@Nullable Object[] alternatingKeysAndValues;
    int size;
    boolean entriesUsed;
    /**
     * If non-null, a duplicate key we found in a previous buildKeepingLast() or buildOrThrow()
     * call. A later buildOrThrow() can simply report this duplicate immediately.
     */
    @Nullable DuplicateKey duplicateKey;

    /**
     * Creates a new builder. The returned builder is equivalent to the builder generated by {@link

   * cannot be overridden by a call to a {@code set*} method, only by a call to {@link #cancel}.
   *
   * <p>If the call {@code setFuture(delegate)} is accepted and this {@code Future} is later
   * cancelled, cancellation will be propagated to {@code delegate}. Additionally, any call to
   * {@code setFuture} after any cancellation will propagate cancellation to the supplied {@code
   * Future}.
   *

   *             complete, the listener runs immediately in that thread.
   *         <li>When a thread attaches a listener to a {@code ListenableFuture} that's
   *             incomplete and the {@code ListenableFuture} later completes normally, the
   *             listener runs in the thread that completes the {@code ListenableFuture}.
   *         <li>When a listener is attached to a {@code ListenableFuture} and the {@code

   *
   * <p>The returned navigable set will be serializable if the specified navigable set is
   * serializable.
   *
   * <p><b>Java 8+ users and later:</b> Prefer {@link Collections#unmodifiableNavigableSet}.
   *
   * @param set the navigable set for which an unmodifiable view is to be returned
   * @return an unmodifiable view of the specified navigable set