Question

I am trying to implement the Fibonacci sequence using callables and seeded the initial values of my Fibonacci callable with 3,4,5,6 and 2000. The output I get is as follows:

3 5 8 13
-820905900187520670

The problem is when I am trying to calculate fib(2000) in my callable. Can somebody take a look at my code provided below to see where I am going wrong with my approach:

import java.util.concurrent.*;
import java.util.*;

class FibonacciGen implements Callable<Long>{
    private Long fib;
    public FibonacciGen(long num){
        this.fib = num;
    }
    public Long call(){
        return calculateFibonacci(fib);
    }

    private long calculateFibonacci(long someNum){
        long firstNum = 0L;
        long secondNum = 1L;
        long counter = 0L;
        while(counter<someNum){
            long fibCalc = secondNum+firstNum;
            firstNum = secondNum;
            secondNum = fibCalc;
            counter= counter+1L;
        }
        return secondNum;
    }   

}

public class FibonacciCallable{
    public static void main(String[] args){
        ExecutorService exec = Executors.newCachedThreadPool();
        ArrayList<Callable<Long>> results = new ArrayList<Callable<Long>>();
        CompletionService<Long> ecs = new ExecutorCompletionService<Long>(exec);
        results.add(new FibonacciGen(3L));
        results.add(new FibonacciGen(4L));
        results.add(new FibonacciGen(5L));
        results.add(new FibonacciGen(6L));
        results.add(new FibonacciGen(2000L));
            try{
                for(Callable<Long> fs:results){
                    ecs.submit(fs);
                }
                System.out.println("Submitted all the tasks");
                int n = results.size();
                for(int i=0;i<n;++i){
                    System.out.println("Taking the first completed task");
                    Long r = ecs.take().get();
                    if(r != null)
                        System.out.println(r);
                    }   


            }
            catch(InterruptedException ex){System.out.println(ex);return;}
            catch(ExecutionException e){System.out.println(e);}
            finally{exec.shutdown();}
        }
}

Thanks

Answer 1

Java doesn’t throw an exception on overflow, just wraps the value around, that’s why you’re getting the strange result. Fibonacci is a fast growing sequence, the 2000. element is way beyond long

Try using BigInteger, it will give you arbitrary precision (at the cost of performance obviously).