Listing 5-1. Calculating Euler’s Number e
import java.math.BigDecimal;
import java.math.MathContext;
import java.math.RoundingMode;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
public class CalculateE
{
    final static int LASTITER = 17;
    public static void main(String[] args)
    {
    ExecutorService executor = Executors.newFixedThreadPool(1);
    Callable<BigDecimal> callable;
        callable = new Callable<BigDecimal>()
        {
            @Override
            public BigDecimal call()
            {
                MathContext mc = new MathContext(100, RoundingMode.HALF_UP);
                BigDecimal result = BigDecimal.ZERO;
                for (int i = 0; i <= LASTITER; i++)
                {
                    BigDecimal factorial =
                    factorial(new BigDecimal(i));
                    BigDecimal res = BigDecimal.ONE.divide(factorial,mc);
                    result = result.add(res);
                }
                return result;
            }

            public BigDecimal factorial(BigDecimal n)
            {
                if (n.equals(BigDecimal.ZERO))
                return BigDecimal.ONE;
                else
                return n.multiply(factorial(n.
                subtract(BigDecimal.ONE)));
            }
        };

        Future<BigDecimal> taskFuture = executor.submit(callable);
        try
        {
            while (!taskFuture.isDone())
            System.out.println("waiting");
            System.out.println(taskFuture.get());
        }
        catch(ExecutionException ee)
        {
            System.err.println("task threw an exception");
            System.err.println(ee);
        }
        catch(InterruptedException ie)
        {
            System.err.println("interrupted while waiting");
        }
         executor.shutdownNow();
        }
}