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In this example, we will calculate the factorial of a number taking the value from the user.
You can also learn to find the Factorial of a Number in Java Using for loop.

Recursion refers to the function calling itself inside its function in a program.

Before we begin, you should have the knowledge of the following:

• Java Data-Types
• Java Operators
• Java Methods

Factorial of n number: Factorial of n number is the product of all the positive descending integers and is denoted by n!.
Example:

factorial of n (n!) = n * (n-1) * (n-2) * (n-3)….1
factorial of 5 (n!) = 5 * 4 * 3 * 2 * 1
NOTE: Factorial of 0 (0!) = 1

Factorial Program using Recursion in java.

 //find the factorial of a number using recursion in java

 import java.util.Scanner;

 public class FactorialRecursion
 {
   public static void main(String args[])
   {

     Scanner scanner = new Scanner(System.in);
     System.out.print("Enter the number: ");

     int num = scanner.nextInt();

     //Called the user defined function fact
     int factorial = fact(num);

     System.out.println("Factorial of entered number is: " + factorial);
   }

   //function
   static int fact(int n)
   {
     int output;
     if (n == 1)
     {
       return 1;
     }
     output = fact(n - 1) *n; //recursion
     return output;
   }
 }

Output:

Enter the number: 4
Factorial of entered number is: 24

Generating Random numbers in java is quite easy as it provides various classes to do that. It comes in handy when needed to apply to any kind of application development that might require random number generation. Let start by knowing random numbers.

What is Randon Number?

We can define Random as something that happens without any conscious decision. Similarly, a random number is a number that is chosen unpredictably or randomly from the set of numbers.

Random Number can be generated using two ways.

• java.util.Random class
• Math.random() method

We will also learn to generate between the range, at last.

Java Program to generate Random Number

1. Using java.util.Random class:

java.util.Random class allows us to generate random numbers of type integers, doubles, floats, longs, and booleans. In the example below, we will see to generate numbers of type Integers and booleans. Then you can apply to other data-types.

Program:

import java.util.Scanner;
import java.util.Random;

public class JavaRandomNumber
{
  public static void main(String[] args)
  {
    Scanner sc = new Scanner(System.in);

    //user Input
    System.out.println("How many random Numbers do you want?");
    int num = sc.nextInt();

    Random random = new Random();

    //Interger Random Numbers
    System.out.println(num + " Random Integers are:");
    for (int i = 0; i < num; i++)
    {
      System.out.println(+random.nextInt());
    }

    System.out.println("\n");

    //Booleans Random Numbers
    System.out.println(num + " Random Booleans are:");
    for (int i = 0; i < num; i++)
    {
      System.out.println(random.nextBoolean());
    }
  }
}

The output of generating random numbers using java.util.Random class:

2. Using Math.random() method:

Math.random() method allows you to create only a random number of type doubles as shown below.

Program:

import java.util.Scanner;

public class JavaRandomNumber
{
  public static void main(String[] args)
  {
    Scanner sc = new Scanner(System.in);
    
    //user Input
    System.out.println("How many random Numbers do you want?");
    int num = sc.nextInt();
    
    //Doubles Random Numbers
    System.out.println(num+" Random Doubles are:");
    for(int i = 0; i < num; i++)
    {
        System.out.println(+Math.random());
    }
  }
}

The output of generating random numbers using Math.random() method:

Java Program to generate Random Number within the range

import java.util.Scanner;
import java.util.Random; 

public class JavaRandomNumber
{
  public static void main(String[] args)
  {
    Scanner sc = new Scanner(System.in);
    
    //user Inputs
    System.out.println("How many random Numbers do you want?");
    int num = sc.nextInt();
    
    System.out.println("What is the last range of number starting from 0 to _?");
    int lastNum = sc.nextInt();
    
    Random random = new Random();
    
    //Doubles Random Numbers
    System.out.println("\n"+num+" Random integers within 0 to "+lastNum+ " are : ");
    for (int i = 0; i < 5; i++)
    {
        System.out.println(+random.nextInt(lastNum));
    }
  }
}

The output to generate random numbers between the range in java:

How many random Numbers do you want?
4
What is the last range of number starting from 0 to _?
30

4 Random integers within 0 to 30 are :
1
15
9
28
4

This random generation of numbers within range can be applied to Math.random() method, we only need to replace the code inside the for loop to System.out.println((int)(Math.random() * lastNum));.

In this tutorial, we will write a java program to calculate the GCD of two numbers.

GCD of two numbers in Java:

GCD(Greatest Common Divisor) or we can say HCF(Highest Common Factor) of two numbers is the largest number(integer) that divides both the number. Or in other words, two numbers that are divisible by the same largest number leaving no remainder.

For example: GCD of 20 and 28 is 4 as shown in the image below.

Java program two calculate GCD of two numbers using for loop and if statement

In this example, we take the user input for the two numbers whose GCD needs to be found and store it in num1 and num2. Then iterate through for loop for i less than equal num1 and i less than equal to num2

Inside for loop, we set if statement to check for the divisibility 0f both num1 and num2 by i. If true then we set the value of current i to gcd. This iteration runs until it finds the largest number that divides both num1 and num2.

Program: GCD of Two Number in Java.

 //calculate the gcd of two number
 
 import java.util.Scanner;

 public class GCDJava
 {
   public static void main(String[] args) 
   {
     int num1 , num2, gcd = 1;
     Scanner sc = new Scanner(System.in);

     System.out.println("Enter the first number:");
     num1 = sc.nextInt();
        
     
     System.out.println("Enter the second number:");
     num2 = sc.nextInt();
        
        
     for(int i = 1; i <= num1 && i <= num2; ++i)
     {
       // Checks if i is factor of both integers
       if(num1 % i==0 && num2 % i==0)
          gcd = i;
     }
    System.out.printf("G.C.D of %d and %d is %d", num1, num2, gcd);
   }
 }

The Output of GCD of two numbers in java

Enter the first number:
81
Enter the second number:
153
G.C.D of 81 and 153 is 99