Java program to check if a string is a prime number or not

Java program to check if a string is a prime number or not:

In this post, we will learn how to check if a string is a prime number or not in Java. The program will take a string as input from the user, verify if it is a prime number string or not and print one result to the user.

We can use any loop to solve this problem. A number is called a prime number if it is greater than 1 and it is divisible by 1 and the number itself. If we can divide the number by any other number, it will not be a prime number.

How to check if a number is prime or not programmatically:

If we want to check if a number is a prime number or not programmatically, we have to run one loop. The loop will run from 2 to number/2 and for each value, it will check if it can divide the number. If yes, it will not be a prime number. Else, it will be a prime number.

We can also run from 2 to the square root of the number. This will reduce the number of steps and for a large number, it will be faster than the previous method.

Let’s learn how to write these algorithms in Java :

Method 1: By using a for loop:

We can use a for loop and run from 2 to number/2. On each iteration, it will check if any value can divide the number or not. If yes, it will not be a prime number.

We will take the value as a string and convert it to a number first.

import java.util.Scanner;

class Main {
    public static boolean isPrime(String n) {
        int number = Integer.parseInt(n);

        if (number == 0 || number == 1) return false;

        for (int i = 2; i <= number / 2; i++) {
            if (number % i == 0) return false;
        }
        return true;
    }

    public static void main(String[] args) {
        String num;

        Scanner sc = new Scanner(System.in);
        System.out.println("Enter the number: ");
        num = sc.next();

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

In this example,

sc is a Scanner object to read the user input values.
We are asking the user to enter the number. The number is stored in the string variable num.
isPrime is a method to check if a string is prime or not. It takes a string value as its parameter and returns one boolean value.
We are parsing the integer value at the start of this method and this value is stored in the variable number.
If the value of a number is equal to 0 or 1, it returns false i.e. it is not a prime number.
It runs one for loop from i = 2 to i = number/2 and for each value of i, it checks if it can divide the number or not. If yes, it returns false i.e. it is not a prime number.
Once the loop ends, it returns true, i.e. the number is a prime number.

Method 2: By using a while loop:

We can also use a while loop to write the same program. It will be similar to the above program. The only difference is that we will initialize a variable before the loop starts and the increment of i will be at the end of each iteration.

import java.util.Scanner;

class Main {
    public static boolean isPrime(String n) {
        int number = Integer.parseInt(n);

        if (number == 0 || number == 1) return false;

        int i = 2;

        while (i <= number / 2) {
            if (number % i == 0) return false;
            i++;
        }
        return true;
    }

    public static void main(String[] args) {
        String num;

        Scanner sc = new Scanner(System.in);
        System.out.println("Enter the number: ");
        num = sc.next();

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

Method 3: By iterating up to the square root of the number:

We can also iterate from 2 to the square root of the number. It will give us the same result. The advantage of this approach is that the number of iterations will be reduced.

import java.util.Scanner;

class Main {
    public static boolean isPrime(String n) {
        int number = Integer.parseInt(n);

        if (number == 0 || number == 1) return false;

        int i = 2;

        while (i <= Math.sqrt(number)) {
            if (number % i == 0) return false;
            i++;
        }
        return true;
    }

    public static void main(String[] args) {
        String num;

        Scanner sc = new Scanner(System.in);
        System.out.println("Enter the number: ");
        num = sc.next();

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

To find the square root of the number, we are using Math.sqrt() method. If you run this program, it will give similar output.

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Java program to check if a string is a prime number or not