/*

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

 * or visit www.oracle.com if you need additional information or have any

 * questions.

 */

/*

 * This file is available under and governed by the GNU General Public

 * License version 2 only, as published by the Free Software Foundation.

 * However, the following notice accompanied the original version of this

 * file:

 *

 * Written by Doug Lea with assistance from members of JCP JSR-166

 * Expert Group and released to the public domain, as explained at

 * http://creativecommons.org/publicdomain/zero/1.0/

 */

package java.util.concurrent;

import java.util.function.BiConsumer;

import java.util.function.BiFunction;

import java.util.function.Consumer;

import java.util.function.Function;

/**

 * A stage of a possibly asynchronous computation, that performs an

 * action or computes a value when another CompletionStage completes.

 * A stage completes upon termination of its computation, but this may

 * in turn trigger other dependent stages.  The functionality defined

 * in this interface takes only a few basic forms, which expand out to

 * a larger set of methods to capture a range of usage styles:

 *

 * <ul>

 *

 * <li>The computation performed by a stage may be expressed as a

 * Function, Consumer, or Runnable (using methods with names including

 * <em>apply</em>, <em>accept</em>, or <em>run</em>, respectively)

 * depending on whether it requires arguments and/or produces results.

 * For example:

 * <pre> {@code

 * stage.thenApply(x -> square(x))

 *      .thenAccept(x -> System.out.print(x))

 *      .thenRun(() -> System.out.println());}</pre>

 *

 * An additional form (<em>compose</em>) allows the construction of

 * computation pipelines from functions returning completion stages.

 *

 * <p>Any argument to a stage's computation is the outcome of a

 * triggering stage's computation.

 *

 * <li>One stage's execution may be triggered by completion of a

 * single stage, or both of two stages, or either of two stages.

 * Dependencies on a single stage are arranged using methods with

 * prefix <em>then</em>. Those triggered by completion of

 * <em>both</em> of two stages may <em>combine</em> their results or

 * effects, using correspondingly named methods. Those triggered by

 * <em>either</em> of two stages make no guarantees about which of the

 * results or effects are used for the dependent stage's computation.

 *

 * <li>Dependencies among stages control the triggering of

 * computations, but do not otherwise guarantee any particular

 * ordering. Additionally, execution of a new stage's computations may

 * be arranged in any of three ways: default execution, default

 * asynchronous execution (using methods with suffix <em>async</em>

 * that employ the stage's default asynchronous execution facility),

 * or custom (via a supplied {@link Executor}).  The execution

 * properties of default and async modes are specified by

 * CompletionStage implementations, not this interface. Methods with

 * explicit Executor arguments may have arbitrary execution

 * properties, and might not even support concurrent execution, but

 * are arranged for processing in a way that accommodates asynchrony.

 *

 * <li>Two method forms ({@link #handle handle} and {@link

 * #whenComplete whenComplete}) support unconditional computation

 * whether the triggering stage completed normally or exceptionally.

 * Method {@link #exceptionally exceptionally} supports computation

 * only when the triggering stage completes exceptionally, computing a

 * replacement result, similarly to the java {@code catch} keyword.

 * In all other cases, if a stage's computation terminates abruptly

 * with an (unchecked) exception or error, then all dependent stages

 * requiring its completion complete exceptionally as well, with a

 * {@link CompletionException} holding the exception as its cause.  If

 * a stage is dependent on <em>both</em> of two stages, and both

 * complete exceptionally, then the CompletionException may correspond

 * to either one of these exceptions.  If a stage is dependent on

 * <em>either</em> of two others, and only one of them completes

 * exceptionally, no guarantees are made about whether the dependent

 * stage completes normally or exceptionally. In the case of method

 * {@code whenComplete}, when the supplied action itself encounters an

 * exception, then the stage completes exceptionally with this

 * exception unless the source stage also completed exceptionally, in

 * which case the exceptional completion from the source stage is

 * given preference and propagated to the dependent stage.

 *

 * </ul>

 *

 * <p>All methods adhere to the above triggering, execution, and

 * exceptional completion specifications (which are not repeated in

 * individual method specifications). Additionally, while arguments

 * used to pass a completion result (that is, for parameters of type

 * {@code T}) for methods accepting them may be null, passing a null

 * value for any other parameter will result in a {@link

 * NullPointerException} being thrown.

 *

 * <p>Method form {@link #handle handle} is the most general way of

 * creating a continuation stage, unconditionally performing a

 * computation that is given both the result and exception (if any) of

 * the triggering CompletionStage, and computing an arbitrary result.

 * Method {@link #whenComplete whenComplete} is similar, but preserves

 * the result of the triggering stage instead of computing a new one.

 * Because a stage's normal result may be {@code null}, both methods

 * should have a computation structured thus:

 *

 * <pre>{@code (result, exception) -> {

 *   if (exception == null) {

 *     // triggering stage completed normally

 *   } else {

 *     // triggering stage completed exceptionally

 *   }

 * }}</pre>

 *

 * <p>This interface does not define methods for initially creating,

 * forcibly completing normally or exceptionally, probing completion

 * status or results, or awaiting completion of a stage.

 * Implementations of CompletionStage may provide means of achieving

 * such effects, as appropriate.  Method {@link #toCompletableFuture}

 * enables interoperability among different implementations of this

 * interface by providing a common conversion type.

 *

 * @author Doug Lea

 * @since 1.8

 */

public interface CompletionStage<T> {

    /**

     * Returns a new CompletionStage that, when this stage completes

     * normally, is executed with this stage's result as the argument

     * to the supplied function.

     *

     * <p>This method is analogous to

     * {@link java.util.Optional#map Optional.map} and

     * {@link java.util.stream.Stream#map Stream.map}.

     *

     * <p>See the {@link CompletionStage} documentation for rules

     * covering exceptional completion.

     *

     * @param fn the function to use to compute the value of the

     * returned CompletionStage

     * @param <U> the function's return type

     * @return the new CompletionStage

     */

    public <U> CompletionStage<U> thenApply(Function<? super T,? extends U> fn);

    /**

     * Returns a new CompletionStage that, when this stage completes

     * normally, is executed using this stage's default asynchronous

     * execution facility, with this stage's result as the argument to

     * the supplied function.

     *

     * See the {@link CompletionStage} documentation for rules

     * covering exceptional completion.

     *

     * @param fn the function to use to compute the value of the

     * returned CompletionStage

     * @param <U> the function's return type

     * @return the new CompletionStage

     */

    public <U> CompletionStage<U> thenApplyAsync

        (Function<? super T,? extends U> fn);

    /**

     * Returns a new CompletionStage that, when this stage completes

     * normally, is executed using the supplied Executor, with this

     * stage's result as the argument to the supplied function.

     *

     * See the {@link CompletionStage} documentation for rules

     * covering exceptional completion.

     *

     * @param fn the function to use to compute the value of the

     * returned CompletionStage

     * @param executor the executor to use for asynchronous execution

     * @param <U> the function's return type

     * @return the new CompletionStage

     */

    public <U> CompletionStage<U> thenApplyAsync

        (Function<? super T,? extends U> fn,

         Executor executor);

    /**

     * Returns a new CompletionStage that, when this stage completes

     * normally, is executed with this stage's result as the argument

     * to the supplied action.

     *

     * See the {@link CompletionStage} documentation for rules

     * covering exceptional completion.

     *

     * @param action the action to perform before completing the

     * returned CompletionStage

     * @return the new CompletionStage

     */

    public CompletionStage<Void> thenAccept(Consumer<? super T> action);

    /**

     * Returns a new CompletionStage that, when this stage completes

     * normally, is executed using this stage's default asynchronous

     * execution facility, with this stage's result as the argument to

     * the supplied action.

     *

     * See the {@link CompletionStage} documentation for rules

     * covering exceptional completion.

     *

     * @param action the action to perform before completing the

     * returned CompletionStage

     * @return the new CompletionStage

     */

    public CompletionStage<Void> thenAcceptAsync(Consumer<? super T> action);

    /**

     * Returns a new CompletionStage that, when this stage completes

     * normally, is executed using the supplied Executor, with this

     * stage's result as the argument to the supplied action.

     *

     * See the {@link CompletionStage} documentation for rules

     * covering exceptional completion.

     *

     * @param action the action to perform before completing the

     * returned CompletionStage

     * @param executor the executor to use for asynchronous execution

     * @return the new CompletionStage

     */

    public CompletionStage<Void> thenAcceptAsync(Consumer<? super T> action,

                                                 Executor executor);

    /**

     * Returns a new CompletionStage that, when this stage completes

     * normally, executes the given action.

     *

     * See the {@link CompletionStage} documentation for rules

     * covering exceptional completion.

     *

     * @param action the action to perform before completing the

     * returned CompletionStage

     * @return the new CompletionStage

     */

    public CompletionStage<Void> thenRun(Runnable action);

    /**

     * Returns a new CompletionStage that, when this stage completes

     * normally, executes the given action using this stage's default

     * asynchronous execution facility.

     *

     * See the {@link CompletionStage} documentation for rules

     * covering exceptional completion.

     *

     * @param action the action to perform before completing the

     * returned CompletionStage

     * @return the new CompletionStage

     */

    public CompletionStage<Void> thenRunAsync(Runnable action);

    /**

     * Returns a new CompletionStage that, when this stage completes

     * normally, executes the given action using the supplied Executor.

     *

     * See the {@link CompletionStage} documentation for rules

     * covering exceptional completion.

     *

     * @param action the action to perform before completing the

     * returned CompletionStage

     * @param executor the executor to use for asynchronous execution

     * @return the new CompletionStage

     */

    public CompletionStage<Void> thenRunAsync(Runnable action,

                                              Executor executor);

    /**

     * Returns a new CompletionStage that, when this and the other

     * given stage both complete normally, is executed with the two

     * results as arguments to the supplied function.

     *

     * See the {@link CompletionStage} documentation for rules

     * covering exceptional completion.

     *

     * @param other the other CompletionStage

     * @param fn the function to use to compute the value of the

     * returned CompletionStage

     * @param <U> the type of the other CompletionStage's result

     * @param <V> the function's return type

     * @return the new CompletionStage

     */

    public <U,V> CompletionStage<V> thenCombine

        (CompletionStage<? extends U> other,

         BiFunction<? super T,? super U,? extends V> fn);

    /**

     * Returns a new CompletionStage that, when this and the other

     * given stage both complete normally, is executed using this

     * stage's default asynchronous execution facility, with the two

     * results as arguments to the supplied function.

     *

     * See the {@link CompletionStage} documentation for rules

     * covering exceptional completion.

     *

     * @param other the other CompletionStage

     * @param fn the function to use to compute the value of the

     * returned CompletionStage

     * @param <U> the type of the other CompletionStage's result

     * @param <V> the function's return type

     * @return the new CompletionStage

     */

    public <U,V> CompletionStage<V> thenCombineAsync

        (CompletionStage<? extends U> other,

         BiFunction<? super T,? super U,? extends V> fn);

    /**

     * Returns a new CompletionStage that, when this and the other

     * given stage both complete normally, is executed using the

     * supplied executor, with the two results as arguments to the

     * supplied function.

     *

     * See the {@link CompletionStage} documentation for rules

     * covering exceptional completion.

     *

     * @param other the other CompletionStage

     * @param fn the function to use to compute the value of the

     * returned CompletionStage

     * @param executor the executor to use for asynchronous execution

     * @param <U> the type of the other CompletionStage's result

     * @param <V> the function's return type

     * @return the new CompletionStage

     */

    public <U,V> CompletionStage<V> thenCombineAsync

        (CompletionStage<? extends U> other,

         BiFunction<? super T,? super U,? extends V> fn,

         Executor executor);

    /**

     * Returns a new CompletionStage that, when this and the other

     * given stage both complete normally, is executed with the two

     * results as arguments to the supplied action.

     *

     * See the {@link CompletionStage} documentation for rules

     * covering exceptional completion.

     *

     * @param other the other CompletionStage

     * @param action the action to perform before completing the

     * returned CompletionStage

     * @param <U> the type of the other CompletionStage's result

     * @return the new CompletionStage

     */

    public <U> CompletionStage<Void> thenAcceptBoth

        (CompletionStage<? extends U> other,

         BiConsumer<? super T, ? super U> action);

    /**

     * Returns a new CompletionStage that, when this and the other

     * given stage both complete normally, is executed using this

     * stage's default asynchronous execution facility, with the two

     * results as arguments to the supplied action.

     *

     * See the {@link CompletionStage} documentation for rules

     * covering exceptional completion.

     *

     * @param other the other CompletionStage

     * @param action the action to perform before completing the

     * returned CompletionStage

     * @param <U> the type of the other CompletionStage's result

     * @return the new CompletionStage

     */

    public <U> CompletionStage<Void> thenAcceptBothAsync

        (CompletionStage<? extends U> other,

         BiConsumer<? super T, ? super U> action);

    /**

     * Returns a new CompletionStage that, when this and the other

     * given stage both complete normally, is executed using the

     * supplied executor, with the two results as arguments to the

     * supplied action.

     *

     * See the {@link CompletionStage} documentation for rules

     * covering exceptional completion.

     *

     * @param other the other CompletionStage

     * @param action the action to perform before completing the

     * returned CompletionStage

     * @param executor the executor to use for asynchronous execution

     * @param <U> the type of the other CompletionStage's result

     * @return the new CompletionStage

     */

    public <U> CompletionStage<Void> thenAcceptBothAsync

        (CompletionStage<? extends U> other,

         BiConsumer<? super T, ? super U> action,

         Executor executor);

    /**

     * Returns a new CompletionStage that, when this and the other

     * given stage both complete normally, executes the given action.

     *

     * See the {@link CompletionStage} documentation for rules

     * covering exceptional completion.

     *

     * @param other the other CompletionStage

     * @param action the action to perform before completing the

     * returned CompletionStage

     * @return the new CompletionStage

     */

    public CompletionStage<Void> runAfterBoth(CompletionStage<?> other,

                                              Runnable action);

    /**

     * Returns a new CompletionStage that, when this and the other

     * given stage both complete normally, executes the given action

     * using this stage's default asynchronous execution facility.

     *

     * See the {@link CompletionStage} documentation for rules

     * covering exceptional completion.

     *

     * @param other the other CompletionStage

     * @param action the action to perform before completing the

     * returned CompletionStage

     * @return the new CompletionStage

     */

    public CompletionStage<Void> runAfterBothAsync(CompletionStage<?> other,

                                                   Runnable action);

    /**

     * Returns a new CompletionStage that, when this and the other

     * given stage both complete normally, executes the given action

     * using the supplied executor.

     *

     * See the {@link CompletionStage} documentation for rules

     * covering exceptional completion.

     *

     * @param other the other CompletionStage

     * @param action the action to perform before completing the

     * returned CompletionStage

     * @param executor the executor to use for asynchronous execution

     * @return the new CompletionStage

     */

    public CompletionStage<Void> runAfterBothAsync(CompletionStage<?> other,

                                                   Runnable action,

                                                   Executor executor);

    /**

     * Returns a new CompletionStage that, when either this or the

     * other given stage complete normally, is executed with the

     * corresponding result as argument to the supplied function.

     *

     * See the {@link CompletionStage} documentation for rules

     * covering exceptional completion.

     *

     * @param other the other CompletionStage

     * @param fn the function to use to compute the value of the

     * returned CompletionStage

     * @param <U> the function's return type

     * @return the new CompletionStage

     */

    public <U> CompletionStage<U> applyToEither

        (CompletionStage<? extends T> other,

         Function<? super T, U> fn);

    /**

     * Returns a new CompletionStage that, when either this or the

     * other given stage complete normally, is executed using this

     * stage's default asynchronous execution facility, with the

     * corresponding result as argument to the supplied function.

     *

     * See the {@link CompletionStage} documentation for rules

     * covering exceptional completion.

     *

     * @param other the other CompletionStage

     * @param fn the function to use to compute the value of the

     * returned CompletionStage

     * @param <U> the function's return type

     * @return the new CompletionStage

     */

    public <U> CompletionStage<U> applyToEitherAsync

        (CompletionStage<? extends T> other,

         Function<? super T, U> fn);

    /**

     * Returns a new CompletionStage that, when either this or the

     * other given stage complete normally, is executed using the

     * supplied executor, with the corresponding result as argument to

     * the supplied function.

     *

     * See the {@link CompletionStage} documentation for rules

     * covering exceptional completion.

     *

     * @param other the other CompletionStage

     * @param fn the function to use to compute the value of the

     * returned CompletionStage

     * @param executor the executor to use for asynchronous execution

     * @param <U> the function's return type

     * @return the new CompletionStage

     */

    public <U> CompletionStage<U> applyToEitherAsync

        (CompletionStage<? extends T> other,

         Function<? super T, U> fn,

         Executor executor);

    /**

     * Returns a new CompletionStage that, when either this or the

     * other given stage complete normally, is executed with the