Added in API level 24

CountedCompleter

abstract class CountedCompleter<T : Any!> : ForkJoinTask<T>

kotlin.Any
↳	java.util.concurrent.ForkJoinTask<T>
	↳	java.util.concurrent.CountedCompleter

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 java.nio.channels.CompletionHandler) except that multiple pending completions may be necessary to trigger the completion action , not just one. Unless initialized otherwise, the pending starts at zero, but may be (atomically) changed using methods setPendingCount, addToPendingCount, and compareAndSetPendingCount. 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 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, 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.

<code>public static &lt;E&gt; void forEach(E[] array, Consumer&lt;E&gt; action) {
    class Task extends CountedCompleter&lt;Void&gt; {
      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 &gt;= 2) {
          int mid = (lo + hi) &gt;&gt;&gt; 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 &gt; lo)
          action.accept(array[lo]);
        tryComplete();
      }
    }
    new Task(null, 0, array.length).invoke();
  }</code>

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".

<code>public void compute() {
        if (hi - lo &gt;= 2) {
          int mid = (lo + hi) &gt;&gt;&gt; 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 &gt; lo)
            action.accept(array[lo]);
          tryComplete();
        }
      }</code>

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.

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

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

<code>public static &lt;E&gt; void forEach(E[] array, Consumer&lt;E&gt; action) {
    class Task extends CountedCompleter&lt;Void&gt; {
      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 &gt;= 2; n /= 2)
          new Task(this, lo + n/2, lo + n).fork();
        action.accept(array[lo]);
        propagateCompletion();
      }
    }
    if (array.length &gt; 0)
      new Task(null, 0, array.length).invoke();
  }</code>

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 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):

<code>class Searcher&lt;E&gt; extends CountedCompleter&lt;E&gt; {
    final E[] array; final AtomicReference&lt;E&gt; result; final int lo, hi;
    Searcher(CountedCompleter&lt;?&gt; p, E[] array, AtomicReference&lt;E&gt; 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 &amp;&amp; h &gt;= l) {
        if (h - l &gt;= 2) {
          int mid = (l + h) &gt;&gt;&gt; 1;
          addToPendingCount(1);
          new Searcher(this, array, result, mid, h).fork();
          h = mid;
        }
        else {
          E x = array[l];
          if (matches(x) &amp;&amp; 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 &lt;E&gt; E search(E[] array) {
        return new Searcher&lt;E&gt;(null, array, new AtomicReference&lt;E&gt;(), 0, array.length).invoke();
    }
  }</code>

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.

<code>class MyMapper&lt;E&gt; { E apply(E v) {  ...  } }
  class MyReducer&lt;E&gt; { E apply(E x, E y) {  ...  } }
  class MapReducer&lt;E&gt; extends CountedCompleter&lt;E&gt; {
    final E[] array; final MyMapper&lt;E&gt; mapper;
    final MyReducer&lt;E&gt; reducer; final int lo, hi;
    MapReducer&lt;E&gt; sibling;
    E result;
    MapReducer(CountedCompleter&lt;?&gt; p, E[] array, MyMapper&lt;E&gt; mapper,
               MyReducer&lt;E&gt; 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 &gt;= 2) {
        int mid = (lo + hi) &gt;&gt;&gt; 1;
        MapReducer&lt;E&gt; left = new MapReducer(this, array, mapper, reducer, lo, mid);
        MapReducer&lt;E&gt; 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 &gt; lo)
            result = mapper.apply(array[lo]);
        tryComplete();
      }
    }
    public void onCompletion(CountedCompleter&lt;?&gt; caller) {
      if (caller != this) {
        MapReducer&lt;E&gt; child = (MapReducer&lt;E&gt;)caller;
        MapReducer&lt;E&gt; 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 &lt;E&gt; E mapReduce(E[] array, MyMapper&lt;E&gt; mapper, MyReducer&lt;E&gt; reducer) {
      return new MapReducer&lt;E&gt;(null, array, mapper, reducer,
                               0, array.length).invoke();
    }
  }</code>

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:

<code>class MapReducer&lt;E&gt; extends CountedCompleter&lt;E&gt; { // version 2
    final E[] array; final MyMapper&lt;E&gt; mapper;
    final MyReducer&lt;E&gt; reducer; final int lo, hi;
    MapReducer&lt;E&gt; forks, next; // record subtask forks in list
    E result;
    MapReducer(CountedCompleter&lt;?&gt; p, E[] array, MyMapper&lt;E&gt; mapper,
               MyReducer&lt;E&gt; reducer, int lo, int hi, MapReducer&lt;E&gt; 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 &gt;= 2) {
        int mid = (l + h) &gt;&gt;&gt; 1;
        addToPendingCount(1);
        (forks = new MapReducer(this, array, mapper, reducer, mid, h, forks)).fork();
        h = mid;
      }
      if (h &gt; l)
        result = mapper.apply(array[l]);
      // process completions by reducing along and advancing subtask links
      for (CountedCompleter&lt;?&gt; 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 &lt;E&gt; E mapReduce(E[] array, MyMapper&lt;E&gt; mapper, MyReducer&lt;E&gt; reducer) {
      return new MapReducer&lt;E&gt;(null, array, mapper, reducer,
                               0, array.length, null).invoke();
    }
  }</code>

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:

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

Summary

Protected constructors
CountedCompleter(completer: CountedCompleter<*>!, initialPendingCount: Int) Creates a new CountedCompleter with the given completer and initial pending count.
CountedCompleter(completer: CountedCompleter<*>!) Creates a new CountedCompleter with the given completer and an initial pending count of zero.
CountedCompleter() Creates a new CountedCompleter with no completer and an initial pending count of zero.

Public methods
Unit	addToPendingCount(delta: Int) Adds (atomically) the given value to the pending count.
Boolean	compareAndSetPendingCount(expected: Int, count: Int) Sets (atomically) the pending count to the given count only if it currently holds the given expected value.
open Unit	complete(rawResult: T) 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 Unit	compute() The main computation performed by this task.
Int	decrementPendingCountUnlessZero() If the pending count is nonzero, (atomically) decrements it.
CountedCompleter<*>!	firstComplete() If this task's pending count is zero, returns this task; otherwise decrements its pending count and returns null.
CountedCompleter<*>!	getCompleter() Returns the completer established in this task's constructor, or null if none.
Int	getPendingCount() Returns the current pending count.
open T	getRawResult() Returns the result of the computation.
CountedCompleter<*>!	getRoot() Returns the root of the current computation; i.
Unit	helpComplete(maxTasks: Int) 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.
CountedCompleter<*>!	nextComplete() If this task does not have a completer, invokes java.util.concurrent.ForkJoinTask#quietlyComplete and returns null.
open Unit	onCompletion(caller: CountedCompleter<*>!) Performs an action when method tryComplete is invoked and the pending count is zero, or when the unconditional method complete is invoked.
open Boolean	onExceptionalCompletion(ex: Throwable!, caller: CountedCompleter<*>!) Performs an action when method completeExceptionally(java.lang.Throwable) is invoked or method compute throws an exception, and this task has not already otherwise completed normally.
Unit	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.
Unit	quietlyCompleteRoot() Equivalent to getRoot().quietlyComplete().
Unit	setPendingCount(count: Int) Sets the pending count to the given value.
Unit	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
Boolean	exec() Implements execution conventions for CountedCompleters.
open Unit	setRawResult(t: T) A method that result-bearing CountedCompleters may optionally use to help maintain result data.

Inherited functions

From class ForkJoinTask 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. ForkJoinTask<T>! adapt(runnable: Runnable!, result: T) Returns a new ForkJoinTask that performs the run method of the given Runnable as its action, and returns the given result upon join. ForkJoinTask<T>! adapt(callable: Callable<out T>!) 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(mayInterruptIfRunning: Boolean) Attempts to cancel execution of this task. This attempt will fail if the task has already completed or could not be cancelled for some other reason. If successful, and this task has not started when cancel is called, execution of this task is suppressed. After this method returns successfully, unless there is an intervening call to reinitialize, subsequent calls to isCancelled, isDone, and cancel will return true and calls to join and related methods will result in CancellationException. This method may be overridden in subclasses, but if so, must still ensure that these properties hold. In particular, the cancel method itself must not throw exceptions. This method is designed to be invoked by other tasks. To terminate the current task, you can just return or throw an unchecked exception from its computation method, or invoke completeExceptionally(java.lang.Throwable). Boolean compareAndSetForkJoinTaskTag(expect: Short, update: Short) Atomically conditionally sets the tag value for this task. Among other applications, tags can be used as visit markers in tasks operating on graphs, as in methods that check: if (task.compareAndSetForkJoinTaskTag((short)0, (short)1)) before processing, otherwise exiting because the node has already been visited. Unit completeExceptionally(ex: Throwable!) Completes this task abnormally, and if not already aborted or cancelled, causes it to throw the given exception upon join and related operations. This method may be used to induce exceptions in asynchronous tasks, or to force completion of tasks that would not otherwise complete. Its use in other situations is discouraged. This method is overridable, but overridden versions must invoke super implementation to maintain guarantees. ForkJoinTask<V>! fork() Arranges to asynchronously execute this task in the pool the current task is running in, if applicable, or using the java.util.concurrent.ForkJoinPool#commonPool() if not inForkJoinPool. While it is not necessarily enforced, it is a usage error to fork a task more than once unless it has completed and been reinitialized. Subsequent modifications to the state of this task or any data it operates on are not necessarily consistently observable by any thread other than the one executing it unless preceded by a call to join or related methods, or a call to isDone returning true. V get() Waits if necessary for the computation to complete, and then retrieves its result. V get(timeout: Long, unit: TimeUnit!) Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available. 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. Short getForkJoinTaskTag() Returns the tag for this task. ForkJoinPool! getPool() Returns the pool hosting the current thread, or null if the current thread is executing outside of any ForkJoinPool. This method returns null if and only if inForkJoinPool returns false. Int getQueuedTaskCount() Returns an estimate of the number of tasks that have been forked by the current worker thread but not yet executed. This value may be useful for heuristic decisions about whether to fork other tasks. 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. This value may be useful for heuristic decisions about whether to fork other tasks. In many usages of ForkJoinTasks, at steady state, each worker should aim to maintain a small constant surplus (for example, 3) of tasks, and to process computations locally if this threshold is exceeded. Unit helpQuiesce() Possibly executes tasks until the pool hosting the current task is quiescent. This method may be of use in designs in which many tasks are forked, but none are explicitly joined, instead executing them until all are processed. Boolean inForkJoinPool() Returns true if the current thread is a ForkJoinWorkerThread executing as a ForkJoinPool computation. V 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. Unit invokeAll(t1: ForkJoinTask<*>!, t2: ForkJoinTask<*>!) Forks the given tasks, returning when isDone holds for each task or an (unchecked) exception is encountered, in which case the exception is rethrown. If more than one task encounters an exception, then this method throws any one of these exceptions. If any task encounters an exception, the other may be cancelled. However, the execution status of individual tasks is not guaranteed upon exceptional return. The status of each task may be obtained using getException() and related methods to check if they have been cancelled, completed normally or exceptionally, or left unprocessed. Unit invokeAll(vararg tasks: ForkJoinTask<*>!) Forks the given tasks, returning when isDone holds for each task or an (unchecked) exception is encountered, in which case the exception is rethrown. If more than one task encounters an exception, then this method throws any one of these exceptions. If any task encounters an exception, others may be cancelled. However, the execution status of individual tasks is not guaranteed upon exceptional return. The status of each task may be obtained using getException() and related methods to check if they have been cancelled, completed normally or exceptionally, or left unprocessed. MutableCollection<T>! invokeAll(tasks: MutableCollection<T>!) 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. If more than one task encounters an exception, then this method throws any one of these exceptions. If any task encounters an exception, others may be cancelled. However, the execution status of individual tasks is not guaranteed upon exceptional return. The status of each task may be obtained using getException() and related methods to check if they have been cancelled, completed normally or exceptionally, or left unprocessed. Boolean isCancelled() Boolean isCompletedAbnormally() Returns true if this task threw an exception or was cancelled. Boolean isCompletedNormally() Returns true if this task completed without throwing an exception and was not cancelled. Boolean isDone() V join() Returns the result of the computation when it is done. This method differs from get() in that abnormal completion results in RuntimeException or Error, not ExecutionException, and that interrupts of the calling thread do not cause the method to abruptly return by throwing InterruptedException. 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. There is no guarantee that this task will actually be polled or executed next. Conversely, this method may return null even if a task exists but cannot be accessed without contention with other threads. This method is designed primarily to support extensions, and is unlikely to be useful otherwise. 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. This method is designed primarily to support extensions, and is unlikely to be useful otherwise. 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. Availability may be transient, so a null result does not necessarily imply quiescence of the pool this task is operating in. This method is designed primarily to support extensions, and is unlikely to be useful otherwise. Unit quietlyComplete() Completes this task normally without setting a value. The most recent value established by setRawResult (or null by default) will be returned as the result of subsequent invocations of join and related operations. Unit quietlyInvoke() Commences performing this task and awaits its completion if necessary, without returning its result or throwing its exception. Unit quietlyJoin() Joins this task, without returning its result or throwing its exception. This method may be useful when processing collections of tasks when some have been cancelled or otherwise known to have aborted. Unit reinitialize() Resets the internal bookkeeping state of this task, allowing a subsequent fork. This method allows repeated reuse of this task, but only if reuse occurs when this task has either never been forked, or has been forked, then completed and all outstanding joins of this task have also completed. Effects under any other usage conditions are not guaranteed. This method may be useful when executing pre-constructed trees of subtasks in loops. Upon completion of this method, isDone() reports false, and getException() reports null. However, the value returned by getRawResult is unaffected. To clear this value, you can invoke setRawResult(null). Short setForkJoinTaskTag(newValue: Short) Atomically sets the tag value for this task and returns the old value. Boolean tryUnfork() Tries to unschedule this task for execution. This method will typically (but is not guaranteed to) succeed if this task is the most recently forked task by the current thread, and has not commenced executing in another thread. This method may be useful when arranging alternative local processing of tasks that could have been, but were not, stolen.

Protected constructors

CountedCompleter

Added in API level 24

protected CountedCompleter(
    completer: CountedCompleter<*>!, 
    initialPendingCount: Int)

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

CountedCompleter

Added in API level 24

protected CountedCompleter(completer: CountedCompleter<*>!)

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()

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

Public methods

addToPendingCount

Added in API level 24

fun addToPendingCount(delta: Int): Unit

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

Parameters
delta	Int: the value to add

compareAndSetPendingCount

Added in API level 24

fun compareAndSetPendingCount(
    expected: Int, 
    count: Int
): Boolean

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

Return
Boolean	true if successful

complete

Added in API level 24

open fun complete(rawResult: T): Unit

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 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
value	the result value for this task
rawResult	T: the raw result

compute

Added in API level 24

abstract fun compute(): Unit

The main computation performed by this task.

decrementPendingCountUnlessZero

Added in API level 24

fun decrementPendingCountUnlessZero(): Int

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

Return
Int	the initial (undecremented) pending count holding on entry to this method

firstComplete

Added in API level 24

fun firstComplete(): CountedCompleter<*>!

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.

Return
CountedCompleter<*>!	this task, if pending count was zero, else null

getCompleter

Added in API level 24

fun getCompleter(): CountedCompleter<*>!

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

Return
CountedCompleter<*>!	the completer

getPendingCount

Added in API level 24

fun getPendingCount(): Int

Returns the current pending count.

Return
Int	the current pending count

getRawResult

Added in API level 24

open fun getRawResult(): T

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.

Return
T	the result of the computation

getRoot

Added in API level 24

fun getRoot(): CountedCompleter<*>!

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

Return
CountedCompleter<*>!	the root of the current computation

helpComplete

Added in API level 24

fun helpComplete(maxTasks: Int): Unit

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

fun nextComplete(): CountedCompleter<*>!

If this task does not have a completer, invokes java.util.concurrent.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:

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

Return
CountedCompleter<*>!	the completer, or null if none

onCompletion

Added in API level 24

open fun onCompletion(caller: CountedCompleter<*>!): Unit

Performs an action when method tryComplete is invoked and the pending count is zero, or when the unconditional method complete 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

open fun onExceptionalCompletion(
    ex: Throwable!, 
    caller: CountedCompleter<*>!
): Boolean

Performs an action when method 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)

Return
Boolean	true if this exception should be propagated to this task's completer, if one exists

propagateCompletion

Added in API level 24

fun propagateCompletion(): Unit

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

fun quietlyCompleteRoot(): Unit

Equivalent to getRoot().quietlyComplete().

setPendingCount

Added in API level 24

fun setPendingCount(count: Int): Unit

Sets the pending count to the given value.

Parameters
count	Int: the count

tryComplete

Added in API level 24

fun tryComplete(): Unit

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 fun exec(): Boolean

Implements execution conventions for CountedCompleters.

Return
Boolean	true if this task is known to have completed normally

setRawResult

Added in API level 24

protected open fun setRawResult(t: T): Unit

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
value	the value