impossible to solve without global synchronization of
   * all future completions. And they should have little practical impact.)
   *

   *
   * <p><b>Comparison to {@code java.util.Optional}:</b> this class leaves the specific choice of
   * hash code unspecified, unlike the Java 8+ equivalent.
   */
  @Override
  public abstract int hashCode();

  /**
   * Returns a string representation for this instance.
   *
   * <p><b>Comparison to {@code java.util.Optional}:</b> this class leaves the specific string

              <logLevel>${gwt.logLevel}</logLevel>
              <mode>htmlunit</mode>
              <htmlunit>FF38</htmlunit>
              <productionMode>true</productionMode>
              <!-- Fix OutOfMemoryError under Travis. -->
              <extraJvmArgs>-Xms3500m -Xmx3500m -Xss1024k</extraJvmArgs>
              <sourceLevel>1.8</sourceLevel>

   * Supports-Loading-Mode}</a> header field name. This can be used to specify, for example, <a
   * href="https://developer.chrome.com/docs/privacy-sandbox/fenced-frame/#server-opt-in">fenced
   * frames</a>.
   *
   * @since 32.0.0
   */
  public static final String SUPPORTS_LOADING_MODE = "Supports-Loading-Mode";

  /**
   * The HTTP <a href="http://tools.ietf.org/html/rfc6797#section-6.1">{@code