The factorial of a non-negative integer is multiplication of all integers smaller than or equal to n. In this article, we will learn how to write a program for the factorial of a number in Java.
Formula for Factorial
n! = n * (n-1) * (n-2) * (n-3) * ........ * 1
Example:
6! == 6*5*4*3*2*1 = 720.
5! == 5*4*3*2*1 = 120
4! == 4*3*2*1 = 24
Methods to Find the Factorial of a Number
1. Iterative Solution for Factorial in Java
class Test {
// Method to find factorial of given number
static int factorial(int n)
{
int res = 1, i;
for (i = 2; i <= n; i++)
res *= i;
return res;
}
// main method
public static void main(String[] args)
{
int num = 5;
System.out.println("Factorial of " + num + " is "
+ factorial(5));
}
}
Output
Factorial of 5 is 120
2. Factorial in Java Using Recursive Method
class Test {
// method to find factorial of given number
static int factorial(int n)
{
if (n == 0)
return 1;
return n * factorial(n - 1);
}
// main method
public static void main(String[] args)
{
int num = 5;
System.out.println("Factorial of " + num + " is "
+ factorial(5));
}
}
Output
Factorial of 5 is 120
3. One-line Solution (Using the Ternary operator)
class Factorial {
// function to find factorial
int factorial(int n)
{
// single line to find factorial
return (n == 1 || n == 0) ? 1
: n * factorial(n - 1);
}
// main function
public static void main(String args[])
{
Factorial obj = new Factorial();
int num = 5;
System.out.println("Factorial of " + num + " is "
+ obj.factorial(num));
}
}
Output
Factorial of 5 is 120
In the previous methods, we used basic data types like int or long to calculate factorials. But there's a problem and the problem is these data types can only store numbers up to a certain size, but what if the number is too big, when the number gets too big they can’t hold the full value, and the result becomes incorrect.
Note: Java provides a special class called BigInteger to handle large number without worrying about overflow.
Now, we are going to discuss how BigInteger class works in Java, It allows us to calculate factorials of very large number, such as 100! which would overflow the int or long types.
4. Factorial Using BigInteger
import java.math.BigInteger;
public class Geeks {
public static BigInteger factorial(int n) {
BigInteger res = BigInteger.ONE;
for (int i = 2; i <= n; i++) {
res = res.multiply(BigInteger.valueOf(i));
}
return res;
}
public static void main(String[] args) {
int num = 100;
System.out.println("Factorial of " + num + " is " + factorial(num));
}
}
Output
Factorial of 100 is 93326215443944152681699238856266700490715968264381621468592963895217599993229915608941463976156518286253697920827223758251185210916864000000000000000000000000
Please refer factorial of large numbers for a solution that works for large numbers. Please refer complete article on the Program for factorial of a number for more details!