Program to Check if a Number is Prime in Java

Checking Prime Number

A prime number is a natural number greater than 1 that has no divisors other than 1 and itself.

We will explore different methods to check if a number is prime using Java programming.

Method 1: Using a for Loop

We iterate from 2 to the square root of the number and check for divisibility.

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter a number: ");
        int num = scanner.nextInt();
        boolean isPrime = true;
        
        if (num < 2) {
            isPrime = false;
        } else {
            for (int i = 2; i * i <= num; 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");
        
        scanner.close();
    }
}
            

Output:

Enter a number: 7
7 is a prime number

Method 2: Using a Function

We create a function to check for primality and return the result.

import java.util.Scanner;

public class Main {
    public static boolean isPrime(int num) {
        if (num < 2) return false;
        for (int i = 2; i * i <= num; i++) {
            if (num % i == 0)
                return false;
        }
        return true;
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter a number: ");
        int num = scanner.nextInt();
        
        if (isPrime(num))
            System.out.println(num + " is a prime number");
        else
            System.out.println(num + " is not a prime number");
        
        scanner.close();
    }
}
            

Output:

Enter a number: 10
10 is not a prime number