Java Project - Prime Number Checker

Efficient Ways to Check for Primes in Java:

Prime Number Checker :

Determine whether a given number is prime or not.

A simple program where the user inputs a number, and the program checks whether the number is prime (a number that can only be divided by 1 and itself).

Input: A number.
Output: Whether the number is prime or not.

Example:

• Input: 29
• Output: "Prime"
• Input: 12
• Output: "Not prime"

Solution 1: Basic Prime Number Checker in Java

Code:

import java.util.Scanner;

public class PrimeChecker {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);  // Initialize scanner for user input

        // Input the number from the user
        System.out.println("Enter a number: ");
        int number = scanner.nextInt();  

        // Check if the number is prime
        if (isPrime(number)) {
            System.out.println(number + " is a Prime number.");
        } else {
            System.out.println(number + " is Not a Prime number.");
        }

        scanner.close();  // Close the scanner
    }

    // Method to check if a number is prime
    public static boolean isPrime(int num) {
        // Numbers less than 2 are not prime
        if (num < 2) {
            return false;
        }
        // Check divisibility from 2 up to the square root of the number
        for (int i = 2; i <= Math.sqrt(num); i++) {
            if (num % i == 0) {
                return false;  // If divisible, it's not prime
            }
        }
        return true;  // Otherwise, it's prime
    }
} 

Output:

Enter a number: 
 17
17 is a Prime number.

Enter a number: 
 15
15 is Not a Prime number.

Explanation :

• Input Handling: The user inputs a number using the Scanner.
• Prime Check Logic: The method isPrime() checks divisibility of the number starting from 2 up to its square root.
• Efficiency: Using the square root reduces unnecessary iterations, improving performance for large numbers.
• Prime Result: Outputs whether the number is prime or not based on the result of the isPrime() method.

Solution 2: Optimized Prime Checker with Edge Case Handling

Code:

import java.util.Scanner;

public class OptimizedPrimeChecker {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);  // Initialize scanner for user input

        // Input number from the user
        System.out.println("Enter a number: ");
        int number = scanner.nextInt();

        // Check if the number is prime
        if (number <= 1) {
            System.out.println(number + " is Not a Prime number.");
        } else if (number == 2 || number == 3) {
            System.out.println(number + " is a Prime number.");  // 2 and 3 are prime
        } else if (number % 2 == 0 || number % 3 == 0) {
            System.out.println(number + " is Not a Prime number.");  // Rule out even numbers and multiples of 3
        } else if (isPrime(number)) {
            System.out.println(number + " is a Prime number.");
        } else {
            System.out.println(number + " is Not a Prime number.");
        }

        scanner.close();  // Close the scanner
    }

    // Optimized method to check if a number is prime
    public static boolean isPrime(int num) {
        // Check from 5 up to the square root of the number, skipping multiples of 2 and 3
        for (int i = 5; i * i <= num; i += 6) {
            if (num % i == 0 || num % (i + 2) == 0) {
                return false;  // If divisible by i or (i + 2), it's not prime
            }
        }
        return true;  // Otherwise, it's prime
    }
}  

Output:

Enter a number: 
 99
99 is Not a Prime number.

Enter a number: 
 19
19 is a Prime number.

Explanation:

• Edge Case Handling: Special cases for numbers less than or equal to 1, and small primes like 2 and 3, are handled separately.
• Optimization: Numbers divisible by 2 or 3 are quickly ruled out before performing more advanced checks.
• Loop Optimization: The method checks divisibility starting from 5 and skips unnecessary checks by jumping by 6 (since multiples of 2 and 3 are already excluded).
• Prime Result: Outputs whether the number is prime based on an optimized check.