import static com.google.common.truth.Truth.assertThat;
import static com.google.common.util.concurrent.Futures.immediateFailedFuture;
import static com.google.common.util.concurrent.Futures.immediateFuture;
import static java.util.Arrays.asList;
import static java.util.concurrent.TimeUnit.MILLISECONDS;
import static java.util.concurrent.TimeUnit.SECONDS;
import static org.junit.Assert.assertThrows;

import static com.google.common.util.concurrent.Futures.immediateCancelledFuture;
import static com.google.common.util.concurrent.Futures.immediateFailedFuture;
import static com.google.common.util.concurrent.Futures.immediateFuture;
import static com.google.common.util.concurrent.MoreExecutors.directExecutor;
import static com.google.common.util.concurrent.SneakyThrows.sneakyThrow;
import static java.util.concurrent.Executors.callable;

      ListenableFuture<? extends O> output = function.apply(input);
      checkNotNull(
          output,
          "AsyncFunction.apply returned null instead of a Future. "
              + "Did you mean to return immediateFuture(null)? %s",
          function);
      return output;
    }

    @Override
    void setResult(ListenableFuture<? extends O> result) {
      setFuture(result);
    }
  }

  /**

   * // counters.
   * ListenableFuture<Integer> faultTolerantFuture =
   *     fetchCounters().catchingAsync(
   *         FetchException.class, x -> immediateFuture(0), directExecutor());
   * }
   *
   * <p>The fallback can also choose to propagate the original exception when desired:
   *
   * {@snippet :

import static com.google.common.util.concurrent.Futures.immediateCancelledFuture;
import static com.google.common.util.concurrent.Futures.immediateFailedFuture;
import static com.google.common.util.concurrent.Futures.immediateFuture;
import static com.google.common.util.concurrent.MoreExecutors.directExecutor;
import static com.google.common.util.concurrent.SneakyThrows.sneakyThrow;
import static java.util.concurrent.Executors.callable;

      ListenableFuture<? extends O> output = function.apply(input);
      checkNotNull(
          output,
          "AsyncFunction.apply returned null instead of a Future. "
              + "Did you mean to return immediateFuture(null)? %s",
          function);
      return output;
    }

    @Override
    void setResult(ListenableFuture<? extends O> result) {
      setFuture(result);
    }
  }

  /**

import static com.google.common.util.concurrent.ClosingFuture.State.WILL_CREATE_VALUE_AND_CLOSER;
import static com.google.common.util.concurrent.Futures.getDone;
import static com.google.common.util.concurrent.Futures.immediateFuture;
import static com.google.common.util.concurrent.Futures.nonCancellationPropagating;
import static com.google.common.util.concurrent.MoreExecutors.directExecutor;

import static com.google.common.util.concurrent.ClosingFuture.State.WILL_CREATE_VALUE_AND_CLOSER;
import static com.google.common.util.concurrent.Futures.getDone;
import static com.google.common.util.concurrent.Futures.immediateFuture;
import static com.google.common.util.concurrent.Futures.nonCancellationPropagating;
import static com.google.common.util.concurrent.MoreExecutors.directExecutor;