return new WeakReference<>(shadowLoader);
  }

  /*
   * TODO(cpovirk): It would be great to run all these tests (including class unloading) in an
   * environment that forces Futures.getChecked to its fallback WeakSetValidator. One awful way of
   * doing so would be to derive a separate test library by using remove_from_jar to strip out
   * ClassValueValidator.
   *

   *
   * <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.
   *

    return new WeakReference<>(shadowLoader);
  }

  /*
   * TODO(cpovirk): It would be great to run all these tests (including class unloading) in an
   * environment that forces Futures.getChecked to its fallback WeakSetValidator. One awful way of
   * doing so would be to derive a separate test library by using remove_from_jar to strip out
   * ClassValueValidator.
   *

         * were running under a new enough version of Android to support NIO.
         *
         * So this is probably just the "Windows Java 8" case. In that case, if we wanted *another*
         * layer of fallback before consulting the system property, we could try
         * com.sun.security.auth.module.NTSystem.
         *
         * But for now, we use the value from the system property as our best guess.
         */

  /** 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;

            && (BYTE_ARRAY_BASE_OFFSET % 8) == 0
            // sanity check - this should never fail
            && theUnsafe.arrayIndexScale(byte[].class) == 1)) {
          throw new Error(); // force fallback to PureJavaComparator
        }
      }

      /**
       * Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package. Replace with a simple
       * call to Unsafe.getUnsafe when integrating into a jdk.

  /** 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;

   *
   * <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.
   *

    return new SingletonImmutableSet<>(element);
  }

  /*
   * TODO: b/315526394 - Skip the Builder entirely for the of(...) methods, since we don't need to
   * worry that we might trigger the fallback to the JDK-backed implementation? (The varargs one
   * _could_, so we could keep it as it is. Or we could convince ourselves that hash flooding is
   * unlikely in practice there, too.)
   */

  /**

    MockCallback callback = new MockCallback("foo");
    addCallback(f, callback, directExecutor());
    f.set("foo");
  }

  public void testExecutorSuccess() {
    CountingSameThreadExecutor ex = new CountingSameThreadExecutor();
    SettableFuture<String> f = SettableFuture.create();
    MockCallback callback = new MockCallback("foo");
    Futures.addCallback(f, callback, ex);
    f.set("foo");