Find factorial for given no Program in Java -

import java.util.Scanner;
class Factorial {
  public static void main(String args[]) {
    int n, c, fact = 1;
    System.out.println("Enter an integer to calculate it's factorial");
    Scanner in = new Scanner(System.in);

    n = in.nextInt();

    if (n < 0)
      System.out.println("Number should be non-negative.");
    else {
      for (c = 1; c <= n; c++) fact = fact * c;
      System.out.println("Factorial of " + n + " is = " + fact);
    }
  }
}

//Calculate factorial for large No
import java.util.Scanner;
import java.math.BigInteger;
class BigFactorial {
  public static void main(String args[]) {
    int n, c;
    BigInteger inc = new BigInteger("1");
    BigInteger fact = new BigInteger("1");
    Scanner input = new Scanner(System.in);

    System.out.println("Input an integer");
    n = input.nextInt();
    for (c = 1; c <= n; c++) {
      fact = fact.multiply(inc);
      inc = inc.add(BigInteger.ONE);
    }

    System.out.println(n + "! = " + fact);
  }
}

Let’s have both programs presented. The first one calculates the factorial for an integer using a simple loop in case the number is not too large, and the second one, which is pretty much the same computation, uses the BigInteger class.

Step-by-Step Explanation for Factorial Class:

1. Import Scanner Class:

It imports the Scanner class from java.util to allow the program to accept a user’s input.

2. Declare Class:

class Factorial {

// The Factorial class declares all the logic to be used for calculating the factorial.

3. Declaration of Main Method:

public static void main(String args[]) {

This is the point where the program begins running.

4. Variable Declarators

int n, c, fact =1;

n: Number on whose factorial shall be calculated.
c: counting variable.
fact: Factorial value: This value is defined initially as 1.

5. Obtain a value input by the user :

Scanner in = new Scanner(System.in);

The program prompts the user for an input.
A Scanner Object is declared and used to scan from the keyboard.

6. Scan the Value of n:

n = in.nextInt();

The integer typed into the keyboard will be stored inside the variable n.

7. Check for a Negative Input:

if (n < 0)

System.out.println(“Number should be non-negative.”);

It determines whether the input is negative. If n is negative, it issues an error message.
Factorials are not defined for negative numbers.

8. Factorial Computation with a Loop:

else {

for (c = 1; c <= n; c++) {

fact = fact * c;

}}

System.out.println(“Factorial of ” + n + ” is = ” + fact);

If the number is nonnegative, a for loop is used to calculate the factorial by multiplying fact with each integer from 1 to n.

9. Print the Answer:

Once the loop is done, the program prints out the calculated factorial.

Execution Example

Input:

Enter an integer to find its factorial

The program calculates 5! = 5 * 4 * 3 * 2 * 1 = 120.

Output:

Factorial of 5 is = 120

Step-by-step Explanation for BigFactorial Class:

1. Import Required Classes:

Scanner to obtain the user’s input.
BigInteger, as the calculations are too huge for large input.
BigInteger is used since the factorials tend to increase so fast that cannot be stored inside int or long primitives.

2. Class declaration:

class BigFactorial {

The BigFactorial class enables us to calculate the factorials of big integer numbers

3. Main Method Definition:

public static void main(String args[]) {

4. Declarations of Variables and Objects:

int n, c;

BigInteger inc = new BigInteger(“1”);

BigInteger fact = new BigInteger(“1”);

n: the value for which will be computed as factorial.
c : the loop counter of.
inc: BigInteger initialized to 1 used for multiplication with fact and to increase by 1 in each cycle.
fact: BigInteger initialized to 1 where the result of factorial will be stored.

5. Declare Scanner Object for Input Reading

Scanner input = new Scanner(System.in);

This declares a scanner object that reads the input from the user.

6. Ask for Input from User

System.out.println(“Input an integer”);

This line asks the user to input a number for which the factorial is to be calculated.

7. Input the Value of n:

n = input.nextInt();

8. Factorial Calculation Using BigInteger:

for (c = 1; c <= n; c++) {

fact = fact.multiply(inc); // Multiply fact by inc

inc = inc.add(BigInteger.ONE); // Increment inc by 1

}

A for loop runs from 1 to n. In each iteration:

fact is multiplied by inc.

inc is added 1 to it using add of BigInteger.

9. Print the Answer

System.out.println(n + “!” + ” = ” + fact);

The program prints the computed factorial using BigInteger.

Running Example for BigFactorial