Using {RecursiveTask} in Java 1.7

[Kevin Simonson]

I'm currently trying to learn how to use Fork and Join with Java 1.7. I started by taking a look at the web page at "http://www.oracle.com/technetwork/articles/java/fork-join-422606.html", and got as far as its mention of class {RecursiveTask}. At that point I thought I'd look {RecursiveTask} up at "http://docs/oracle.com/javase/7/docs/api", so I did, and noticed it gave an example of how to use {RecursiveTask} to calculate a Fibonacci number. I used its example, and just added a few more lines of code to come up with:

import java.util.concurrent.RecursiveTask;
import java.io.PrintStream;

public class Fibonacci extends RecursiveTask< Long>
{
  private static final PrintStream SY_O = System.out;

  private final Long NUMBER;

  private Fibonacci ( Long number)
  {
    NUMBER = number;
  }

  protected Long compute ()
  {
    if (NUMBER <= 1)
    { return NUMBER;
    }
    Fibonacci minusOne = new Fibonacci( NUMBER - 1);
    minusOne.fork();
    Fibonacci minusTwo = new Fibonacci( NUMBER - 2);
    return minusTwo.compute() + minusOne.join();
  }

  public static void main ( String[] arguments)
  {
    if (arguments.length == 1)
    { try
      { Fibonacci fib = new Fibonacci( Long.parseLong( arguments[ 0]));
        SY_O.println
          ( "Fibonacci number #" + arguments[ 0] + " is " + fib.compute()
                                 + '.');
      }
      catch (NumberFormatException nfExc)
      { SY_O.println
          ( "Couldn't convert argument \"" + arguments[ 0]
                                           + "\" to an integer!");
      }
    }
    else
    { SY_O.println( "Usage is");
      SY_O.println( "  java Fibonacci <number>");
    }
  }
}

This works just fine when I execute "java Fibonacci 0" or "java Fibonacci 1", but when I try "java Fibonacci 2" I get the error message:

Exception in thread "main" java.lang.ClassCastException: java.lang.Thread cannot
be cast to java.util.concurrent.ForkJoinWorkerThread
at java.util.concurrent.ForkJoinTask.fork(ForkJoinTask.java:622)
at Fibonacci.compute(Fibonacci.java:21)
at Fibonacci.main(Fibonacci.java:31)

Can anybody tell me what I'm doing wrong?

Kevin S

[Steve Luke]

You are running the task from the main thread, you need to run it from a ForkJoinPool

[Kevin Simonson]

Steve Luke wrote:You are running the task from the main thread, you need to run it from a ForkJoinPool

Thanks, Steve; that fixed it for me.