Find the sum of the digits in the number 100!

FactorialDigitSum.java

import java.math.BigInteger;
public class FactorialDigitSum {
	public FactorialDigitSum() {
	}
	public static int  calculate(int n){
        	return digitSum(factorial(new BigInteger(""+n))).intValue();
	}
	private static BigInteger digitSum(BigInteger  n){
	        if(n.compareTo(BigInteger.TEN)==-1) return n;
	        else return n.mod(BigInteger.TEN).add(digitSum(n.divide(BigInteger.TEN)));
	}
	private static BigInteger factorial(BigInteger n){
        	if(n.equals(BigInteger.ONE)) return BigInteger.ONE;
        	else return n.multiply(factorial(n.subtract(BigInteger.ONE)));
	}
}

FactorialDigitSumTest.java

public class FactorialDigitSumTest {
	@Test
	public void testCalculate() {
		assertEquals(1,FactorialDigitSum.calculate(1));
		assertEquals(27,FactorialDigitSum.calculate(10));
		assertEquals(648,FactorialDigitSum.calculate(100));
	}
}