private static final Logger log = Logger.getLogger(AbstractFuture.class.getName());

  private State state;
  private V value;
  private @Nullable Future<? extends V> delegate;
  private @Nullable Throwable throwable;
  private boolean mayInterruptIfRunning;
  private List<Listener> listeners;

  protected AbstractFuture() {
    state = State.PENDING;
    listeners = new ArrayList<Listener>();
  }

  @CanIgnoreReturnValue

  /**
   * This field encodes the current state of the future.
   *
   * <p>The valid values are:
   *
   * <ul>
   *   <li>{@code null} initial state, nothing has happened.
   *   <li>{@link Cancellation} terminal state, {@code cancel} was called.
   *   <li>{@link Failure} terminal state, {@code setException} was called.
   *   <li>{@link SetFuture} intermediate state, {@code setFuture} was called.