Java Program to Check Prime Number

WAP to check whether a number is prime or not in Java

What are Prime Numbers ?

A prime number is a natural number greater than 1 that has only two factors: 1 and itself.
Examples: 2, 3, 5, 7, 11, 13, 17, etc.
Non-prime numbers (composite): 4, 6, 8, 9, 10 (since they have more than two factors).

Logical Steps:

If the number is less than or equal to 1, it is not prime.
Loop from 2 to number and check divisibility:
- If the number is divisible by any number in this range, it is not prime.
- Otherwise, it is prime.

Programs:

Below is the simple program:

public class CheckPrimeNumber
{
    public static void main(String[] args)
    {
        int num = 23;

        // Combined check for 0 and 1
        if (num <= 1)
        {
            System.out.println(num + " is not a prime number.");
            return; // Exit early for 0 and 1
        }

        boolean isPrime = true;

        for (int i=2; i < num/2; i++)
        {
            if (num % i == 0)
            {
                isPrime = false;
                break;
            }
        }

        if (isPrime)
        {
            System.out.println(num + " is a prime number.");
        }
        else
        {
            System.out.println(num + " is not a prime number.");
        }
    }
}

Output:

23 is a prime number.

Below is the program by taking user input:

import java.util.Scanner;

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

        System.out.print("Enter the number: ");
        int num = s.nextInt();

        // Combined check for 0 and 1
        if (num <= 1)
        {
            System.out.println(num + " is not a prime number.");
            return; // Exit early for 0 and 1
        }

        boolean isPrime = true;

        for (int i=2; i < num/2; i++)
        {
            if (num % i == 0)
            {
                isPrime = false;
                break;
            }
        }

        if (isPrime)
        {
            System.out.println(num + " is a prime number.");
        }
        else
        {
            System.out.println(num + " is not a prime number.");
        }
    }
}

Output:

Enter the number: 31
31 is a prime number.

Program Explanation:

Input and Variables:
- Takes an integer input from the user.
- Use a boolean flag (isPrime) initialized to true, assuming the number is prime unless proven otherwise.
Special Case Handling:
- If num <= 1, it is not a prime number:
  - Prints: "num is not a prime number."
  - Uses return; to exit early (skips checking for factors).
Prime Check Logic:
- Loop from 2 to num / 2.
- If num % i == 0, the number is divisible by i, so:
  - Set isPrime = false (not prime).
  - break; to exit the loop early (no need for further checks).
Display Result:
- If isPrime == true, print "num is a prime number."
- Else, print "num is not a prime number."
Close Scanner:
- Releases input resources with scanner.close().

Note: For program better efficiency, we can use i <= Math.sqrt(num) in place of i < num / 2 because a number cannot have factors greater than its square root.

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WAP to check whether a number is prime or not in Java

What are Prime Numbers ?

Logical Steps:

Programs:

Output:

Output:

Program Explanation:

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