PrimeNumber.java

package chapter_five.samples;

/** Listing 5.15 PrimeNumber.java */
public class PrimeNumber
{
    public static void main(String[] args)
    {
        final int NUMBER_OF_PRIMES = 50; // Number of primes to display
        final int NUMBER_OF_PRIMES_PER_LINE = 10; // Display 10 per line
        int count = 0; // Count the number of prime numbers
        int number = 2; // A number to be tested for primeness

        System.out.println("The first 50 prime numbers are \n");

        // Repeatedly find prime numbers
        while (count < NUMBER_OF_PRIMES)
        {
            // Assume the number is prime
            boolean isPrime = true; // Is the current number prime?

            // Test whether number is prime
            for (int divisor = 2; divisor <= number / 2; divisor++)
            {
                if (number % divisor == 0) { // If true, number is not prime
                    isPrime = false; // Set isPrime to false
                    break; // Exit the for loop
                }
            }

            // Display the prime number and increase the count
            if (isPrime)
            {
                count++; // Increase the count

                if (count % NUMBER_OF_PRIMES_PER_LINE == 0)
                {
                    // Display the number and advance to the new line
                    System.out.println(number);
                }
                else
                    System.out.print(number + " ");
            }

            // Check if the next number is prime
            number++;
        }
    }
}