https://github.com/google/guava/guava: Guava is a suite of core and expanded libraries that include utility classes, google's collections, io classes, and much much more. 
/*
 * Copyright (C) 2006 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.util.concurrent;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.common.annotations.GwtIncompatible;
import com.google.common.base.Function;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Executor;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;


A ListenableFuture that supports fluent chains of operations. For example: 

ListenableFuture<Boolean> adminIsLoggedIn =
    FluentFuture.from(usersDatabase.getAdminUser())
        .transform(User::getId, directExecutor())
        .transform(ActivityService::isLoggedIn, threadPool)
        .catching(RpcException.class, e -> false, directExecutor());



Alternatives


Frameworks


When chaining together a graph of asynchronous operations, you will often find it easier to
use a framework. Frameworks automate the process, often adding features like monitoring,
debugging, and cancellation. Examples of frameworks include:



CompletableFuture / CompletionStage 


Users of CompletableFuture will likely want to continue using 
CompletableFuture. FluentFuture is targeted at people who use ListenableFuture, who can't use Java 8, or who want an API more focused than CompletableFuture. (If you need to adapt between CompletableFuture and ListenableFuture, consider Future Converter.)

Extension
 If you want a class like FluentFuture but with extra methods, we recommend declaring your own subclass of ListenableFuture, complete with a method like from to adapt an existing ListenableFuture, implemented atop a ForwardingListenableFuture that forwards to that future and adds the desired methods. 
Since:23.0
/**
 * A {@link ListenableFuture} that supports fluent chains of operations. For example:
 *
 * <pre>{@code
 * ListenableFuture<Boolean> adminIsLoggedIn =
 *     FluentFuture.from(usersDatabase.getAdminUser())
 *         .transform(User::getId, directExecutor())
 *         .transform(ActivityService::isLoggedIn, threadPool)
 *         .catching(RpcException.class, e -> false, directExecutor());
 * }</pre>
 *
 * <h3>Alternatives</h3>
 *
 * <h4>Frameworks</h4>
 *
 * <p>When chaining together a graph of asynchronous operations, you will often find it easier to
 * use a framework. Frameworks automate the process, often adding features like monitoring,
 * debugging, and cancellation. Examples of frameworks include:
 *
 * <ul>
 *   <li><a href="http://google.github.io/dagger/producers.html">Dagger Producers</a>
 * </ul>
 *
 * <h4>{@link java.util.concurrent.CompletableFuture} / {@link java.util.concurrent.CompletionStage}
 * </h4>
 *
 * <p>Users of {@code CompletableFuture} will likely want to continue using {@code
 * CompletableFuture}. {@code FluentFuture} is targeted at people who use {@code ListenableFuture},
 * who can't use Java 8, or who want an API more focused than {@code CompletableFuture}. (If you
 * need to adapt between {@code CompletableFuture} and {@code ListenableFuture}, consider <a
 * href="https://github.com/lukas-krecan/future-converter">Future Converter</a>.)
 *
 * <h3>Extension</h3>
 *
 * If you want a class like {@code FluentFuture} but with extra methods, we recommend declaring your
 * own subclass of {@link ListenableFuture}, complete with a method like {@link #from} to adapt an
 * existing {@code ListenableFuture}, implemented atop a {@link ForwardingListenableFuture} that
 * forwards to that future and adds the desired methods.
 *
 * @since 23.0
 */
@Beta
@GwtCompatible(emulated = true)
public abstract class FluentFuture<V> extends GwtFluentFutureCatchingSpecialization<V> {

  
A less abstract subclass of AbstractFuture. This can be used to optimize setFuture by ensuring that get calls exactly the implementation of AbstractFuture.get. 
/**
   * A less abstract subclass of AbstractFuture. This can be used to optimize setFuture by ensuring
   * that {@link #get} calls exactly the implementation of {@link AbstractFuture#get}.
   */
  abstract static class TrustedFuture<V> extends FluentFuture<V>
      implements AbstractFuture.Trusted<V> {
    @CanIgnoreReturnValue
    @Override
    public final V get() throws InterruptedException, ExecutionException {
      return super.get();
    }

    @CanIgnoreReturnValue
    @Override
    public final V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException {
      return super.get(timeout, unit);
    }

    @Override
    public final boolean isDone() {
      return super.isDone();
    }

    @Override
    public final boolean isCancelled() {
      return super.isCancelled();
    }

    @Override
    public final void addListener(Runnable listener, Executor executor) {
      super.addListener(listener, executor);
    }

    @CanIgnoreReturnValue
    @Override
    public final boolean cancel(boolean mayInterruptIfRunning) {
      return super.cancel(mayInterruptIfRunning);
    }
  }

  FluentFuture() {}

  
Converts the given ListenableFuture to an equivalent FluentFuture. 
If the given ListenableFuture is already a FluentFuture, it is returned directly. If not, it is wrapped in a FluentFuture that delegates all calls to the original ListenableFuture. 
/**
   * Converts the given {@code ListenableFuture} to an equivalent {@code FluentFuture}.
   *
   * <p>If the given {@code ListenableFuture} is already a {@code FluentFuture}, it is returned
   * directly. If not, it is wrapped in a {@code FluentFuture} that delegates all calls to the
   * original {@code ListenableFuture}.
   */
  public static <V> FluentFuture<V> from(ListenableFuture<V> future) {
    return future instanceof FluentFuture
        ? (FluentFuture<V>) future
        : new ForwardingFluentFuture<V>(future);
  }

  
Returns a Future whose result is taken from this Future or, if this 
Future fails with the given exceptionType, from the result provided by the 
fallback. Function.apply is not invoked until the primary input has failed, so if the primary input succeeds, it is never invoked. If, during the invocation of fallback, an exception is thrown, this exception is used as the result of the output Future. 
Usage example:


// Falling back to a zero counter in case an exception happens when processing the RPC to fetch
// counters.
ListenableFuture<Integer> faultTolerantFuture =
    fetchCounters().catching(FetchException.class, x -> 0, directExecutor());



When selecting an executor, note that directExecutor is dangerous in some cases. See the discussion in the AbstractFuture.addListener documentation. All its warnings about heavyweight listeners are also applicable to heavyweight functions passed to this method. 
This method is similar to CompletableFuture.exceptionally. It can also serve some of the use cases of CompletableFuture.handle and CompletableFuture.handleAsync when used along with transform. 
Params:
  • exceptionType – the exception type that triggers use of fallback. The exception type is matched against the input's exception. "The input's exception" means the cause of the ExecutionException thrown by input.get() or, if get() throws a different kind of exception, that exception itself. To avoid hiding bugs and other unrecoverable errors, callers should prefer more specific types, avoiding 
    Throwable.class in particular.
  • fallback – the Function to be called if the input fails with the expected exception type. The function's argument is the input's exception. "The input's exception" means the cause of the ExecutionException thrown by this.get() or, if get() throws a different kind of exception, that exception itself.
  • executor – the executor that runs fallback if the input fails
/**
   * Returns a {@code Future} whose result is taken from this {@code Future} or, if this {@code
   * Future} fails with the given {@code exceptionType}, from the result provided by the {@code
   * fallback}. {@link Function#apply} is not invoked until the primary input has failed, so if the
   * primary input succeeds, it is never invoked. If, during the invocation of {@code fallback}, an
   * exception is thrown, this exception is used as the result of the output {@code Future}.
   *
   * <p>Usage example:
   *
   * <pre>{@code
   * // Falling back to a zero counter in case an exception happens when processing the RPC to fetch
   * // counters.
   * ListenableFuture<Integer> faultTolerantFuture =
   *     fetchCounters().catching(FetchException.class, x -> 0, directExecutor());
   * }</pre>
   *
   * <p>When selecting an executor, note that {@code directExecutor} is dangerous in some cases. See
   * the discussion in the {@link #addListener} documentation. All its warnings about heavyweight
   * listeners are also applicable to heavyweight functions passed to this method.
   *
   * <p>This method is similar to {@link java.util.concurrent.CompletableFuture#exceptionally}. It
   * can also serve some of the use cases of {@link java.util.concurrent.CompletableFuture#handle}
   * and {@link java.util.concurrent.CompletableFuture#handleAsync} when used along with {@link
   * #transform}.
   *
   * @param exceptionType the exception type that triggers use of {@code fallback}. The exception
   *     type is matched against the input's exception. "The input's exception" means the cause of
   *     the {@link ExecutionException} thrown by {@code input.get()} or, if {@code get()} throws a
   *     different kind of exception, that exception itself. To avoid hiding bugs and other
   *     unrecoverable errors, callers should prefer more specific types, avoiding {@code
   *     Throwable.class} in particular.
   * @param fallback the {@link Function} to be called if the input fails with the expected
   *     exception type. The function's argument is the input's exception. "The input's exception"
   *     means the cause of the {@link ExecutionException} thrown by {@code this.get()} or, if
   *     {@code get()} throws a different kind of exception, that exception itself.
   * @param executor the executor that runs {@code fallback} if the input fails
   */
  @Partially.GwtIncompatible("AVAILABLE but requires exceptionType to be Throwable.class")
  public final <X extends Throwable> FluentFuture<V> catching(
      Class<X> exceptionType, Function<? super X, ? extends V> fallback, Executor executor) {
    return (FluentFuture<V>) Futures.catching(this, exceptionType, fallback, executor);
  }

  
Returns a Future whose result is taken from this Future or, if this 
Future fails with the given exceptionType, from the result provided by the 
fallback. AsyncFunction.apply is not invoked until the primary input has failed, so if the primary input succeeds, it is never invoked. If, during the invocation of fallback, an exception is thrown, this exception is used as the result of the output Future. 
Usage examples:


// Falling back to a zero counter in case an exception happens when processing the RPC to fetch
// counters.
ListenableFuture<Integer> faultTolerantFuture =
    fetchCounters().catchingAsync(
        FetchException.class, x -> immediateFuture(0), directExecutor());



The fallback can also choose to propagate the original exception when desired:


// Falling back to a zero counter only in case the exception was a
// TimeoutException.
ListenableFuture<Integer> faultTolerantFuture =
    fetchCounters().catchingAsync(
        FetchException.class,
        e -> {
          if (omitDataOnFetchFailure) {
            return immediateFuture(0);
          }
          throw e;
        },
        directExecutor());



When selecting an executor, note that directExecutor is dangerous in some cases. See the discussion in the AbstractFuture.addListener documentation. All its warnings about heavyweight listeners are also applicable to heavyweight functions passed to this method. (Specifically, directExecutor functions should avoid heavyweight operations inside 
AsyncFunction.apply. Any heavyweight operations should occur in other threads responsible for completing the returned Future.) 
This method is similar to CompletableFuture.exceptionally. It can also serve some of the use cases of CompletableFuture.handle and CompletableFuture.handleAsync when used along with transform. 
Params:
  • exceptionType – the exception type that triggers use of fallback. The exception type is matched against the input's exception. "The input's exception" means the cause of the ExecutionException thrown by this.get() or, if get() throws a different kind of exception, that exception itself. To avoid hiding bugs and other unrecoverable errors, callers should prefer more specific types, avoiding 
    Throwable.class in particular.
  • fallback – the AsyncFunction to be called if the input fails with the expected exception type. The function's argument is the input's exception. "The input's exception" means the cause of the ExecutionException thrown by input.get() or, if get() throws a different kind of exception, that exception itself.
  • executor – the executor that runs fallback if the input fails
/**
   * Returns a {@code Future} whose result is taken from this {@code Future} or, if this {@code
   * Future} fails with the given {@code exceptionType}, from the result provided by the {@code
   * fallback}. {@link AsyncFunction#apply} is not invoked until the primary input has failed, so if
   * the primary input succeeds, it is never invoked. If, during the invocation of {@code fallback},
   * an exception is thrown, this exception is used as the result of the output {@code Future}.
   *
   * <p>Usage examples:
   *
   * <pre>{@code
   * // Falling back to a zero counter in case an exception happens when processing the RPC to fetch
   * // counters.
   * ListenableFuture<Integer> faultTolerantFuture =
   *     fetchCounters().catchingAsync(
   *         FetchException.class, x -> immediateFuture(0), directExecutor());
   * }</pre>
   *
   * <p>The fallback can also choose to propagate the original exception when desired:
   *
   * <pre>{@code
   * // Falling back to a zero counter only in case the exception was a
   * // TimeoutException.
   * ListenableFuture<Integer> faultTolerantFuture =
   *     fetchCounters().catchingAsync(
   *         FetchException.class,
   *         e -> {
   *           if (omitDataOnFetchFailure) {
   *             return immediateFuture(0);
   *           }
   *           throw e;
   *         },
   *         directExecutor());
   * }</pre>
   *
   * <p>When selecting an executor, note that {@code directExecutor} is dangerous in some cases. See
   * the discussion in the {@link #addListener} documentation. All its warnings about heavyweight
   * listeners are also applicable to heavyweight functions passed to this method. (Specifically,
   * {@code directExecutor} functions should avoid heavyweight operations inside {@code
   * AsyncFunction.apply}. Any heavyweight operations should occur in other threads responsible for
   * completing the returned {@code Future}.)
   *
   * <p>This method is similar to {@link java.util.concurrent.CompletableFuture#exceptionally}. It
   * can also serve some of the use cases of {@link java.util.concurrent.CompletableFuture#handle}
   * and {@link java.util.concurrent.CompletableFuture#handleAsync} when used along with {@link
   * #transform}.
   *
   * @param exceptionType the exception type that triggers use of {@code fallback}. The exception
   *     type is matched against the input's exception. "The input's exception" means the cause of
   *     the {@link ExecutionException} thrown by {@code this.get()} or, if {@code get()} throws a
   *     different kind of exception, that exception itself. To avoid hiding bugs and other
   *     unrecoverable errors, callers should prefer more specific types, avoiding {@code
   *     Throwable.class} in particular.
   * @param fallback the {@link AsyncFunction} to be called if the input fails with the expected
   *     exception type. The function's argument is the input's exception. "The input's exception"
   *     means the cause of the {@link ExecutionException} thrown by {@code input.get()} or, if
   *     {@code get()} throws a different kind of exception, that exception itself.
   * @param executor the executor that runs {@code fallback} if the input fails
   */
  @Partially.GwtIncompatible("AVAILABLE but requires exceptionType to be Throwable.class")
  public final <X extends Throwable> FluentFuture<V> catchingAsync(
      Class<X> exceptionType, AsyncFunction<? super X, ? extends V> fallback, Executor executor) {
    return (FluentFuture<V>) Futures.catchingAsync(this, exceptionType, fallback, executor);
  }

  
Returns a future that delegates to this future but will finish early (via a TimeoutException wrapped in an ExecutionException) if the specified timeout expires. If the timeout expires, not only will the output future finish, but also the input future (this) will be cancelled and interrupted. 
Params:
  • timeout – when to time out the future
  • unit – the time unit of the time parameter
  • scheduledExecutor – The executor service to enforce the timeout.
/**
   * Returns a future that delegates to this future but will finish early (via a {@link
   * TimeoutException} wrapped in an {@link ExecutionException}) if the specified timeout expires.
   * If the timeout expires, not only will the output future finish, but also the input future
   * ({@code this}) will be cancelled and interrupted.
   *
   * @param timeout when to time out the future
   * @param unit the time unit of the time parameter
   * @param scheduledExecutor The executor service to enforce the timeout.
   */
  @GwtIncompatible // ScheduledExecutorService
  @SuppressWarnings("GoodTime") // should accept a java.time.Duration
  public final FluentFuture<V> withTimeout(
      long timeout, TimeUnit unit, ScheduledExecutorService scheduledExecutor) {
    return (FluentFuture<V>) Futures.withTimeout(this, timeout, unit, scheduledExecutor);
  }

  
Returns a new Future whose result is asynchronously derived from the result of this Future. If the input Future fails, the returned Future fails with the same exception (and the function is not invoked). 
More precisely, the returned Future takes its result from a Future produced by applying the given AsyncFunction to the result of the original Future. Example usage: 

FluentFuture<RowKey> rowKeyFuture = FluentFuture.from(indexService.lookUp(query));
ListenableFuture<QueryResult> queryFuture =
    rowKeyFuture.transformAsync(dataService::readFuture, executor);



When selecting an executor, note that directExecutor is dangerous in some cases. See the discussion in the AbstractFuture.addListener documentation. All its warnings about heavyweight listeners are also applicable to heavyweight functions passed to this method. (Specifically, directExecutor functions should avoid heavyweight operations inside 
AsyncFunction.apply. Any heavyweight operations should occur in other threads responsible for completing the returned Future.) 
The returned Future attempts to keep its cancellation state in sync with that of the input future and that of the future returned by the chain function. That is, if the returned Future is cancelled, it will attempt to cancel the other two, and if either of the other two is cancelled, the returned Future will receive a callback in which it will attempt to cancel itself. 
This method is similar to CompletableFuture.thenCompose and CompletableFuture.thenComposeAsync. It can also serve some of the use cases of CompletableFuture.handle and CompletableFuture.handleAsync when used along with catching. 
Params:
  • function – A function to transform the result of this future to the result of the output
    future
  • executor – Executor to run the function in.
Returns:A future that holds result of the function (if the input succeeded) or the original
    input's failure (if not)
/**
   * Returns a new {@code Future} whose result is asynchronously derived from the result of this
   * {@code Future}. If the input {@code Future} fails, the returned {@code Future} fails with the
   * same exception (and the function is not invoked).
   *
   * <p>More precisely, the returned {@code Future} takes its result from a {@code Future} produced
   * by applying the given {@code AsyncFunction} to the result of the original {@code Future}.
   * Example usage:
   *
   * <pre>{@code
   * FluentFuture<RowKey> rowKeyFuture = FluentFuture.from(indexService.lookUp(query));
   * ListenableFuture<QueryResult> queryFuture =
   *     rowKeyFuture.transformAsync(dataService::readFuture, executor);
   * }</pre>
   *
   * <p>When selecting an executor, note that {@code directExecutor} is dangerous in some cases. See
   * the discussion in the {@link #addListener} documentation. All its warnings about heavyweight
   * listeners are also applicable to heavyweight functions passed to this method. (Specifically,
   * {@code directExecutor} functions should avoid heavyweight operations inside {@code
   * AsyncFunction.apply}. Any heavyweight operations should occur in other threads responsible for
   * completing the returned {@code Future}.)
   *
   * <p>The returned {@code Future} attempts to keep its cancellation state in sync with that of the
   * input future and that of the future returned by the chain function. That is, if the returned
   * {@code Future} is cancelled, it will attempt to cancel the other two, and if either of the
   * other two is cancelled, the returned {@code Future} will receive a callback in which it will
   * attempt to cancel itself.
   *
   * <p>This method is similar to {@link java.util.concurrent.CompletableFuture#thenCompose} and
   * {@link java.util.concurrent.CompletableFuture#thenComposeAsync}. It can also serve some of the
   * use cases of {@link java.util.concurrent.CompletableFuture#handle} and {@link
   * java.util.concurrent.CompletableFuture#handleAsync} when used along with {@link #catching}.
   *
   * @param function A function to transform the result of this future to the result of the output
   *     future
   * @param executor Executor to run the function in.
   * @return A future that holds result of the function (if the input succeeded) or the original
   *     input's failure (if not)
   */
  public final <T> FluentFuture<T> transformAsync(
      AsyncFunction<? super V, T> function, Executor executor) {
    return (FluentFuture<T>) Futures.transformAsync(this, function, executor);
  }

  
Returns a new Future whose result is derived from the result of this Future. If this input Future fails, the returned Future fails with the same exception (and the function is not invoked). Example usage: 

ListenableFuture<List<Row>> rowsFuture =
    queryFuture.transform(QueryResult::getRows, executor);



When selecting an executor, note that directExecutor is dangerous in some cases. See the discussion in the AbstractFuture.addListener documentation. All its warnings about heavyweight listeners are also applicable to heavyweight functions passed to this method. 
The returned Future attempts to keep its cancellation state in sync with that of the input future. That is, if the returned Future is cancelled, it will attempt to cancel the input, and if the input is cancelled, the returned Future will receive a callback in which it will attempt to cancel itself. 
An example use of this method is to convert a serializable object returned from an RPC into
a POJO.

This method is similar to CompletableFuture.thenApply and CompletableFuture.thenApplyAsync. It can also serve some of the use cases of CompletableFuture.handle and CompletableFuture.handleAsync when used along with catching. 
Params:
  • function – A Function to transform the results of this future to the results of the
    returned future.
  • executor – Executor to run the function in.
Returns:A future that holds result of the transformation.
/**
   * Returns a new {@code Future} whose result is derived from the result of this {@code Future}. If
   * this input {@code Future} fails, the returned {@code Future} fails with the same exception (and
   * the function is not invoked). Example usage:
   *
   * <pre>{@code
   * ListenableFuture<List<Row>> rowsFuture =
   *     queryFuture.transform(QueryResult::getRows, executor);
   * }</pre>
   *
   * <p>When selecting an executor, note that {@code directExecutor} is dangerous in some cases. See
   * the discussion in the {@link #addListener} documentation. All its warnings about heavyweight
   * listeners are also applicable to heavyweight functions passed to this method.
   *
   * <p>The returned {@code Future} attempts to keep its cancellation state in sync with that of the
   * input future. That is, if the returned {@code Future} is cancelled, it will attempt to cancel
   * the input, and if the input is cancelled, the returned {@code Future} will receive a callback
   * in which it will attempt to cancel itself.
   *
   * <p>An example use of this method is to convert a serializable object returned from an RPC into
   * a POJO.
   *
   * <p>This method is similar to {@link java.util.concurrent.CompletableFuture#thenApply} and
   * {@link java.util.concurrent.CompletableFuture#thenApplyAsync}. It can also serve some of the
   * use cases of {@link java.util.concurrent.CompletableFuture#handle} and {@link
   * java.util.concurrent.CompletableFuture#handleAsync} when used along with {@link #catching}.
   *
   * @param function A Function to transform the results of this future to the results of the
   *     returned future.
   * @param executor Executor to run the function in.
   * @return A future that holds result of the transformation.
   */
  public final <T> FluentFuture<T> transform(Function<? super V, T> function, Executor executor) {
    return (FluentFuture<T>) Futures.transform(this, function, executor);
  }

  
Registers separate success and failure callbacks to be run when this Future's computation is complete or, if the computation is already complete, immediately. 
The callback is run on executor. There is no guaranteed ordering of execution of callbacks, but any callback added through this method is guaranteed to be called once the computation is complete. 
Example:


future.addCallback(
    new FutureCallback<QueryResult>() {
      public void onSuccess(QueryResult result) {
        storeInCache(result);
      }
      public void onFailure(Throwable t) {
        reportError(t);
      }
    }, executor);



When selecting an executor, note that directExecutor is dangerous in some cases. See the discussion in the AbstractFuture.addListener documentation. All its warnings about heavyweight listeners are also applicable to heavyweight callbacks passed to this method. 
For a more general interface to attach a completion listener, see AbstractFuture.addListener. 
This method is similar to CompletableFuture.whenComplete and CompletableFuture.whenCompleteAsync. It also serves the use case of CompletableFuture.thenAccept and CompletableFuture.thenAcceptAsync. 
Params:
  • callback – The callback to invoke when this Future is completed.
  • executor – The executor to run callback when the future completes.
/**
   * Registers separate success and failure callbacks to be run when this {@code Future}'s
   * computation is {@linkplain java.util.concurrent.Future#isDone() complete} or, if the
   * computation is already complete, immediately.
   *
   * <p>The callback is run on {@code executor}. There is no guaranteed ordering of execution of
   * callbacks, but any callback added through this method is guaranteed to be called once the
   * computation is complete.
   *
   * <p>Example:
   *
   * <pre>{@code
   * future.addCallback(
   *     new FutureCallback<QueryResult>() {
   *       public void onSuccess(QueryResult result) {
   *         storeInCache(result);
   *       }
   *       public void onFailure(Throwable t) {
   *         reportError(t);
   *       }
   *     }, executor);
   * }</pre>
   *
   * <p>When selecting an executor, note that {@code directExecutor} is dangerous in some cases. See
   * the discussion in the {@link #addListener} documentation. All its warnings about heavyweight
   * listeners are also applicable to heavyweight callbacks passed to this method.
   *
   * <p>For a more general interface to attach a completion listener, see {@link #addListener}.
   *
   * <p>This method is similar to {@link java.util.concurrent.CompletableFuture#whenComplete} and
   * {@link java.util.concurrent.CompletableFuture#whenCompleteAsync}. It also serves the use case
   * of {@link java.util.concurrent.CompletableFuture#thenAccept} and {@link
   * 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);
  }
}