Added in API level 24

Summary: Protected Ctors | Methods | Protected Methods | Inherited Methods

CountedCompleter

public abstract class CountedCompleter
extends ForkJoinTask<T>

java.lang.Object
↳	java.util.concurrent.ForkJoinTask<T>
	↳	java.util.concurrent.CountedCompleter<T>

A ForkJoinTask with a completion action performed when triggered and there are no remaining pending actions. CountedCompleters are in general more robust in the presence of subtask stalls and blockage than are other forms of ForkJoinTasks, but are less intuitive to program. Uses of CountedCompleter are similar to those of other completion based components (such as CompletionHandler) except that multiple pending completions may be necessary to trigger the completion action onCompletion(java.util.concurrent.CountedCompleter), not just one. Unless initialized otherwise, the pending count starts at zero, but may be (atomically) changed using methods setPendingCount(int), addToPendingCount(int), and compareAndSetPendingCount(int, int). Upon invocation of tryComplete(), if the pending action count is nonzero, it is decremented; otherwise, the completion action is performed, and if this completer itself has a completer, the process is continued with its completer. As is the case with related synchronization components such as Phaser and Semaphore, these methods affect only internal counts; they do not establish any further internal bookkeeping. In particular, the identities of pending tasks are not maintained. As illustrated below, you can create subclasses that do record some or all pending tasks or their results when needed. As illustrated below, utility methods supporting customization of completion traversals are also provided. However, because CountedCompleters provide only basic synchronization mechanisms, it may be useful to create further abstract subclasses that maintain linkages, fields, and additional support methods appropriate for a set of related usages.

A concrete CountedCompleter class must define method compute(), that should in most cases (as illustrated below), invoke tryComplete() once before returning. The class may also optionally override method onCompletion(java.util.concurrent.CountedCompleter) to perform an action upon normal completion, and method onExceptionalCompletion(java.lang.Throwable, java.util.concurrent.CountedCompleter) to perform an action upon any exception.

CountedCompleters most often do not bear results, in which case they are normally declared as CountedCompleter<Void>, and will always return null as a result value. In other cases, you should override method getRawResult() to provide a result from join(), invoke(), and related methods. In general, this method should return the value of a field (or a function of one or more fields) of the CountedCompleter object that holds the result upon completion. Method setRawResult(T) by default plays no role in CountedCompleters. It is possible, but rarely applicable, to override this method to maintain other objects or fields holding result data.

A CountedCompleter that does not itself have a completer (i.e., one for which getCompleter() returns null) can be used as a regular ForkJoinTask with this added functionality. However, any completer that in turn has another completer serves only as an internal helper for other computations, so its own task status (as reported in methods such as ForkJoinTask.isDone) is arbitrary; this status changes only upon explicit invocations of complete(T), ForkJoinTask.cancel, ForkJoinTask.completeExceptionally(Throwable) or upon exceptional completion of method compute. Upon any exceptional completion, the exception may be relayed to a task's completer (and its completer, and so on), if one exists and it has not otherwise already completed. Similarly, cancelling an internal CountedCompleter has only a local effect on that completer, so is not often useful.

Sample Usages.

Parallel recursive decomposition. CountedCompleters may be arranged in trees similar to those often used with RecursiveActions, although the constructions involved in setting them up typically vary. Here, the completer of each task is its parent in the computation tree. Even though they entail a bit more bookkeeping, CountedCompleters may be better choices when applying a possibly time-consuming operation (that cannot be further subdivided) to each element of an array or collection; especially when the operation takes a significantly different amount of time to complete for some elements than others, either because of intrinsic variation (for example I/O) or auxiliary effects such as garbage collection. Because CountedCompleters provide their own continuations, other tasks need not block waiting to perform them.

For example, here is an initial version of a utility method that uses divide-by-two recursive decomposition to divide work into single pieces (leaf tasks). Even when work is split into individual calls, tree-based techniques are usually preferable to directly forking leaf tasks, because they reduce inter-thread communication and improve load balancing. In the recursive case, the second of each pair of subtasks to finish triggers completion of their parent (because no result combination is performed, the default no-op implementation of method onCompletion is not overridden). The utility method sets up the root task and invokes it (here, implicitly using the ForkJoinPool.commonPool()). It is straightforward and reliable (but not optimal) to always set the pending count to the number of child tasks and call tryComplete() immediately before returning.

 public static <E> void forEach(E[] array, Consumer<E> action) {
   class Task extends CountedCompleter<Void> {
     final int lo, hi;
     Task(Task parent, int lo, int hi) {
       super(parent); this.lo = lo; this.hi = hi;
     }

     public void compute() {
       if (hi - lo >= 2) {
         int mid = (lo + hi) >>> 1;
         // must set pending count before fork
         setPendingCount(2);
         new Task(this, mid, hi).fork(); // right child
         new Task(this, lo, mid).fork(); // left child
       }
       else if (hi > lo)
         action.accept(array[lo]);
       tryComplete();
     }
   }
   new Task(null, 0, array.length).invoke();
 }

This design can be improved by noticing that in the recursive case, the task has nothing to do after forking its right task, so can directly invoke its left task before returning. (This is an analog of tail recursion removal.) Also, when the last action in a task is to fork or invoke a subtask (a "tail call"), the call to tryComplete() can be optimized away, at the cost of making the pending count look "off by one".

 public void compute() {
       if (hi - lo >= 2) {
         int mid = (lo + hi) >>> 1;
         setPendingCount(1); // looks off by one, but correct!
         new Task(this, mid, hi).fork(); // right child
         new Task(this, lo, mid).compute(); // direct invoke
       } else {
         if (hi > lo)
           action.accept(array[lo]);
         tryComplete();
       }
     }

As a further optimization, notice that the left task need not even exist. Instead of creating a new one, we can continue using the original task, and add a pending count for each fork. Additionally, because no task in this tree implements an onCompletion(java.util.concurrent.CountedCompleter) method, tryComplete can be replaced with propagateCompletion().

 public void compute() {
       int n = hi - lo;
       for (; n >= 2; n /= 2) {
         addToPendingCount(1);
         new Task(this, lo + n/2, lo + n).fork();
       }
       if (n > 0)
         action.accept(array[lo]);
       propagateCompletion();
     }

When pending counts can be precomputed, they can be established in the constructor:

 public static <E> void forEach(E[] array, Consumer<E> action) {
   class Task extends CountedCompleter<Void> {
     final int lo, hi;
     Task(Task parent, int lo, int hi) {
       super(parent, 31 - Integer.numberOfLeadingZeros(hi - lo));
       this.lo = lo; this.hi = hi;
     }

     public void compute() {
       for (int n = hi - lo; n >= 2; n /= 2)
         new Task(this, lo + n/2, lo + n).fork();
       action.accept(array[lo]);
       propagateCompletion();
     }
   }
   if (array.length > 0)
     new Task(null, 0, array.length).invoke();
 }

Additional optimizations of such classes might entail specializing classes for leaf steps, subdividing by say, four, instead of two per iteration, and using an adaptive threshold instead of always subdividing down to single elements.

Searching. A tree of CountedCompleters can search for a value or property in different parts of a data structure, and report a result in an AtomicReference as soon as one is found. The others can poll the result to avoid unnecessary work. (You could additionally cancel other tasks, but it is usually simpler and more efficient to just let them notice that the result is set and if so skip further processing.) Illustrating again with an array using full partitioning (again, in practice, leaf tasks will almost always process more than one element):

 class Searcher<E> extends CountedCompleter<E> {
   final E[] array; final AtomicReference<E> result; final int lo, hi;
   Searcher(CountedCompleter<?> p, E[] array, AtomicReference<E> result, int lo, int hi) {
     super(p);
     this.array = array; this.result = result; this.lo = lo; this.hi = hi;
   }
   public E getRawResult() { return result.get(); }
   public void compute() { // similar to ForEach version 3
     int l = lo, h = hi;
     while (result.get() == null && h >= l) {
       if (h - l >= 2) {
         int mid = (l + h) >>> 1;
         addToPendingCount(1);
         new Searcher(this, array, result, mid, h).fork();
         h = mid;
       }
       else {
         E x = array[l];
         if (matches(x) && result.compareAndSet(null, x))
           quietlyCompleteRoot(); // root task is now joinable
         break;
       }
     }
     tryComplete(); // normally complete whether or not found
   }
   boolean matches(E e) { ... } // return true if found

   public static <E> E search(E[] array) {
       return new Searcher<E>(null, array, new AtomicReference<E>(), 0, array.length).invoke();
   }
 }

In this example, as well as others in which tasks have no other effects except to compareAndSet a common result, the trailing unconditional invocation of tryComplete could be made conditional (if (result.get() == null) tryComplete();) because no further bookkeeping is required to manage completions once the root task completes.

Recording subtasks. CountedCompleter tasks that combine results of multiple subtasks usually need to access these results in method onCompletion(java.util.concurrent.CountedCompleter). As illustrated in the following class (that performs a simplified form of map-reduce where mappings and reductions are all of type E), one way to do this in divide and conquer designs is to have each subtask record its sibling, so that it can be accessed in method onCompletion. This technique applies to reductions in which the order of combining left and right results does not matter; ordered reductions require explicit left/right designations. Variants of other streamlinings seen in the above examples may also apply.

 class MyMapper<E> { E apply(E v) {  ...  } }
 class MyReducer<E> { E apply(E x, E y) {  ...  } }
 class MapReducer<E> extends CountedCompleter<E> {
   final E[] array; final MyMapper<E> mapper;
   final MyReducer<E> reducer; final int lo, hi;
   MapReducer<E> sibling;
   E result;
   MapReducer(CountedCompleter<?> p, E[] array, MyMapper<E> mapper,
              MyReducer<E> reducer, int lo, int hi) {
     super(p);
     this.array = array; this.mapper = mapper;
     this.reducer = reducer; this.lo = lo; this.hi = hi;
   }
   public void compute() {
     if (hi - lo >= 2) {
       int mid = (lo + hi) >>> 1;
       MapReducer<E> left = new MapReducer(this, array, mapper, reducer, lo, mid);
       MapReducer<E> right = new MapReducer(this, array, mapper, reducer, mid, hi);
       left.sibling = right;
       right.sibling = left;
       setPendingCount(1); // only right is pending
       right.fork();
       left.compute();     // directly execute left
     }
     else {
       if (hi > lo)
           result = mapper.apply(array[lo]);
       tryComplete();
     }
   }
   public void onCompletion(CountedCompleter<?> caller) {
     if (caller != this) {
       MapReducer<E> child = (MapReducer<E>)caller;
       MapReducer<E> sib = child.sibling;
       if (sib == null || sib.result == null)
         result = child.result;
       else
         result = reducer.apply(child.result, sib.result);
     }
   }
   public E getRawResult() { return result; }

   public static <E> E mapReduce(E[] array, MyMapper<E> mapper, MyReducer<E> reducer) {
     return new MapReducer<E>(null, array, mapper, reducer,
                              0, array.length).invoke();
   }
 }

Here, method onCompletion takes a form common to many completion designs that combine results. This callback-style method is triggered once per task, in either of the two different contexts in which the pending count is, or becomes, zero: (1) by a task itself, if its pending count is zero upon invocation of tryComplete, or (2) by any of its subtasks when they complete and decrement the pending count to zero. The caller argument distinguishes cases. Most often, when the caller is this, no action is necessary. Otherwise the caller argument can be used (usually via a cast) to supply a value (and/or links to other values) to be combined. Assuming proper use of pending counts, the actions inside onCompletion occur (once) upon completion of a task and its subtasks. No additional synchronization is required within this method to ensure thread safety of accesses to fields of this task or other completed tasks.

Completion Traversals. If using onCompletion to process completions is inapplicable or inconvenient, you can use methods firstComplete() and nextComplete() to create custom traversals. For example, to define a MapReducer that only splits out right-hand tasks in the form of the third ForEach example, the completions must cooperatively reduce along unexhausted subtask links, which can be done as follows:

 class MapReducer<E> extends CountedCompleter<E> { // version 2
   final E[] array; final MyMapper<E> mapper;
   final MyReducer<E> reducer; final int lo, hi;
   MapReducer<E> forks, next; // record subtask forks in list
   E result;
   MapReducer(CountedCompleter<?> p, E[] array, MyMapper<E> mapper,
              MyReducer<E> reducer, int lo, int hi, MapReducer<E> next) {
     super(p);
     this.array = array; this.mapper = mapper;
     this.reducer = reducer; this.lo = lo; this.hi = hi;
     this.next = next;
   }
   public void compute() {
     int l = lo, h = hi;
     while (h - l >= 2) {
       int mid = (l + h) >>> 1;
       addToPendingCount(1);
       (forks = new MapReducer(this, array, mapper, reducer, mid, h, forks)).fork();
       h = mid;
     }
     if (h > l)
       result = mapper.apply(array[l]);
     // process completions by reducing along and advancing subtask links
     for (CountedCompleter<?> c = firstComplete(); c != null; c = c.nextComplete()) {
       for (MapReducer t = (MapReducer)c, s = t.forks; s != null; s = t.forks = s.next)
         t.result = reducer.apply(t.result, s.result);
     }
   }
   public E getRawResult() { return result; }

   public static <E> E mapReduce(E[] array, MyMapper<E> mapper, MyReducer<E> reducer) {
     return new MapReducer<E>(null, array, mapper, reducer,
                              0, array.length, null).invoke();
   }
 }

Triggers. Some CountedCompleters are themselves never forked, but instead serve as bits of plumbing in other designs; including those in which the completion of one or more async tasks triggers another async task. For example:

 class HeaderBuilder extends CountedCompleter<...> { ... }
 class BodyBuilder extends CountedCompleter<...> { ... }
 class PacketSender extends CountedCompleter<...> {
   PacketSender(...) { super(null, 1); ... } // trigger on second completion
   public void compute() { } // never called
   public void onCompletion(CountedCompleter<?> caller) { sendPacket(); }
 }
 // sample use:
 PacketSender p = new PacketSender();
 new HeaderBuilder(p, ...).fork();
 new BodyBuilder(p, ...).fork();

Summary

Protected constructors
`CountedCompleter()` Creates a new CountedCompleter with no completer and an initial pending count of zero.
`CountedCompleter(CountedCompleter<?> completer)` Creates a new CountedCompleter with the given completer and an initial pending count of zero.
`CountedCompleter(CountedCompleter<?> completer, int initialPendingCount)` Creates a new CountedCompleter with the given completer and initial pending count.

Public methods
`final void`	`addToPendingCount(int delta)` Adds (atomically) the given value to the pending count.
`final boolean`	`compareAndSetPendingCount(int expected, int count)` Sets (atomically) the pending count to the given count only if it currently holds the given expected value.
`void`	`complete(T rawResult)` Regardless of pending count, invokes `onCompletion(java.util.concurrent.CountedCompleter)`, marks this task as complete and further triggers `tryComplete()` on this task's completer, if one exists.
`abstract void`	`compute()` The main computation performed by this task.
`final int`	`decrementPendingCountUnlessZero()` If the pending count is nonzero, (atomically) decrements it.
`final CountedCompleter<?>`	`firstComplete()` If this task's pending count is zero, returns this task; otherwise decrements its pending count and returns `null`.
`final CountedCompleter<?>`	`getCompleter()` Returns the completer established in this task's constructor, or `null` if none.
`final int`	`getPendingCount()` Returns the current pending count.
`T`	`getRawResult()` Returns the result of the computation.
`final CountedCompleter<?>`	`getRoot()` Returns the root of the current computation; i.e., this task if it has no completer, else its completer's root.
`final void`	`helpComplete(int maxTasks)` If this task has not completed, attempts to process at most the given number of other unprocessed tasks for which this task is on the completion path, if any are known to exist.
`final CountedCompleter<?>`	`nextComplete()` If this task does not have a completer, invokes `ForkJoinTask.quietlyComplete()` and returns `null`.
`void`	`onCompletion(CountedCompleter<?> caller)` Performs an action when method `tryComplete()` is invoked and the pending count is zero, or when the unconditional method `complete(T)` is invoked.
`boolean`	`onExceptionalCompletion(Throwable ex, CountedCompleter<?> caller)` Performs an action when method `ForkJoinTask.completeExceptionally(java.lang.Throwable)` is invoked or method `compute()` throws an exception, and this task has not already otherwise completed normally.
`final void`	`propagateCompletion()` Equivalent to `tryComplete()` but does not invoke `onCompletion(java.util.concurrent.CountedCompleter)` along the completion path: If the pending count is nonzero, decrements the count; otherwise, similarly tries to complete this task's completer, if one exists, else marks this task as complete.
`final void`	`quietlyCompleteRoot()` Equivalent to `getRoot().quietlyComplete()`.
`final void`	`setPendingCount(int count)` Sets the pending count to the given value.
`final void`	`tryComplete()` If the pending count is nonzero, decrements the count; otherwise invokes `onCompletion(java.util.concurrent.CountedCompleter)` and then similarly tries to complete this task's completer, if one exists, else marks this task as complete.

Protected methods
`final boolean`	`exec()` Implements execution conventions for CountedCompleters.
`void`	`setRawResult(T t)` A method that result-bearing CountedCompleters may optionally use to help maintain result data.

Inherited methods

From class


        
          java.util.concurrent.ForkJoinTask

`static <T> ForkJoinTask<T>`	`adapt(Runnable runnable, T result)` Returns a new `ForkJoinTask` that performs the `run` method of the given `Runnable` as its action, and returns the given result upon `join()`.
`static ForkJoinTask<?>`	`adapt(Runnable runnable)` Returns a new `ForkJoinTask` that performs the `run` method of the given `Runnable` as its action, and returns a null result upon `join()`.
`static <T> ForkJoinTask<T>`	`adapt(Callable<? extends T> callable)` Returns a new `ForkJoinTask` that performs the `call` method of the given `Callable` as its action, and returns its result upon `join()`, translating any checked exceptions encountered into `RuntimeException`.
`static <T> ForkJoinTask<T>`	`adaptInterruptible(Callable<? extends T> callable)` Returns a new `ForkJoinTask` that performs the `call` method of the given `Callable` as its action, and returns its result upon `join()`, translating any checked exceptions encountered into `RuntimeException`.
`boolean`	`cancel(boolean mayInterruptIfRunning)` Attempts to cancel execution of this task.
`final boolean`	`compareAndSetForkJoinTaskTag(short expect, short update)` Atomically conditionally sets the tag value for this task.
`void`	`complete(T value)` Completes this task, and if not already aborted or cancelled, returning the given value as the result of subsequent invocations of `join` and related operations.
`void`	`completeExceptionally(Throwable ex)` Completes this task abnormally, and if not already aborted or cancelled, causes it to throw the given exception upon `join` and related operations.
`Throwable`	`exceptionNow()` Returns the exception thrown by the task, without waiting.
`abstract boolean`	`exec()` Immediately performs the base action of this task and returns true if, upon return from this method, this task is guaranteed to have completed.
`final ForkJoinTask<T>`	`fork()` Arranges to asynchronously execute this task in the pool the current task is running in, if applicable, or using the `ForkJoinPool.commonPool()` if not `inForkJoinPool()`.
`final T`	`get(long timeout, TimeUnit unit)` Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available.
`final T`	`get()` Waits if necessary for the computation to complete, and then retrieves its result.
`final Throwable`	`getException()` Returns the exception thrown by the base computation, or a `CancellationException` if cancelled, or `null` if none or if the method has not yet completed.
`final short`	`getForkJoinTaskTag()` Returns the tag for this task.
`static ForkJoinPool`	`getPool()` Returns the pool hosting the current thread, or `null` if the current thread is executing outside of any ForkJoinPool.
`static int`	`getQueuedTaskCount()` Returns an estimate of the number of tasks that have been forked by the current worker thread but not yet executed.
`abstract T`	`getRawResult()` Returns the result that would be returned by `join()`, even if this task completed abnormally, or `null` if this task is not known to have been completed.
`static int`	`getSurplusQueuedTaskCount()` Returns an estimate of how many more locally queued tasks are held by the current worker thread than there are other worker threads that might steal them, or zero if this thread is not operating in a ForkJoinPool.
`static void`	`helpQuiesce()` Possibly executes tasks until the pool hosting the current task is quiescent.
`static boolean`	`inForkJoinPool()` Returns `true` if the current thread is a `ForkJoinWorkerThread` executing as a ForkJoinPool computation.
`final T`	`invoke()` Commences performing this task, awaits its completion if necessary, and returns its result, or throws an (unchecked) `RuntimeException` or `Error` if the underlying computation did so.
`static void`	`invokeAll(ForkJoinTask<?> t1, ForkJoinTask<?> t2)` Forks the given tasks, returning when `isDone` holds for each task or an (unchecked) exception is encountered, in which case the exception is rethrown.
`static <T extends ForkJoinTask<?>> Collection<T>`	`invokeAll(Collection<T> tasks)` Forks all tasks in the specified collection, returning when `isDone` holds for each task or an (unchecked) exception is encountered, in which case the exception is rethrown.
`static void`	`invokeAll(ForkJoinTask...<?> tasks)` Forks the given tasks, returning when `isDone` holds for each task or an (unchecked) exception is encountered, in which case the exception is rethrown.
`final boolean`	`isCancelled()` Returns `true` if this task was cancelled before it completed normally.
`final boolean`	`isCompletedAbnormally()` Returns `true` if this task threw an exception or was cancelled.
`final boolean`	`isCompletedNormally()` Returns `true` if this task completed without throwing an exception and was not cancelled.
`final boolean`	`isDone()` Returns `true` if this task completed.
`final T`	`join()` Returns the result of the computation when it is done.
`static ForkJoinTask<?>`	`peekNextLocalTask()` Returns, but does not unschedule or execute, a task queued by the current thread but not yet executed, if one is immediately available.
`static ForkJoinTask<?>`	`pollNextLocalTask()` Unschedules and returns, without executing, the next task queued by the current thread but not yet executed, if the current thread is operating in a ForkJoinPool.
`static ForkJoinTask<?>`	`pollTask()` If the current thread is operating in a ForkJoinPool, unschedules and returns, without executing, the next task queued by the current thread but not yet executed, if one is available, or if not available, a task that was forked by some other thread, if available.
`final void`	`quietlyComplete()` Completes this task normally without setting a value.
`final void`	`quietlyInvoke()` Commences performing this task and awaits its completion if necessary, without returning its result or throwing its exception.
`final boolean`	`quietlyJoin(long timeout, TimeUnit unit)` Tries to join this task, returning true if it completed (possibly exceptionally) before the given timeout and the current thread has not been interrupted.
`final void`	`quietlyJoin()` Joins this task, without returning its result or throwing its exception.
`final boolean`	`quietlyJoinUninterruptibly(long timeout, TimeUnit unit)` Tries to join this task, returning true if it completed (possibly exceptionally) before the given timeout.
`void`	`reinitialize()` Resets the internal bookkeeping state of this task, allowing a subsequent `fork`.
`T`	`resultNow()` Returns the computed result, without waiting.
`final short`	`setForkJoinTaskTag(short newValue)` Atomically sets the tag value for this task and returns the old value.
`abstract void`	`setRawResult(T value)` Forces the given value to be returned as a result.
`Future.State`	`state()`
`boolean`	`tryUnfork()` Tries to unschedule this task for execution.

From class


        
          java.lang.Object

`Object`	`clone()` Creates and returns a copy of this object.
`boolean`	`equals(Object obj)` Indicates whether some other object is "equal to" this one.
`void`	`finalize()` Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.
`final Class<?>`	`getClass()` Returns the runtime class of this `Object`.
`int`	`hashCode()` Returns a hash code value for the object.
`final void`	`notify()` Wakes up a single thread that is waiting on this object's monitor.
`final void`	`notifyAll()` Wakes up all threads that are waiting on this object's monitor.
`String`	`toString()` Returns a string representation of the object.
`final void`	`wait(long timeoutMillis, int nanos)` Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.
`final void`	`wait(long timeoutMillis)` Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.
`final void`	`wait()` Causes the current thread to wait until it is awakened, typically by being notified or interrupted.

From interface


        
          java.util.concurrent.Future

`abstract boolean`	`cancel(boolean mayInterruptIfRunning)` Attempts to cancel execution of this task.
`default Throwable`	`exceptionNow()` Returns the exception thrown by the task, without waiting.
`abstract T`	`get(long timeout, TimeUnit unit)` Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available.
`abstract T`	`get()` Waits if necessary for the computation to complete, and then retrieves its result.
`abstract boolean`	`isCancelled()` Returns `true` if this task was cancelled before it completed normally.
`abstract boolean`	`isDone()` Returns `true` if this task completed.
`default T`	`resultNow()` Returns the computed result, without waiting.
`default Future.State`	`state()`

Protected constructors

CountedCompleter

Added in API level 24

protected CountedCompleter ()

Creates a new CountedCompleter with no completer and an initial pending count of zero.

CountedCompleter

Added in API level 24

protected CountedCompleter (CountedCompleter<?> completer)

Creates a new CountedCompleter with the given completer and an initial pending count of zero.

Parameters
`completer`	`CountedCompleter`: this task's completer, or `null` if none

CountedCompleter

Added in API level 24

protected CountedCompleter (CountedCompleter<?> completer, 
                int initialPendingCount)

Creates a new CountedCompleter with the given completer and initial pending count.

Parameters
`completer`	`CountedCompleter`: this task's completer, or `null` if none
`initialPendingCount`	`int`: the initial pending count

Public methods

addToPendingCount

Added in API level 24

public final void addToPendingCount (int delta)

Adds (atomically) the given value to the pending count.

Parameters
`delta`	`int`: the value to add

compareAndSetPendingCount

Added in API level 24

public final boolean compareAndSetPendingCount (int expected, 
                int count)

Sets (atomically) the pending count to the given count only if it currently holds the given expected value.

Parameters
`expected`	`int`: the expected value
`count`	`int`: the new value

Returns
`boolean`	`true` if successful

complete

Added in API level 24

public void complete (T rawResult)

Regardless of pending count, invokes onCompletion(java.util.concurrent.CountedCompleter), marks this task as complete and further triggers tryComplete() on this task's completer, if one exists. The given rawResult is used as an argument to setRawResult(T) before invoking onCompletion(java.util.concurrent.CountedCompleter) or marking this task as complete; its value is meaningful only for classes overriding setRawResult. This method does not modify the pending count.

This method may be useful when forcing completion as soon as any one (versus all) of several subtask results are obtained. However, in the common (and recommended) case in which setRawResult is not overridden, this effect can be obtained more simply using quietlyCompleteRoot().

Parameters
`rawResult`	`T`: the raw result

compute

Added in API level 24

public abstract void compute ()

The main computation performed by this task.

decrementPendingCountUnlessZero

Added in API level 24

public final int decrementPendingCountUnlessZero ()

If the pending count is nonzero, (atomically) decrements it.

Returns
`int`	the initial (undecremented) pending count holding on entry to this method

firstComplete

Added in API level 24

public final CountedCompleter<?> firstComplete ()

If this task's pending count is zero, returns this task; otherwise decrements its pending count and returns null. This method is designed to be used with nextComplete() in completion traversal loops.

Returns
`CountedCompleter<?>`	this task, if pending count was zero, else `null`

getCompleter

Added in API level 24

public final CountedCompleter<?> getCompleter ()

Returns the completer established in this task's constructor, or null if none.

Returns
`CountedCompleter<?>`	the completer

getPendingCount

Added in API level 24

public final int getPendingCount ()

Returns the current pending count.

Returns
`int`	the current pending count

getRawResult

Added in API level 24

public T getRawResult ()

Returns the result of the computation. By default, returns null, which is appropriate for Void actions, but in other cases should be overridden, almost always to return a field or function of a field that holds the result upon completion.

Returns
`T`	the result of the computation

getRoot

Added in API level 24

public final CountedCompleter<?> getRoot ()

Returns the root of the current computation; i.e., this task if it has no completer, else its completer's root.

Returns
`CountedCompleter<?>`	the root of the current computation

helpComplete

Added in API level 24

public final void helpComplete (int maxTasks)

If this task has not completed, attempts to process at most the given number of other unprocessed tasks for which this task is on the completion path, if any are known to exist.

Parameters
`maxTasks`	`int`: the maximum number of tasks to process. If less than or equal to zero, then no tasks are processed.

nextComplete

Added in API level 24

public final CountedCompleter<?> nextComplete ()

If this task does not have a completer, invokes ForkJoinTask.quietlyComplete() and returns null. Or, if the completer's pending count is non-zero, decrements that pending count and returns null. Otherwise, returns the completer. This method can be used as part of a completion traversal loop for homogeneous task hierarchies:

 for (CountedCompleter<?> c = firstComplete();
      c != null;
      c = c.nextComplete()) {
   // ... process c ...
 }

Returns
`CountedCompleter<?>`	the completer, or `null` if none

onCompletion

Added in API level 24

public void onCompletion (CountedCompleter<?> caller)

Performs an action when method tryComplete() is invoked and the pending count is zero, or when the unconditional method complete(T) is invoked. By default, this method does nothing. You can distinguish cases by checking the identity of the given caller argument. If not equal to this, then it is typically a subtask that may contain results (and/or links to other results) to combine.

Parameters
`caller`	`CountedCompleter`: the task invoking this method (which may be this task itself)

onExceptionalCompletion

Added in API level 24

public boolean onExceptionalCompletion (Throwable ex, 
                CountedCompleter<?> caller)

Performs an action when method ForkJoinTask.completeExceptionally(java.lang.Throwable) is invoked or method compute() throws an exception, and this task has not already otherwise completed normally. On entry to this method, this task ForkJoinTask.isCompletedAbnormally. The return value of this method controls further propagation: If true and this task has a completer that has not completed, then that completer is also completed exceptionally, with the same exception as this completer. The default implementation of this method does nothing except return true.

Parameters
`ex`	`Throwable`: the exception
`caller`	`CountedCompleter`: the task invoking this method (which may be this task itself)

Returns
`boolean`	`true` if this exception should be propagated to this task's completer, if one exists

propagateCompletion

Added in API level 24

public final void propagateCompletion ()

Equivalent to tryComplete() but does not invoke onCompletion(java.util.concurrent.CountedCompleter) along the completion path: If the pending count is nonzero, decrements the count; otherwise, similarly tries to complete this task's completer, if one exists, else marks this task as complete. This method may be useful in cases where onCompletion should not, or need not, be invoked for each completer in a computation.

quietlyCompleteRoot

Added in API level 24

public final void quietlyCompleteRoot ()

Equivalent to getRoot().quietlyComplete().

setPendingCount

Added in API level 24

public final void setPendingCount (int count)

Sets the pending count to the given value.

Parameters
`count`	`int`: the count

tryComplete

Added in API level 24

public final void tryComplete ()

If the pending count is nonzero, decrements the count; otherwise invokes onCompletion(java.util.concurrent.CountedCompleter) and then similarly tries to complete this task's completer, if one exists, else marks this task as complete.

Protected methods

exec

Added in API level 24

protected final boolean exec ()

Implements execution conventions for CountedCompleters.

Returns
`boolean`	`true` if this task is known to have completed normally

setRawResult

Added in API level 24

protected void setRawResult (T t)

A method that result-bearing CountedCompleters may optionally use to help maintain result data. By default, does nothing. Overrides are not recommended. However, if this method is overridden to update existing objects or fields, then it must in general be defined to be thread-safe.

Parameters
`t`	`T`: the value