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/* |
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* Written by Doug Lea with assistance from members of JCP JSR-166 |
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* Expert Group and released to the public domain, as explained at |
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* http://creativecommons.org/licenses/publicdomain |
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* http://creativecommons.org/publicdomain/zero/1.0/ |
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*/ |
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package jsr166y; |
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* A recursive resultless {@link ForkJoinTask}. This class |
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* establishes conventions to parameterize resultless actions as |
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* {@code Void} {@code ForkJoinTask}s. Because {@code null} is the |
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* only valid value of type {@code Void}, methods such as join always |
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* return {@code null} upon completion. |
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* only valid value of type {@code Void}, methods such as {@code join} |
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* always return {@code null} upon completion. |
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* |
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* <p><b>Sample Usages.</b> Here is a sketch of a ForkJoin sort that |
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* sorts a given {@code long[]} array: |
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* |
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* <pre> {@code |
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* class SortTask extends RecursiveAction { |
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* final long[] array; final int lo; final int hi; |
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* final long[] array; final int lo, hi; |
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* SortTask(long[] array, int lo, int hi) { |
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* this.array = array; this.lo = lo; this.hi = hi; |
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* } |
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* int mid = (lo + hi) >>> 1; |
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* invokeAll(new SortTask(array, lo, mid), |
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* new SortTask(array, mid, hi)); |
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* merge(array, lo, hi); |
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* merge(array, lo, mid, hi); |
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* } |
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* } |
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* }}</pre> |
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* |
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* You could then sort {@code anArray} by creating {@code new |
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* SortTask(anArray, 0, anArray.length-1) } and invoking it in a |
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* SortTask(anArray, 0, anArray.length) } and invoking it in a |
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* ForkJoinPool. As a more concrete simple example, the following |
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* task increments each element of an array: |
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* <pre> {@code |
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* class IncrementTask extends RecursiveAction { |
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* final long[] array; final int lo; final int hi; |
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* final long[] array; final int lo, hi; |
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* IncrementTask(long[] array, int lo, int hi) { |
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* this.array = array; this.lo = lo; this.hi = hi; |
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* } |
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* <pre> {@code |
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* double sumOfSquares(ForkJoinPool pool, double[] array) { |
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* int n = array.length; |
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* int seqSize = 1 + n / (8 * pool.getParallelism()); |
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* Applyer a = new Applyer(array, 0, n, seqSize, null); |
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* Applyer a = new Applyer(array, 0, n, null); |
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* pool.invoke(a); |
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* return a.result; |
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* } |
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* |
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* class Applyer extends RecursiveAction { |
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* final double[] array; |
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* final int lo, hi, seqSize; |
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* final int lo, hi; |
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* double result; |
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* Applyer next; // keeps track of right-hand-side tasks |
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* Applyer(double[] array, int lo, int hi, int seqSize, Applyer next) { |
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* Applyer(double[] array, int lo, int hi, Applyer next) { |
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* this.array = array; this.lo = lo; this.hi = hi; |
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* this.seqSize = seqSize; this.next = next; |
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* this.next = next; |
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* } |
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* |
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* double atLeaf(int l, int r) { |
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* double atLeaf(int l, int h) { |
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* double sum = 0; |
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* for (int i = l; i < h; ++i) // perform leftmost base step |
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* sum += array[i] * array[i]; |
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* Applyer right = null; |
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* while (h - l > 1 && getSurplusQueuedTaskCount() <= 3) { |
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* int mid = (l + h) >>> 1; |
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* right = new Applyer(array, mid, h, seqSize, right); |
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* right = new Applyer(array, mid, h, right); |
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* right.fork(); |
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* h = mid; |
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* } |
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* if (right.tryUnfork()) // directly calculate if not stolen |
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* sum += right.atLeaf(right.lo, right.hi); |
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* else { |
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* right.helpJoin(); |
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* right.join(); |
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* sum += right.result; |
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* } |
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* right = right.next; |
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protected abstract void compute(); |
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/** |
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* Always returns null. |
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* Always returns {@code null}. |
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* |
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* @return {@code null} always |
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*/ |
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public final Void getRawResult() { return null; } |
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