Java Program to Check if a Number is Prime

Introduction

A prime number is a natural number greater than 1 that has no positive divisors other than 1 and itself. Checking if a number is prime is a common task in programming, especially in algorithms related to number theory. This guide will walk you through writing a Java program that checks if a given number is prime.

Problem Statement

Create a Java program that:

• Prompts the user to enter a number.
• Checks if the number is a prime number.
• Displays whether the number is prime or not.

Example:

• Input: 17

• Output: "17 is a prime number."

• Input: 18

• Output: "18 is not a prime number."

Solution Steps

1. Read the Number: Use the Scanner class to take the number as input from the user.
2. Check if the Number is Prime: Implement logic to determine if the number is prime.
3. Display the Result: Print whether the number is prime or not.

Java Program

// Java Program to Check if a Number is Prime
// Author: https://www.rameshfadatare.com/

import java.util.Scanner;

public class PrimeNumberChecker {
    public static void main(String[] args) {
        // Step 1: Read the number from the user
        try (Scanner scanner = new Scanner(System.in)) {
            System.out.print("Enter a number: ");
            int number = scanner.nextInt();
            
            // Step 2: Check if the number is prime
            boolean isPrime = checkPrime(number);
            
            // Step 3: Display 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 checkPrime(int number) {
        // Prime numbers are greater than 1
        if (number <= 1) {
            return false;
        }
        // Check divisibility from 2 to the square root of the number
        for (int i = 2; i <= Math.sqrt(number); i++) {
            if (number % i == 0) {
                return false;
            }
        }
        return true;
    }
}

Explanation

Step 1: Read the Number

• The Scanner class is used to read an integer input from the user. The nextInt() method captures the input number.

Step 2: Check if the Number is Prime

• The checkPrime() method is used to determine if the number is prime.
• Numbers less than or equal to 1 are not prime.
• For numbers greater than 1, the method checks if there are any divisors other than 1 and the number itself by testing divisibility from 2 up to the square root of the number. If a divisor is found, the number is not prime.

Step 3: Display the Result

• The program prints whether the number is prime based on the result returned by the checkPrime() method.

Output Example

Example 1:

Enter a number: 17
17 is a prime number.

Example 2:

Enter a number: 18
18 is not a prime number.

Conclusion

This Java program demonstrates how to check if a given number is prime. It covers essential concepts such as loops, conditionals, and mathematical operations, making it a valuable exercise for beginners learning Java programming.