Java – Check Prime Number

Write a Java program to check a given number is a prime number or not.

What is a Prime Number?

A prime number is a number that is fully divisible by itself or 1 only.
Such as 13 is a prime number, it is not fully divisible by any number from 2-12.

You may like:

List all prime numbers between 1 to N

Example

This is a sample Java program to check 13 is a prime number or not.

public class CheckPrimeNumber {

    public static void main(String[] args) {
	
        int num = 13;
        boolean isPrime = true;

        for (int i=2; i <= num/2; i++)
        {
            if ( num % i == 0)
            {
                isPrime = false;
                break;
            }
        }
		
        if ( isPrime == true )
            System.out.println(num + "is a prime number");
        else
            System.out.println(num + "is not a prime number");
    }
}

public class CheckPrimeNumber {

public static void main(String[] args) {

int num = 13;

boolean isPrime = true;

for (int i=2; i <= num/2; i++)

{

if ( num % i == 0)

{

isPrime = false;

break;

}

if ( isPrime == true )

System.out.println(num + "is a prime number");

else

System.out.println(num + "is not a prime number");

}

Compile and run this program:

javac CheckPrimeNumber.java
java CheckPrimeNumber

Output:

13 is a prime number

Check Prime Number with User Input

The following Java program will prompt a user to input a number and check if the entered number is a prime number.

import java.util.Scanner;

public class CheckPrimeNumber {

    private static Scanner scanner = new Scanner( System.in );

    public static void main(String[] args) {

        System.out.println("Input a number: ");

        String input = scanner.nextLine();
        int num = Integer.parseInt( input );

        boolean isPrime = true;

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

        if ( isPrime == true )
            System.out.println("Yes " + num + " is a prime number");
        else
            System.out.println("No " + num + " is not a prime number");
    }
}

import java.util.Scanner;

public class CheckPrimeNumber {

private static Scanner scanner = new Scanner( System.in );

public static void main(String[] args) {

System.out.println("Input a number: ");

String input = scanner.nextLine();

int num = Integer.parseInt( input );