How to find factorial of a BigIntegers using Multithreads

import java.math.BigInteger;
import java.util.ArrayList;
import java.util.List;

public class FactorialOfBigNumbers {

	public static void main(String args[]) throws InterruptedException {

		List<BigInteger> numList = new ArrayList<BigInteger>();
		numList.add(BigInteger.valueOf(33));
		numList.add(BigInteger.valueOf(19));
		numList.add(BigInteger.valueOf(79));
		numList.add(BigInteger.valueOf(109));

		List<FactThread> threads = new ArrayList<FactThread>();

		for (BigInteger bI : numList) {
			threads.add(new FactThread(bI));
		}

		for (Thread thread : threads) {
			thread.start();
		}
		// Main thread wait until all the child threads completes.
		for (Thread thread : threads) {
			thread.join();
		}

		for (int i = 0; i < numList.size(); i++) {
			FactThread factThread = threads.get(i);
			if (factThread.isFinished()) {
				System.out.println("The factorial of " + numList.get(i) + " is " + factThread.retAnswer());
			} else {
				System.out.println("The factorial of " + numList.get(i) + " is in Progress");
			}
			Thread.sleep(1000);
		}
	}

	public static class FactThread extends Thread {

		private BigInteger FactOfNum = BigInteger.ONE;
		private BigInteger numVal;
		private boolean isFinished;

		// Initialize the constructor with the give number
		FactThread(BigInteger bI) {
			this.numVal = bI;
		}

		// call the FactorialOfbI routine from the run, and set finished to TRUE
		public void run() {
			FactOfNum = FactorialOfbI(numVal);
			isFinished = true;
		}

		private BigInteger FactorialOfbI(BigInteger num) {
			// Iterate the loop to find factorial of given BigInteger
			while (num.compareTo(BigInteger.ONE) != 0) {
				FactOfNum = FactOfNum.multiply(num);
				num = num.subtract(BigInteger.ONE);
			}
			return FactOfNum;
		}

		private boolean isFinished() {
			return isFinished;
		}

		private BigInteger retAnswer() {
			return FactOfNum;
		}
	}
}