Re: Better RecursiveTask Example

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No "else"s are needed in compute() since the blocks have a return.

On Thu, Nov 25, 2021 at 11:43 AM Doug Lea via Concurrency-interest <
[email protected]> wrote:

> Thanks for the comments and suggestions. Here's pass 2, that tries to
> find a better compromise across concerns. Also pasted below without the
> leading "*"s in case you'd like to experience the copy-paste-run that
> people will be able to do.
>
>   * <p>For example, here is a task-based program for computing Factorials:
>   *
>   * <pre> {@code
>   * import java.util.concurrent.RecursiveTask;
>   * import java.math.BigInteger;
>   * public class Factorial {
>   *   static class FactorialTask extends RecursiveTask<BigInteger> {
>   *     private final int from, to;
>   *     FactorialTask(int from, int to) { this.from = from; this.to = to;
> }
>   *     protected BigInteger compute() {
>   *       if (from == to) {                       // base case
>   *         return BigInteger.valueOf(from);
>   *       } else if (to - from < 2) {             // too small to
> parallelize
>   *          return
> BigInteger.valueOf(from).multiply(BigInteger.valueOf(to));
>   *       } else {                                // split in half
>   *         int mid = from + (to - from) / 2;
>   *         FactorialTask leftTask = new FactorialTask(from, mid);
>   *         leftTask.fork();         // perform about half the work locally
>   *         return new FactorialTask(mid + 1, to).compute()
>   *                .multiply(leftTask.join());
>   *       }
>   *     }
>   *   }
>   *   static BigInteger factorial(int n) { // uses
> ForkJoinPool.commonPool()
>   *     return (n <= 1) ? BigInteger.ONE : new FactorialTask(1,
> n).invoke();
>   *   }
>   *   public static void main(String[] args) {
>   *     System.out.println(factorial(Integer.parseInt(args[0])));
>   *   }
>   * }}</pre>
>
>
> import java.util.concurrent.RecursiveTask;
> import java.math.BigInteger;
> public class Factorial {
>    static class FactorialTask extends RecursiveTask<BigInteger> {
>      private final int from, to;
>      FactorialTask(int from, int to) { this.from = from; this.to = to; }
>      protected BigInteger compute() {
>        if (to == from) {                       // base case
>          return BigInteger.valueOf(from);
>        } else if (to - from < 2) {             // too small to parallelize
>           return BigInteger.valueOf(from).multiply(BigInteger.valueOf(to));
>        } else {                                // split in half
>          int mid = from + (to - from) / 2;
>          FactorialTask leftTask = new FactorialTask(from, mid);
>          leftTask.fork();         // perform about half the work locally
>          return new FactorialTask(mid + 1, to).compute()
>                 .multiply(leftTask.join());
>        }
>      }
>    }
>    static BigInteger factorial(int n) { // uses ForkJoinPool.commonPool()
>      return (n <= 1) ? BigInteger.ONE : new FactorialTask(1, n).invoke();
>    }
>    public static void main(String[] args) {
>      System.out.println(factorial(Integer.parseInt(args[0])));
>    }
> }
>
> _______________________________________________
> Concurrency-interest mailing list
> [email protected]
> http://cs.oswego.edu/mailman/listinfo/concurrency-interest
>

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