* java.util.concurrent.CompletableFuture#thenAcceptAsync}.
   *
   * @param callback The callback to invoke when this {@code Future} is completed.
   * @param executor The executor to run {@code callback} when the future completes.
   */
  public final void addCallback(FutureCallback<? super V> callback, Executor executor) {
    Futures.addCallback(this, callback, executor);
  }

  public void testOr_supplier_present() {
    assertEquals("a", Optional.of("a").or(Suppliers.ofInstance("fallback")));
  }

  public void testOr_supplier_absent() {
    assertEquals("fallback", Optional.absent().or(Suppliers.ofInstance("fallback")));
  }

  public void testOr_nullSupplier_absent() {

   * for bridge methods.
   */
  public void testRegistrationWithBridgeMethod() {
    final AtomicInteger calls = new AtomicInteger();
    bus.register(
        new Callback<String>() {
          @Subscribe
          @Override
          public void call(String s) {
            calls.incrementAndGet();
          }
        });

    bus.post("hello");

   * for bridge methods.
   */
  public void testRegistrationWithBridgeMethod() {
    final AtomicInteger calls = new AtomicInteger();
    bus.register(
        new Callback<String>() {
          @Subscribe
          @Override
          public void call(String s) {
            calls.incrementAndGet();
          }
        });

    bus.post("hello");

  }

  /**
   * This tests for a bug where if {@link Service#stopAsync()} was called while the service was
   * {@link State#STARTING} more than once, the {@link Listener#stopping(State)} callback would get
   * called multiple times.
   */
  public void testManualServiceStopMultipleTimesWhileStarting() throws Exception {
    ManualSwitchedService service = new ManualSwitchedService();

  public void testOr_supplier_present() {
    assertEquals("a", Optional.of("a").or(Suppliers.ofInstance("fallback")));
  }

  public void testOr_supplier_absent() {
    assertEquals("fallback", Optional.absent().or(Suppliers.ofInstance("fallback")));
  }

  public void testOr_nullSupplier_absent() {

  /** Table of cells. When non-null, size is a power of 2. */
  @CheckForNull transient volatile Cell[] cells;

  /**
   * Base value, used mainly when there is no contention, but also as a fallback during table
   * initialization races. Updated via CAS.
   */
  transient volatile long base;

  /** Spinlock (locked via CAS) used when resizing and/or creating Cells. */
  transient volatile int busy;

   * logged.
   *
   * @param listener the listener to run when the service changes state is complete
   * @param executor the executor in which the listeners callback methods will be run. For fast,
   *     lightweight listeners that would be safe to execute in any thread, consider {@link
   *     MoreExecutors#directExecutor}.
   * @since 13.0
   */

   *
   * <p><b>Note:</b> if {@code first} is represented as an {@link Optional}, this can be
   * accomplished with {@link Optional#or(Object) first.or(second)}. That approach also allows for
   * lazy evaluation of the fallback instance, using {@link Optional#or(Supplier)
   * first.or(supplier)}.
   *
   * <p><b>Java 9 users:</b> use {@code java.util.Objects.requireNonNullElse(first, second)}
   * instead.
   *

  }

  /**
   * This tests for a bug where if {@link Service#stopAsync()} was called while the service was
   * {@link State#STARTING} more than once, the {@link Listener#stopping(State)} callback would get
   * called multiple times.
   */
  public void testManualServiceStopMultipleTimesWhileStarting() throws Exception {
    ManualSwitchedService service = new ManualSwitchedService();