JavaHowTo MathFactorial - Rhea

/**
 * Factorial calculation with straightforward way.
 * @param num
 * @return
 */
public static double mathFactorial(int num) {
	//----- to prevent abnormal result
	Ts.assertTrue(num >= 0 && num <= 143, "factorial: the range is from 0 to 143");
	double result = 1;
	for (int i = 1; i <= num; i++) {
		result *= i;
	}
	return result;
}
 
/**
 * Factorial calculation with Stirling approximation.
 * @param num
 * @return
 */
public static double mathFactorialStirling(int num) {
	if (num == 0) {
		return 1.0;
	}
	Ts.assertTrue(num >= 0 && num <= 142, "factorial: the range is from 0 to 142");
	return Math.sqrt(2 * Math.PI * num) * Math.pow(num, num) * Math.pow(Math.E, -num);
}
 
public static void assertTrue(boolean isTrue, String message) {
	if (!isTrue) {
		System.out.println("---> Assert fail : " + message);
		new Exception().printStackTrace();
		System.exit(-1);
	}
}