Prime Number Checker in Java

This program checks if a given number is prime. A prime number is a natural number greater than 1 that is not a product of two smaller natural numbers. For example, 5 is prime because the only ways of writing it as a product, 1 × 5 or 5 × 1, involve 5 itself.

Java Program

/**
 * This class contains a method to check if a given number is prime.
 */
public class PrimeNumberChecker {

    /**
     * This method checks if the given number is prime.
     * 
     * @param number The number to check.
     * @return true if the number is prime, false otherwise.
     */
    public static boolean isPrime(int number) {
        // Check if number is less than 2, as prime numbers are greater than 1
        if (number < 2) {
            return false;
        }
        // Loop to check if the number has any divisor other than 1 and itself
        for (int i = 2; i <= Math.sqrt(number); i++) {
            if (number % i == 0) {
                return false; // number is not prime
            }
        }
        return true; // number is prime
    }

    /**
     * Main method to test the isPrime method.
     * 
     * @param args Command line arguments.
     */
    public static void main(String[] args) {
        int numberToCheck = 29; // Example number to check

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

Explanation

The isPrime method works as follows:

1. First, it checks if the number is less than 2. If so, it returns false because prime numbers are greater than 1.
2. Next, it loops from 2 to the square root of the number. The reason for checking up to the square root is that if n = a * b, then one of the factors a or b must be less than or equal to the square root of n.
3. If the number is divisible by any number in this range, it returns false because the number is not prime.
4. If the loop completes without finding any divisors, the number is prime, and the method returns true.

How to Use the Program

1. Copy the Code: Copy the Java code provided in the <code> tags.
2. Create a Java File: Create a new file named PrimeNumberChecker.java.
3. Paste the Code: Paste the copied code into the PrimeNumberChecker.java file.
4. Compile the Program: Open a terminal or command prompt, navigate to the directory containing the PrimeNumberChecker.java file, and compile the program using the following command:
   javac PrimeNumberChecker.java

5. Run the Program: After compilation, run the program using the following command:
   java PrimeNumberChecker


Example Output

For the example number 29, the program will output:

You can change the value of numberToCheck in the main method to test other numbers.