class OneShotPublisher implements Publisher<Boolean> {
    private final ExecutorService executor = ForkJoinPool.commonPool(); // daemon-based
    private boolean subscribed; // true after first subscribe
    public synchronized void subscribe(Subscriber<? super Boolean> subscriber) {
      if (subscribed)
        subscriber.onError(new IllegalStateException()); // only one allowed
      else {
        subscribed = true;
        subscriber.onSubscribe(new OneShotSubscription(subscriber, executor));
      }
    }

  static class OneShotSubscription implements Subscription {

     private final Subscriber<? super Boolean> subscriber;
     private final ExecutorService executor;
     private Future<?> future; // to allow cancellation
     private boolean completed;

     OneShotSubscription(Subscriber<? super Boolean> subscriber,
                          ExecutorService executor) {
        this.subscriber = subscriber;
        this.executor = executor;
      }

      public synchronized void request(long n) {

        if (!completed) {
          completed = true;
          if (n <= 0) {
            IllegalArgumentException ex = new IllegalArgumentException();
            executor.execute(() -> subscriber.onError(ex));
          } else {
           future = executor.submit(() -> {
              subscriber.onNext(Boolean.TRUE);
              subscriber.onComplete();
            });
          }

        }
      }

      public synchronized void cancel() {
        completed = true;
        if (future != null) future.cancel(false);
      }
    }
  }}

  class SampleSubscriber<T> implements Subscriber<T> {

    final Consumer<? super T> consumer;
    Subscription subscription;
    final long bufferSize;
    long count;

    SampleSubscriber(long bufferSize, Consumer<? super T> consumer) {
      this.bufferSize = bufferSize;
      this.consumer = consumer;
    }
   
public void onSubscribe(Subscription subscription) {

      long initialRequestSize = bufferSize;
      count = bufferSize - bufferSize / 2; // re-request when half consumed
      (this.subscription = subscription).request(initialRequestSize);
    }

    public void onNext(T item) {
      if (--count <= 0)
        subscription.request(count = bufferSize - bufferSize / 2);
      consumer.accept(item);
    }

    public void onError(Throwable ex) { ex.printStackTrace(); }
    public void onComplete() {}
 }

}

class UnboundedSubscriber<T> implements Subscriber<T> {
   
 public void onSubscribe(Subscription subscription) {
      subscription.request(Long.MAX_VALUE); // effectively unbounded
    }

    public void onNext(T item) { use(item); }
    public void onError(Throwable ex) { ex.printStackTrace(); }
    public void onComplete() {}
    void use(T item) { ... }

public final class Flow {
    private Flow() {} // uninstantiable
}

 /**
     * A producer of items (and related control messages) received by
     * Subscribers.  Each current {@link Subscriber} receives the same
     * items (via method {@code onNext}) in the same order, unless
     * drops or errors are encountered. If a Publisher encounters an
     * error that does not allow items to be issued to a Subscriber,
     * that Subscriber receives {@code onError}, and then receives no
     * further messages.  Otherwise, when it is known that no further
     * messages will be issued to it, a subscriber receives {@code
     * onComplete}.  Publishers ensure that Subscriber method
     * invocations for each subscription are strictly ordered in <a
     * href="package-summary.html#MemoryVisibility"><i>happens-before</i></a>
     * order.
     *
     * <p>Publishers may vary in policy about whether drops (failures
     * to issue an item because of resource limitations) are treated
     * as unrecoverable errors.  Publishers may also vary about
     * whether Subscribers receive items that were produced or
     * available before they subscribed.
     *
     * @param <T> the published item type
     */

   @FunctionalInterface
   public static interface Publisher<T> {
        /**
         * Adds the given Subscriber if possible.  If already
         * subscribed, or the attempt to subscribe fails due to policy
         * violations or errors, the Subscriber's {@code onError}
         * method is invoked with an {@link IllegalStateException}.
         * Otherwise, the Subscriber's {@code onSubscribe} method is
         * invoked with a new {@link Subscription}.  Subscribers may
         * enable receiving items by invoking the {@code request}
         * method of this Subscription, and may unsubscribe by
         * invoking its {@code cancel} method.
         *
         * @param subscriber the subscriber
         * @throws NullPointerException if subscriber is null
         */
        public void subscribe(Subscriber<? super T> subscriber);
    }

    /**
     * A receiver of messages.  The methods in this interface are
     * invoked in strict sequential order for each {@link
     * Subscription}.
     *
     * @param <T> the subscribed item type
     */
    public static interface Subscriber<T> {
        /**
         * Method invoked prior to invoking any other Subscriber
         * methods for the given Subscription. If this method throws
         * an exception, resulting behavior is not guaranteed, but may
         * cause the Subscription not to be established or to be cancelled.
         *
         * <p>Typically, implementations of this method invoke {@code
         * subscription.request} to enable receiving items.
         *
         * @param subscription a new subscription
         */
        public void onSubscribe(Subscription subscription);

        /**
         * Method invoked with a Subscription's next item.  If this
         * method throws an exception, resulting behavior is not
         * guaranteed, but may cause the Subscription to be cancelled.
         *
         * @param item the item
         */
        public void onNext(T item);

        /**
         * Method invoked upon an unrecoverable error encountered by a
         * Publisher or Subscription, after which no other Subscriber
         * methods are invoked by the Subscription.  If this method
         * itself throws an exception, resulting behavior is
         * undefined.
         *
         * @param throwable the exception
         */
        public void onError(Throwable throwable);

        /**
         * Method invoked when it is known that no additional
         * Subscriber method invocations will occur for a Subscription
         * that is not already terminated by error, after which no
         * other Subscriber methods are invoked by the Subscription.
         * If this method throws an exception, resulting behavior is
         * undefined.
         */
        public void onComplete();
    }

    /**
     * Message control linking a {@link Publisher} and {@link
     * Subscriber}.  Subscribers receive items only when requested,
     * and may cancel at any time. The methods in this interface are
     * intended to be invoked only by their Subscribers; usages in
     * other contexts have undefined effects.
     */
    public static interface Subscription {
        /**
         * Adds the given number {@code n} of items to the current
         * unfulfilled demand for this subscription.  If {@code n} is
         * less than or equal to zero, the Subscriber will receive an
         * {@code onError} signal with an {@link
         * IllegalArgumentException} argument.  Otherwise, the
         * Subscriber will receive up to {@code n} additional {@code
         * onNext} invocations (or fewer if terminated).
         *
         * @param n the increment of demand; a value of {@code
         * Long.MAX_VALUE} may be considered as effectively unbounded
         */
        public void request(long n);

        /**
         * Causes the Subscriber to (eventually) stop receiving
         * messages.  Implementation is best-effort -- additional
         * messages may be received after invoking this method.
         * A cancelled subscription need not ever receive an
         * {@code onComplete} or {@code onError} signal.
         */
        public void cancel();
    }

    /**
     * A component that acts as both a Subscriber and Publisher.
     *
     * @param <T> the subscribed item type
     * @param <R> the published item type
     */
    public static interface Processor<T,R> extends Subscriber<T>, Publisher<R> {
    }

    static final int DEFAULT_BUFFER_SIZE = 256;

    /**
     * Returns a default value for Publisher or Subscriber buffering,
     * that may be used in the absence of other constraints.
     *
     * @implNote
     * The current value returned is 256.
     *
     * @return the buffer size value
     */
    public static int defaultBufferSize() {
        return DEFAULT_BUFFER_SIZE;
    }
}