Java Program to Find Factorial of a Number

In this post, we will develop the Factorial program in Java. We will see what are the different ways to find the factorial of a number in Java.

In mathematics, the factorial of a positive integer n, denoted by n!, is the product of all positive integers less than or equal to n. For example:- The factorial of 4= 4! = 4*3*2*1 or 1*2*3*4 = 24

Factorial (n) = 
1 * 2 * ... * (n-1) * n 
Or,
n * (n-1) * ... * 2 * 1

The factorial of a negative number doesn’t exist. The value of 0! is 1, according to the convention for an empty product.

Procedure to find the factorial of a number in Java,

1) Take a number
2) Declare a temporary variable fact and initialize it with 1. long fact = 1;
3) Take an iterator variable i, starting from 1
4) Multiply fact variable and iterator variable. Store the result into fact variable. fact = fact * i;
5) Increase the iterator variable by 1.
6) Repeat 4 and 5 steps until the iterator variable is less than the given number.

import java.util.Scanner;
public class Factorial {

  public static long findFactorial(int number){

     // declare variable
     long fact = 1;

     for(int i=1; i<=number; i++) {
       fact = fact * i;
     }

     return fact;
  }

  public static void main(String[] args) {

     // declare variables
     int number = 0;
     long result = 0;

     //create Scanner class object
     // to take input
     Scanner scan = new Scanner(System.in);

     // take input from end-user
     System.out.print("Enter integer number:");
     number = scan.nextInt();

     // Find the factorial of 
     // the given number
     result = findFactorial(number);

     // display result
     System.out.println("Factorial = "+ result);

     // close Scanner class object
     scan.close();
  }
}

The output for different test-cases:-

Enter an integer number:: 5
Factorial = 120

Enter an integer number:: 4
Factorial = 24

It was a basic Java program to calculate factorial of a given number, which calls findFactorial() method to calculate the factorial value. The findFactorial method uses for loop and increment operator.

The for loop calculates the factorial of number 5 as = 1*2*3*4*5 because it uses increment operator. In place of increment operator, we can also use decrement operator, then it will calculate the factorial value as 5! = 5*4*3*2*1

The factorial method in Java using for loop and decrement operator,

public static long findFactorial(int number) {
     // declare variable
     long fact = 1;
     for(int i=number; i>1; i--) {
       fact = fact * i;
     }
     return fact;
}

Factorial for Large Values

The store the factorial value instead of int data type we are using a variable of long data type because the factorial value will be a larger number. But for a large number (like 50, 100) long data type is also not enough to store those values. So, we need to use a variable of BigInteger type which is defined in java.math package (not java.lang.Math class).

To multiply we can’t use the multiplication (*) operator because it is an object of BigInteger class so, we should use multiply methods of BigInteger class.

import java.math.BigInteger;
import java.util.Scanner;
public class Factorial {

  public static BigInteger findFactorial(int number){

     // declare variable
     BigInteger fact = BigInteger.ONE;

     for(int i=1; i<=number; i++) {
       fact = fact.multiply(BigInteger.valueOf(i));
     }
     return fact;
  }

  public static void main(String[] args) {

     // declare variables
     int number = 0;

     //create Scanner class object
     // to take input
     Scanner scan = new Scanner(System.in);

     // take input from end-user
     System.out.print("Enter integer number:");
     number = scan.nextInt();

     // display result
     System.out.println("Factorial = "+ 
		        findFactorial(number));

     // close Scanner class object
     scan.close();
  }
}

Output:-

Enter an integer number:: 50
Factorial = 30414093201713378043612608166064768844377641568960512000000000000

Factorial Program in Java using While loop

import java.util.Scanner;
public class Factorial {

  public static long findFactorial(int number) {
     // declare variable
     long fact = 1;
     int i = 1;

     while(i<=number) {
          fact = fact * i;
          i++;
     }
     return fact;
  }

  public static void main(String[] args) {

     // declare variables
     int number = 0;
     long result = 0;

     //create Scanner class object
     Scanner scan = new Scanner(System.in);
     // take input from end-user
     System.out.print("Enter integer number:");
     number = scan.nextInt();

     // Find factorial 
     result = findFactorial(number);

     // display result
     System.out.println("Factorial = "+ result);

     // close Scanner class object
     scan.close();
   }
}

This program calls findFactorial() method which is developed by using a while loop and increment operator. It calculates the factorial value as 5! = 1*2*3*4*5

Similar to the previous case, we can also use the decrement operator to calculate the factorial value. It will calculate the factorial value as 5! = 5*4*3*2*1

Java method to find factorial using while loop and decrement operator,

public static long findFactorial(int number) {
  // declare variable
  long fact = 1;
  int i = number;
  // loop to repeat the process
  while(i>1) {
    fact = fact * i;
    i--;
  }
  return fact;
}

Now we will see the recursive method to find the factorial value.

Java Factorial Method

Java factorial method using recursion technique

Formula,

n! = n * (n-1)!

The base case for finding factorial

factorial(0) = 1; 
Or,
factorial(1) = 1;

General case for finding factorial

factorial(n) = n * factorial(n-1);

Java code for factorial using recursion method and if-else statement,

// recursive Java method to 
// find factorial of a number
// using if-else statement
public static long findFactorial(int number) {
  if(number == 0) return 1;
  else 
  return number*findFactorial(number-1);
}

Java code for factorial using recusion method and ternary operator,

This method can find factorial of a given number in a single line.

// recursive Java method to 
// find factorial of a number
// using ternary operator
public static long findFactorial(int n) {
   return (n==0) ? 1 : n*findFactorial(n-1);
}

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