/* * Written by Doug Lea with assistance from members of JCP JSR-166 * Expert Group and released to the public domain, as explained at * http://creativecommons.org/publicdomain/zero/1.0/ */ package java.util.concurrent; /** * A recursive resultless {@link ForkJoinTask}. This class * establishes conventions to parameterize resultless actions as * {@code Void} {@code ForkJoinTask}s. Because {@code null} is the * only valid value of type {@code Void}, methods such as {@code join} * always return {@code null} upon completion. * *

Sample Usages. Here is a simple but complete ForkJoin * sort that sorts a given {@code long[]} array: * *

 {@code
 * static class SortTask extends RecursiveAction {
 *   final long[] array; final int lo, hi;
 *   SortTask(long[] array, int lo, int hi) {
 *     this.array = array; this.lo = lo; this.hi = hi;
 *   }
 *   SortTask(long[] array) { this(array, 0, array.length); }
 *   protected void compute() {
 *     if (hi - lo < THRESHOLD)
 *       sortSequentially(lo, hi);
 *     else {
 *       int mid = (lo + hi) >>> 1;
 *       invokeAll(new SortTask(array, lo, mid),
 *                 new SortTask(array, mid, hi));
 *       merge(lo, mid, hi);
 *     }
 *   }
 *   // implementation details follow:
 *   static final int THRESHOLD = 1000;
 *   void sortSequentially(int lo, int hi) {
 *     Arrays.sort(array, lo, hi);
 *   }
 *   void merge(int lo, int mid, int hi) {
 *     long[] buf = Arrays.copyOfRange(array, lo, mid);
 *     for (int i = 0, j = lo, k = mid; i < buf.length; j++)
 *       array[j] = (k == hi || buf[i] < array[k]) ?
 *         buf[i++] : array[k++];
 *   }
 * }}
* * You could then sort {@code anArray} by creating {@code new * SortTask(anArray)} and invoking it in a ForkJoinPool. As a more * concrete simple example, the following task increments each element * of an array: *
 {@code
 * class IncrementTask extends RecursiveAction {
 *   final long[] array; final int lo, hi;
 *   IncrementTask(long[] array, int lo, int hi) {
 *     this.array = array; this.lo = lo; this.hi = hi;
 *   }
 *   protected void compute() {
 *     if (hi - lo < THRESHOLD) {
 *       for (int i = lo; i < hi; ++i)
 *         array[i]++;
 *     }
 *     else {
 *       int mid = (lo + hi) >>> 1;
 *       invokeAll(new IncrementTask(array, lo, mid),
 *                 new IncrementTask(array, mid, hi));
 *     }
 *   }
 * }}
* *

The following example illustrates some refinements and idioms * that may lead to better performance: RecursiveActions need not be * fully recursive, so long as they maintain the basic * divide-and-conquer approach. Here is a class that sums the squares * of each element of a double array, by subdividing out only the * right-hand-sides of repeated divisions by two, and keeping track of * them with a chain of {@code next} references. It uses a dynamic * threshold based on method {@code getSurplusQueuedTaskCount}, but * counterbalances potential excess partitioning by directly * performing leaf actions on unstolen tasks rather than further * subdividing. * *

 {@code
 * double sumOfSquares(ForkJoinPool pool, double[] array) {
 *   int n = array.length;
 *   Applyer a = new Applyer(array, 0, n, null);
 *   pool.invoke(a);
 *   return a.result;
 * }
 *
 * class Applyer extends RecursiveAction {
 *   final double[] array;
 *   final int lo, hi;
 *   double result;
 *   Applyer next; // keeps track of right-hand-side tasks
 *   Applyer(double[] array, int lo, int hi, Applyer next) {
 *     this.array = array; this.lo = lo; this.hi = hi;
 *     this.next = next;
 *   }
 *
 *   double atLeaf(int l, int h) {
 *     double sum = 0;
 *     for (int i = l; i < h; ++i) // perform leftmost base step
 *       sum += array[i] * array[i];
 *     return sum;
 *   }
 *
 *   protected void compute() {
 *     int l = lo;
 *     int h = hi;
 *     Applyer right = null;
 *     while (h - l > 1 && getSurplusQueuedTaskCount() <= 3) {
 *       int mid = (l + h) >>> 1;
 *       right = new Applyer(array, mid, h, right);
 *       right.fork();
 *       h = mid;
 *     }
 *     double sum = atLeaf(l, h);
 *     while (right != null) {
 *       if (right.tryUnfork()) // directly calculate if not stolen
 *         sum += right.atLeaf(right.lo, right.hi);
 *       else {
 *         right.join();
 *         sum += right.result;
 *       }
 *       right = right.next;
 *     }
 *     result = sum;
 *   }
 * }}
* * @since 1.7 * @author Doug Lea */ public abstract class RecursiveAction extends ForkJoinTask { private static final long serialVersionUID = 5232453952276485070L; /** * The main computation performed by this task. */ protected abstract void compute(); /** * Always returns {@code null}. * * @return {@code null} always */ public final Void getRawResult() { return null; } /** * Requires null completion value. */ protected final void setRawResult(Void mustBeNull) { } /** * Implements execution conventions for RecursiveActions. */ protected final boolean exec() { compute(); return true; } }