Write a Java program to check if a number is prime – Java FAQ Solution

Java program to check if a number is prime. Checking if a number is prime is a common programming exercise that helps you understand loops and conditional statements. A prime number is a natural number greater than 1 that has no positive divisors other than 1 and itself. In other words, a prime number is only divisible by 1 and itself.

Java Program to Check if a Number is Prime

Let’s write a Java program to check if a number is prime. We will use a loop to check divisibility and determine if the number has any divisors other than 1 and itself.

Here’s the complete code:

import java.util.Scanner;

public class PrimeCheck {
    public static void main(String[] args) {
        // Create a Scanner object to read input from the user
        Scanner scanner = new Scanner(System.in);

        // Prompt the user to enter a number
        System.out.print("Enter a number: ");
        int number = scanner.nextInt();

        // Check if the number is prime
        boolean isPrime = isPrime(number);

        // Print the result
        if (isPrime) {
            System.out.println(number + " is a prime number.");
        } else {
            System.out.println(number + " is not a prime number.");
        }
    }

    // Method to check if a number is prime
    public static boolean isPrime(int n) {
        // Edge cases: check if the number is less than 2
        if (n <= 1) {
            return false;
        }

        // Check for factors from 2 to the square root of n
        for (int i = 2; i <= Math.sqrt(n); i++) {
            if (n % i == 0) {
                return false; // n is divisible by i, so it's not a prime number
            }
        }

        return true; // n is a prime number
    }
}

Explanation of the Code: (Java program to check if a number is prime)

Let’s break down the program step-by-step to understand how it works.

1. Importing the Scanner Class: We import the Scanner class from the java.util package to read input from the user.

import java.util.Scanner;

2. Creating the Main Class: We create a public class named PrimeCheck. In Java, the class name should match the filename.

public class PrimeCheck {

3. Main Method: The main method is the entry point of the program. Inside this method, we will write the code to check if a number is prime.

public static void main(String[] args) {

4. Creating a Scanner Object: We create a Scanner object to read input from the user.

Scanner scanner = new Scanner(System.in);

5. Reading Input from the User: We prompt the user to enter a number and store this value in a variable.

System.out.print("Enter a number: ");
int number = scanner.nextInt();

6. Checking if the Number is Prime: We call the isPrime method to check if the number is prime. The result is stored in a boolean variable isPrime.

boolean isPrime = isPrime(number);

7. Printing the Result: We print whether the number is prime based on the value of isPrime.

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

8. Method to Check if a Number is Prime: The isPrime method checks if a number is prime. The method returns true if the number is prime and false otherwise.

public static boolean isPrime(int n) {
    // Edge cases: check if the number is less than 2
    if (n <= 1) {
        return false;
    }

    // Check for factors from 2 to the square root of n
    for (int i = 2; i <= Math.sqrt(n); i++) {
        if (n % i == 0) {
            return false; // n is divisible by i, so it's not a prime number
        }
    }

    return true; // n is a prime number
}

Java program to check if a number is prime

Explanation of the isPrime Method

1. Edge Cases: We check if the number is less than or equal to 1. If so, the number is not prime.

if (n <= 1) {
    return false;
}

2. Loop to Check Divisibility: We use a for loop to check if the number is divisible by any number from 2 to the square root of the number. If the number is divisible by any of these numbers, it is not prime.

for (int i = 2; i <= Math.sqrt(n); i++) {
    if (n % i == 0) {
        return false; // n is divisible by i, so it's not a prime number
    }
}

3. Return True if No Divisors Found: If the loop completes without finding any divisors, the number is prime, and we return true.

return true; // n is a prime number

Running the Program – Java program to check if a number is prime

To run the program, follow these steps:

1. Save the code in a file named PrimeCheck.java.
2. Open a command prompt or terminal and navigate to the directory where you saved the file.
3. Compile the program using the following command:

javac PrimeCheck.java

4. Run the compiled program using the following command:

java PrimeCheck

5. Follow the prompt to enter a number. The program will display whether the number is prime or not.

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What did we learn from this article?

Checking if a number is prime is a fundamental exercise that helps in understanding loops and conditional statements in programming. This Java program demonstrates how to implement a prime check using a simple loop and mathematical logic. By practicing such basic programs, you can strengthen your problem-solving skills and gain confidence in your ability to write efficient Java code. Keep practicing and exploring more problems to enhance your programming skills.