Module java.base
Package java.util.concurrent

## Class CompletableFuture<T>

- java.lang.Object
- - java.util.concurrent.CompletableFuture<T>

- Type Parameters:
    `T` - The result type returned by this future's `join` and `get` methods
  All Implemented Interfaces:
    `CompletionStage<T>`, `Future<T>`

      public class CompletableFuture<T>
      extends Object
      implements Future<T>, CompletionStage<T>

  A `Future` that may be explicitly completed (setting its value and status), and may be used as a `CompletionStage`, supporting dependent functions and actions that trigger upon its completion.

  When two or more threads attempt to `complete`, `completeExceptionally`, or `cancel` a CompletableFuture, only one of them succeeds.

  In addition to these and related methods for directly manipulating status and results, CompletableFuture implements interface `CompletionStage` with the following policies:

  - Actions supplied for dependent completions of non-async methods may be performed by the thread that completes the current CompletableFuture, or by any other caller of a completion method.
  - All async methods without an explicit Executor argument are performed using the `ForkJoinPool.commonPool()` (unless it does not support a parallelism level of at least two, in which case, a new Thread is created to run each task). This may be overridden for non-static methods in subclasses by defining method `defaultExecutor()`. To simplify monitoring, debugging, and tracking, all generated asynchronous tasks are instances of the marker interface `CompletableFuture.AsynchronousCompletionTask`. Operations with time-delays can use adapter methods defined in this class, for example: `supplyAsync(supplier, delayedExecutor(timeout, timeUnit))`. To support methods with delays and timeouts, this class maintains at most one daemon thread for triggering and cancelling actions, not for running them.
  - All CompletionStage methods are implemented independently of other public methods, so the behavior of one method is not impacted by overrides of others in subclasses.
  - All CompletionStage methods return CompletableFutures. To restrict usages to only those methods defined in interface CompletionStage, use method `minimalCompletionStage()`. Or to ensure only that clients do not themselves modify a future, use method `copy()`.

  CompletableFuture also implements `Future` with the following policies:

  - Since (unlike `FutureTask`) this class has no direct control over the computation that causes it to be completed, cancellation is treated as just another form of exceptional completion. Method `cancel` has the same effect as `completeExceptionally(new CancellationException())`. Method `isCompletedExceptionally()` can be used to determine if a CompletableFuture completed in any exceptional fashion.
  - In case of exceptional completion with a CompletionException, methods `get()` and `get(long, TimeUnit)` throw an `ExecutionException` with the same cause as held in the corresponding CompletionException. To simplify usage in most contexts, this class also defines methods `join()` and `getNow(T)` that instead throw the CompletionException directly in these cases.

  Arguments used to pass a completion result (that is, for parameters of type `T`) for methods accepting them may be null, but passing a null value for any other parameter will result in a `NullPointerException` being thrown.

  Subclasses of this class should normally override the "virtual constructor" method `newIncompleteFuture()`, which establishes the concrete type returned by CompletionStage methods. For example, here is a class that substitutes a different default Executor and disables the `obtrude` methods:

       class MyCompletableFuture<T> extends CompletableFuture<T> {
         static final Executor myExecutor = ...;
         public MyCompletableFuture() { }
         public <U> CompletableFuture<U> newIncompleteFuture() {
           return new MyCompletableFuture<U>(); }
         public Executor defaultExecutor() {
           return myExecutor; }
         public void obtrudeValue(T value) {
           throw new UnsupportedOperationException(); }
         public void obtrudeException(Throwable ex) {
           throw new UnsupportedOperationException(); }
       }

  Since:
    1.8

- - ### Nested Class Summary

    Nested Classes
    Modifier and Type | Class | Description
    --- | --- | ---
    `static interface` | `CompletableFuture.AsynchronousCompletionTask` | A marker interface identifying asynchronous tasks produced by `async` methods.

  - ### Constructor Summary

    Constructors
    Constructor | Description
    --- | ---
    `CompletableFuture()` | Creates a new incomplete CompletableFuture.

  - ### Method Summary

    Modifier and Type | Method | Description
    --- | --- | ---
    `static CompletableFuture<Void>` | `allOf​(CompletableFuture<?>... cfs)` | Returns a new CompletableFuture that is completed when all of the given CompletableFutures complete.
    `static CompletableFuture<Object>` | `anyOf​(CompletableFuture<?>... cfs)` | Returns a new CompletableFuture that is completed when any of the given CompletableFutures complete, with the same result.
    `boolean` | `cancel​(boolean mayInterruptIfRunning)` | If not already completed, completes this CompletableFuture with a `CancellationException`.
    `boolean` | `complete​(T value)` | If not already completed, sets the value returned by `get()` and related methods to the given value.
    `CompletableFuture<T>` | `completeAsync​(Supplier<? extends T> supplier)` | Completes this CompletableFuture with the result of the given Supplier function invoked from an asynchronous task using the default executor.
    `CompletableFuture<T>` | `completeAsync​(Supplier<? extends T> supplier, Executor executor)` | Completes this CompletableFuture with the result of the given Supplier function invoked from an asynchronous task using the given executor.
    `static <U> CompletableFuture<U>` | `completedFuture​(U value)` | Returns a new CompletableFuture that is already completed with the given value.
    `static <U> CompletionStage<U>` | `completedStage​(U value)` | Returns a new CompletionStage that is already completed with the given value and supports only those methods in interface `CompletionStage`.
    `boolean` | `completeExceptionally​(Throwable ex)` | If not already completed, causes invocations of `get()` and related methods to throw the given exception.
    `CompletableFuture<T>` | `completeOnTimeout​(T value, long timeout, TimeUnit unit)` | Completes this CompletableFuture with the given value if not otherwise completed before the given timeout.
    `CompletableFuture<T>` | `copy()` | Returns a new CompletableFuture that is completed normally with the same value as this CompletableFuture when it completes normally.
    `Executor` | `defaultExecutor()` | Returns the default Executor used for async methods that do not specify an Executor.
    `static Executor` | `delayedExecutor​(long delay, TimeUnit unit)` | Returns a new Executor that submits a task to the default executor after the given delay (or no delay if non-positive).
    `static Executor` | `delayedExecutor​(long delay, TimeUnit unit, Executor executor)` | Returns a new Executor that submits a task to the given base executor after the given delay (or no delay if non-positive).
    `static <U> CompletableFuture<U>` | `failedFuture​(Throwable ex)` | Returns a new CompletableFuture that is already completed exceptionally with the given exception.
    `static <U> CompletionStage<U>` | `failedStage​(Throwable ex)` | Returns a new CompletionStage that is already completed exceptionally with the given exception and supports only those methods in interface `CompletionStage`.
    `T` | `get()` | Waits if necessary for this future to complete, and then returns its result.
    `T` | `get​(long timeout, TimeUnit unit)` | Waits if necessary for at most the given time for this future to complete, and then returns its result, if available.
    `T` | `getNow​(T valueIfAbsent)` | Returns the result value (or throws any encountered exception) if completed, else returns the given valueIfAbsent.
    `int` | `getNumberOfDependents()` | Returns the estimated number of CompletableFutures whose completions are awaiting completion of this CompletableFuture.
    `boolean` | `isCancelled()` | Returns `true` if this CompletableFuture was cancelled before it completed normally.
    `boolean` | `isCompletedExceptionally()` | Returns `true` if this CompletableFuture completed exceptionally, in any way.
    `boolean` | `isDone()` | Returns `true` if completed in any fashion: normally, exceptionally, or via cancellation.
    `T` | `join()` | Returns the result value when complete, or throws an (unchecked) exception if completed exceptionally.
    `CompletionStage<T>` | `minimalCompletionStage()` | Returns a new CompletionStage that is completed normally with the same value as this CompletableFuture when it completes normally, and cannot be independently completed or otherwise used in ways not defined by the methods of interface `CompletionStage`.
    `<U> CompletableFuture<U>` | `newIncompleteFuture()` | Returns a new incomplete CompletableFuture of the type to be returned by a CompletionStage method.
    `void` | `obtrudeException​(Throwable ex)` | Forcibly causes subsequent invocations of method `get()` and related methods to throw the given exception, whether or not already completed.
    `void` | `obtrudeValue​(T value)` | Forcibly sets or resets the value subsequently returned by method `get()` and related methods, whether or not already completed.
    `CompletableFuture<T>` | `orTimeout​(long timeout, TimeUnit unit)` | Exceptionally completes this CompletableFuture with a `TimeoutException` if not otherwise completed before the given timeout.
    `static CompletableFuture<Void>` | `runAsync​(Runnable runnable)` | Returns a new CompletableFuture that is asynchronously completed by a task running in the `ForkJoinPool.commonPool()` after it runs the given action.
    `static CompletableFuture<Void>` | `runAsync​(Runnable runnable, Executor executor)` | Returns a new CompletableFuture that is asynchronously completed by a task running in the given executor after it runs the given action.
    `static <U> CompletableFuture<U>` | `supplyAsync​(Supplier<U> supplier)` | Returns a new CompletableFuture that is asynchronously completed by a task running in the `ForkJoinPool.commonPool()` with the value obtained by calling the given Supplier.
    `static <U> CompletableFuture<U>` | `supplyAsync​(Supplier<U> supplier, Executor executor)` | Returns a new CompletableFuture that is asynchronously completed by a task running in the given executor with the value obtained by calling the given Supplier.
    `CompletableFuture<T>` | `toCompletableFuture()` | Returns this CompletableFuture.
    `String` | `toString()` | Returns a string identifying this CompletableFuture, as well as its completion state.

    - ### Methods declared in class java.lang.Object

      `clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait`

    - ### Methods declared in interface java.util.concurrent.CompletionStage

      `acceptEither, acceptEitherAsync, acceptEitherAsync, applyToEither, applyToEitherAsync, applyToEitherAsync, exceptionally, exceptionallyAsync, exceptionallyAsync, exceptionallyCompose, exceptionallyComposeAsync, exceptionallyComposeAsync, handle, handleAsync, handleAsync, runAfterBoth, runAfterBothAsync, runAfterBothAsync, runAfterEither, runAfterEitherAsync, runAfterEitherAsync, thenAccept, thenAcceptAsync, thenAcceptAsync, thenAcceptBoth, thenAcceptBothAsync, thenAcceptBothAsync, thenApply, thenApplyAsync, thenApplyAsync, thenCombine, thenCombineAsync, thenCombineAsync, thenCompose, thenComposeAsync, thenComposeAsync, thenRun, thenRunAsync, thenRunAsync, whenComplete, whenCompleteAsync, whenCompleteAsync`

- - ### Constructor Detail

    - #### CompletableFuture

          public CompletableFuture()

      Creates a new incomplete CompletableFuture.

  - ### Method Detail

    - #### supplyAsync

          public static <U> CompletableFuture<U> supplyAsync​(Supplier<U> supplier)

      Returns a new CompletableFuture that is asynchronously completed by a task running in the `ForkJoinPool.commonPool()` with the value obtained by calling the given Supplier.
      Type Parameters:
        `U` - the function's return type
      Parameters:
        `supplier` - a function returning the value to be used to complete the returned CompletableFuture
      Returns:
        the new CompletableFuture

    - #### supplyAsync

          public static <U> CompletableFuture<U> supplyAsync​(Supplier<U> supplier,
                                                             Executor executor)

      Returns a new CompletableFuture that is asynchronously completed by a task running in the given executor with the value obtained by calling the given Supplier.
      Type Parameters:
        `U` - the function's return type
      Parameters:
        `supplier` - a function returning the value to be used to complete the returned CompletableFuture
        `executor` - the executor to use for asynchronous execution
      Returns:
        the new CompletableFuture

    - #### runAsync

          public static CompletableFuture<Void> runAsync​(Runnable runnable)

      Returns a new CompletableFuture that is asynchronously completed by a task running in the `ForkJoinPool.commonPool()` after it runs the given action.
      Parameters:
        `runnable` - the action to run before completing the returned CompletableFuture
      Returns:
        the new CompletableFuture

    - #### runAsync

          public static CompletableFuture<Void> runAsync​(Runnable runnable,
                                                         Executor executor)

      Returns a new CompletableFuture that is asynchronously completed by a task running in the given executor after it runs the given action.
      Parameters:
        `runnable` - the action to run before completing the returned CompletableFuture
        `executor` - the executor to use for asynchronous execution
      Returns:
        the new CompletableFuture

    - #### completedFuture

          public static <U> CompletableFuture<U> completedFuture​(U value)

      Returns a new CompletableFuture that is already completed with the given value.
      Type Parameters:
        `U` - the type of the value
      Parameters:
        `value` - the value
      Returns:
        the completed CompletableFuture

    - #### isDone

          public boolean isDone()

      Returns `true` if completed in any fashion: normally, exceptionally, or via cancellation.
      Specified by:
        `isDone` in interface `Future<T>`
      Returns:
        `true` if completed

    - #### get

          public T get()
                throws InterruptedException,
                       ExecutionException

      Waits if necessary for this future to complete, and then returns its result.
      Specified by:
        `get` in interface `Future<T>`
      Returns:
        the result value
      Throws:
        `CancellationException` - if this future was cancelled
        `ExecutionException` - if this future completed exceptionally
        `InterruptedException` - if the current thread was interrupted while waiting

    - #### get

          public T get​(long timeout,
                       TimeUnit unit)
                throws InterruptedException,
                       ExecutionException,
                       TimeoutException

      Waits if necessary for at most the given time for this future to complete, and then returns its result, if available.
      Specified by:
        `get` in interface `Future<T>`
      Parameters:
        `timeout` - the maximum time to wait
        `unit` - the time unit of the timeout argument
      Returns:
        the result value
      Throws:
        `CancellationException` - if this future was cancelled
        `ExecutionException` - if this future completed exceptionally
        `InterruptedException` - if the current thread was interrupted while waiting
        `TimeoutException` - if the wait timed out

    - #### join

          public T join()

      Returns the result value when complete, or throws an (unchecked) exception if completed exceptionally. To better conform with the use of common functional forms, if a computation involved in the completion of this CompletableFuture threw an exception, this method throws an (unchecked) `CompletionException` with the underlying exception as its cause.
      Returns:
        the result value
      Throws:
        `CancellationException` - if the computation was cancelled
        `CompletionException` - if this future completed exceptionally or a completion computation threw an exception

    - #### getNow

          public T getNow​(T valueIfAbsent)

      Returns the result value (or throws any encountered exception) if completed, else returns the given valueIfAbsent.
      Parameters:
        `valueIfAbsent` - the value to return if not completed
      Returns:
        the result value, if completed, else the given valueIfAbsent
      Throws:
        `CancellationException` - if the computation was cancelled
        `CompletionException` - if this future completed exceptionally or a completion computation threw an exception

    - #### complete

          public boolean complete​(T value)

      If not already completed, sets the value returned by `get()` and related methods to the given value.
      Parameters:
        `value` - the result value
      Returns:
        `true` if this invocation caused this CompletableFuture to transition to a completed state, else `false`

    - #### completeExceptionally

          public boolean completeExceptionally​(Throwable ex)

      If not already completed, causes invocations of `get()` and related methods to throw the given exception.
      Parameters:
        `ex` - the exception
      Returns:
        `true` if this invocation caused this CompletableFuture to transition to a completed state, else `false`

    - #### toCompletableFuture

          public CompletableFuture<T> toCompletableFuture()

      Returns this CompletableFuture.
      Specified by:
        `toCompletableFuture` in interface `CompletionStage<T>`
      Returns:
        this CompletableFuture

    - #### allOf

          public static CompletableFuture<Void> allOf​(CompletableFuture<?>... cfs)

      Returns a new CompletableFuture that is completed when all of the given CompletableFutures complete. If any of the given CompletableFutures complete exceptionally, then the returned CompletableFuture also does so, with a CompletionException holding this exception as its cause. Otherwise, the results, if any, of the given CompletableFutures are not reflected in the returned CompletableFuture, but may be obtained by inspecting them individually. If no CompletableFutures are provided, returns a CompletableFuture completed with the value `null`.

      Among the applications of this method is to await completion of a set of independent CompletableFutures before continuing a program, as in: `CompletableFuture.allOf(c1, c2, c3).join();`.

      Parameters:
        `cfs` - the CompletableFutures
      Returns:
        a new CompletableFuture that is completed when all of the given CompletableFutures complete
      Throws:
        `NullPointerException` - if the array or any of its elements are `null`

    - #### anyOf

          public static CompletableFuture<Object> anyOf​(CompletableFuture<?>... cfs)

      Returns a new CompletableFuture that is completed when any of the given CompletableFutures complete, with the same result. Otherwise, if it completed exceptionally, the returned CompletableFuture also does so, with a CompletionException holding this exception as its cause. If no CompletableFutures are provided, returns an incomplete CompletableFuture.
      Parameters:
        `cfs` - the CompletableFutures
      Returns:
        a new CompletableFuture that is completed with the result or exception of any of the given CompletableFutures when one completes
      Throws:
        `NullPointerException` - if the array or any of its elements are `null`

    - #### cancel

          public boolean cancel​(boolean mayInterruptIfRunning)

      If not already completed, completes this CompletableFuture with a `CancellationException`. Dependent CompletableFutures that have not already completed will also complete exceptionally, with a `CompletionException` caused by this `CancellationException`.
      Specified by:
        `cancel` in interface `Future<T>`
      Parameters:
        `mayInterruptIfRunning` - this value has no effect in this implementation because interrupts are not used to control processing.
      Returns:
        `true` if this task is now cancelled

    - #### isCancelled

          public boolean isCancelled()

      Returns `true` if this CompletableFuture was cancelled before it completed normally.
      Specified by:
        `isCancelled` in interface `Future<T>`
      Returns:
        `true` if this CompletableFuture was cancelled before it completed normally

    - #### isCompletedExceptionally

          public boolean isCompletedExceptionally()

      Returns `true` if this CompletableFuture completed exceptionally, in any way. Possible causes include cancellation, explicit invocation of `completeExceptionally`, and abrupt termination of a CompletionStage action.
      Returns:
        `true` if this CompletableFuture completed exceptionally

    - #### obtrudeValue

          public void obtrudeValue​(T value)

      Forcibly sets or resets the value subsequently returned by method `get()` and related methods, whether or not already completed. This method is designed for use only in error recovery actions, and even in such situations may result in ongoing dependent completions using established versus overwritten outcomes.
      Parameters:
        `value` - the completion value

    - #### obtrudeException

          public void obtrudeException​(Throwable ex)

      Forcibly causes subsequent invocations of method `get()` and related methods to throw the given exception, whether or not already completed. This method is designed for use only in error recovery actions, and even in such situations may result in ongoing dependent completions using established versus overwritten outcomes.
      Parameters:
        `ex` - the exception
      Throws:
        `NullPointerException` - if the exception is null

    - #### getNumberOfDependents

          public int getNumberOfDependents()

      Returns the estimated number of CompletableFutures whose completions are awaiting completion of this CompletableFuture. This method is designed for use in monitoring system state, not for synchronization control.
      Returns:
        the number of dependent CompletableFutures

    - #### toString

          public String toString()

      Returns a string identifying this CompletableFuture, as well as its completion state. The state, in brackets, contains the String `"Completed Normally"` or the String `"Completed Exceptionally"`, or the String `"Not completed"` followed by the number of CompletableFutures dependent upon its completion, if any.
      Overrides:
        `toString` in class `Object`
      Returns:
        a string identifying this CompletableFuture, as well as its state

    - #### newIncompleteFuture

          public <U> CompletableFuture<U> newIncompleteFuture()

      Returns a new incomplete CompletableFuture of the type to be returned by a CompletionStage method. Subclasses should normally override this method to return an instance of the same class as this CompletableFuture. The default implementation returns an instance of class CompletableFuture.
      Type Parameters:
        `U` - the type of the value
      Returns:
        a new CompletableFuture
      Since:
        9

    - #### defaultExecutor

          public Executor defaultExecutor()

      Returns the default Executor used for async methods that do not specify an Executor. This class uses the `ForkJoinPool.commonPool()` if it supports more than one parallel thread, or else an Executor using one thread per async task. This method may be overridden in subclasses to return an Executor that provides at least one independent thread.
      Returns:
        the executor
      Since:
        9

    - #### copy

          public CompletableFuture<T> copy()

      Returns a new CompletableFuture that is completed normally with the same value as this CompletableFuture when it completes normally. If this CompletableFuture completes exceptionally, then the returned CompletableFuture completes exceptionally with a CompletionException with this exception as cause. The behavior is equivalent to `thenApply(x -> x)`. This method may be useful as a form of "defensive copying", to prevent clients from completing, while still being able to arrange dependent actions.
      Returns:
        the new CompletableFuture
      Since:
        9

    - #### minimalCompletionStage

          public CompletionStage<T> minimalCompletionStage()

      Returns a new CompletionStage that is completed normally with the same value as this CompletableFuture when it completes normally, and cannot be independently completed or otherwise used in ways not defined by the methods of interface `CompletionStage`. If this CompletableFuture completes exceptionally, then the returned CompletionStage completes exceptionally with a CompletionException with this exception as cause.

      Unless overridden by a subclass, a new non-minimal CompletableFuture with all methods available can be obtained from a minimal CompletionStage via `toCompletableFuture()`. For example, completion of a minimal stage can be awaited by

           minimalStage.toCompletableFuture().join();

      Returns:
        the new CompletionStage
      Since:
        9

    - #### completeAsync

          public CompletableFuture<T> completeAsync​(Supplier<? extends T> supplier,
                                                    Executor executor)

      Completes this CompletableFuture with the result of the given Supplier function invoked from an asynchronous task using the given executor.
      Parameters:
        `supplier` - a function returning the value to be used to complete this CompletableFuture
        `executor` - the executor to use for asynchronous execution
      Returns:
        this CompletableFuture
      Since:
        9

    - #### completeAsync

          public CompletableFuture<T> completeAsync​(Supplier<? extends T> supplier)

      Completes this CompletableFuture with the result of the given Supplier function invoked from an asynchronous task using the default executor.
      Parameters:
        `supplier` - a function returning the value to be used to complete this CompletableFuture
      Returns:
        this CompletableFuture
      Since:
        9

    - #### orTimeout

          public CompletableFuture<T> orTimeout​(long timeout,
                                                TimeUnit unit)

      Exceptionally completes this CompletableFuture with a `TimeoutException` if not otherwise completed before the given timeout.
      Parameters:
        `timeout` - how long to wait before completing exceptionally with a TimeoutException, in units of `unit`
        `unit` - a `TimeUnit` determining how to interpret the `timeout` parameter
      Returns:
        this CompletableFuture
      Since:
        9

    - #### completeOnTimeout

          public CompletableFuture<T> completeOnTimeout​(T value,
                                                        long timeout,
                                                        TimeUnit unit)

      Completes this CompletableFuture with the given value if not otherwise completed before the given timeout.
      Parameters:
        `value` - the value to use upon timeout
        `timeout` - how long to wait before completing normally with the given value, in units of `unit`
        `unit` - a `TimeUnit` determining how to interpret the `timeout` parameter
      Returns:
        this CompletableFuture
      Since:
        9

    - #### delayedExecutor

          public static Executor delayedExecutor​(long delay,
                                                 TimeUnit unit,
                                                 Executor executor)

      Returns a new Executor that submits a task to the given base executor after the given delay (or no delay if non-positive). Each delay commences upon invocation of the returned executor's `execute` method.
      Parameters:
        `delay` - how long to delay, in units of `unit`
        `unit` - a `TimeUnit` determining how to interpret the `delay` parameter
        `executor` - the base executor
      Returns:
        the new delayed executor
      Since:
        9

    - #### delayedExecutor

          public static Executor delayedExecutor​(long delay,
                                                 TimeUnit unit)

      Returns a new Executor that submits a task to the default executor after the given delay (or no delay if non-positive). Each delay commences upon invocation of the returned executor's `execute` method.
      Parameters:
        `delay` - how long to delay, in units of `unit`
        `unit` - a `TimeUnit` determining how to interpret the `delay` parameter
      Returns:
        the new delayed executor
      Since:
        9

    - #### completedStage

          public static <U> CompletionStage<U> completedStage​(U value)

      Returns a new CompletionStage that is already completed with the given value and supports only those methods in interface `CompletionStage`.
      Type Parameters:
        `U` - the type of the value
      Parameters:
        `value` - the value
      Returns:
        the completed CompletionStage
      Since:
        9

    - #### failedFuture

          public static <U> CompletableFuture<U> failedFuture​(Throwable ex)

      Returns a new CompletableFuture that is already completed exceptionally with the given exception.
      Type Parameters:
        `U` - the type of the value
      Parameters:
        `ex` - the exception
      Returns:
        the exceptionally completed CompletableFuture
      Since:
        9

    - #### failedStage

          public static <U> CompletionStage<U> failedStage​(Throwable ex)

      Returns a new CompletionStage that is already completed exceptionally with the given exception and supports only those methods in interface `CompletionStage`.
      Type Parameters:
        `U` - the type of the value
      Parameters:
        `ex` - the exception
      Returns:
        the exceptionally completed CompletionStage
      Since:
        9