Java Program to Check Whether a Number is Prime or Not

 In this article, you'll learn to check whether a number is prime or not. This is done using a for loop and while loop in Java.

To understand this example, you should have the knowledge of the following Java programming topics:

• Java while and do...while Loop
• Java for Loop

A prime number is a number that is divisible by only two numbers: 1 and itself. So, if any number is divisible by any other number, it is not a prime number.

Note : 0 and 1 are not Prime Numbers. The 2 is the only even prime number because all the other even numbers can be divided by 2.                                                                              

Example : Check Prime Number using For Loop

import java.util.Scanner;
public class Prime {
    public static void main(String args[]){
        Scanner s1 = new Scanner(System.in);
        System.out.println("Enter a number");
        int a = s1.nextInt();
        int i,fact,ctr=0;
        for(i=1;i<=a;i++){
            if (a % i == 0) {
                ctr++;//Counts the number of factors
            }
        }
        if(ctr==2){
            System.out.println("Prime");
        }
        else{
            System.out.println("Not Prime");
        }
    }
}

Video:

Example : Check Prime Number using While Loop

import java.util.Scanner;
class PrimeNumbers {
    public static void main(String[] args) {
        Scanner s1 = new Scanner(System.in);
        System.out.println("Enter a number");
        int a = s1.nextInt();
        int n = 2;
        int ctr = 0;
        while (n <= a / 2) {
            if (a % n == 0) {
                ctr++;
                break;
            }
            n++;
        }
        if (ctr == 0 ) {
            System.out.print(a + " is a prime number ");
        } else {
            System.out.print(a + " is not a prime number ");
        }
    }
}