用Java中的线程计算e

Question

我试图用线程计算java中的neper数e。

我正在使用收敛行和（（3-4 * k ^ 2）/（2 * k + 1）！）。

用户可以确定程序将使用多少个线程以及将完成多少次迭代。
每个线程的间隔为1 / numberOfThreads的一部分。我必须尽快做到这一点。
但是我遇到了阶乘函数的问题;在输出文件中，我得到0E-20 ...如果有人可以帮我解决这个问题，我将非常感激。

public class CalculateE {

    static BigDecimal result = BigDecimal.ZERO;
    static BigDecimal f = BigDecimal.ONE;
    static BigDecimal f1 = BigDecimal.ONE;
    static BigDecimal f_rev = BigDecimal.ONE;
    static BigDecimal fact = BigDecimal.ONE;
    static BigDecimal fact_rev = BigDecimal.ONE;

    public static BigDecimal factorial(int x){
        BigDecimal prod = BigDecimal.ONE;
        for (int i=x; i>1; i--){
            prod = prod.multiply(new BigDecimal(i));
        }
        return prod;
    }

    public static Runnable getRunner(final int from, final int to, int iter){
        Runnable runner = new Runnable(){
            @Override
            public void run(){

                BigDecimal e = BigDecimal.ZERO;
                for (int i=from; i<to && i<=iter/2; i++){
                    if(i == from){

                    fact = factorial(2*i+1);

                    }else{
                        fact=fact.multiply(BigDecimal
                                            .valueOf((2*i)*(2*i+1)));

                    }

                    e = e.add(BigDecimal
                                .valueOf(3 - 4*Math.pow(i, 2))
                                .divide(fact, 
                                        new MathContext(10000,RoundingMode.HALF_EVEN)));

                } 
                addResult(e); 
            }
        };
        return runner;
    }

    //public static synchronized void addResult(BigDecimal e){
    //  result = result.add(e);
    //}
    public static synchronized void addResult(BigDecimal e){
        result = result.add(e);
    }

    public static void main(String args[]){

        //Default values
        // Number of Threads used
        int NumOfth = 1;
        // Number of Iterations
        int p = 1000;
        //Quiet mode
        Boolean quiet = false;
        //Output file
        String nameOffile = "result.txt";


        // A check for command line arguments
        for (int i = 0; i < args.length; i++) {
            if (args[i].equals("-p")) {
                p = new Integer(args[i + 1]);
            }
            if (args[i].equals("-t")){
                NumOfth = new Integer(args[i+1]);
            }
            if (args[i].equals("-q")){
                quiet = true;
                System.out.println("Quiet mode enabled.");
            }
            if (args[i].equals("-o")){
                nameOffile = args[i+1];
            }
        }

        //Check for correct input
        if (p <= 0 || NumOfth <= 0 ||  args.length > 7) {
            System.out.println("ERROR: Args are not correct!!!");
        } else{

            long start1 = System.currentTimeMillis();

            if (quiet == false){
                System.out.println("Threads used in current run: " + NumOfth);
            }

            Runnable r[] = new Runnable[NumOfth];
            Thread[] myThreads; 
            myThreads = new Thread[NumOfth];
            int from = 0;
            int to = (p/(NumOfth*2));

            int interval = to - from;

            for (int i = 0; i<NumOfth; i++){
                //Create a runnable object
                r[i] = getRunner(from, to, p);

                // Create our threads over our runnable object

                Thread thrd = new Thread(r[i]);
                if (quiet == false){
                    System.out.println("Thread <" + i + "> created.");
                }

                thrd.start();

                if (quiet == false){
                    System.out.println("Thread <" + i + "> started.");
                }
                myThreads[i] = thrd;
                from = to + 1;
                to = to + interval+1;
            }

            for (int i = 0; i<NumOfth; i++){
                try{
                    //System.out.println("Thread <" + i + "> started.");
                    myThreads[i].join();
                }   catch (InterruptedException e){
                    e.printStackTrace();
                }
            }

            if(quiet == false){
                System.out.println("e = " + result.setScale(20, BigDecimal.ROUND_HALF_UP));
            }

            //Write the result to a file
            //File myfile = new File(nameOffile);
            Writer writer = null;
            try{

                writer = new BufferedWriter(new OutputStreamWriter(
                        new FileOutputStream(nameOffile), "utf-8"));

                writer.write("" + result.setScale(20, BigDecimal.ROUND_HALF_UP));
            } catch(IOException e) {

                e.printStackTrace();

            } finally {
                try { 
                    // close the file
                    writer.close(); 
                    } catch(Exception ex) {
                        //ignore
                    }
            }
            //Current time
            long stop = System.currentTimeMillis();
            //Time for execution
            long diff = stop - start1;
            System.out.println(diff + " ms");
            System.out.println(count);
        }
    }
}