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Geekpedia » Java Code » Find Twin Primes

Find Twin Primes

Find the first 1000 twin prime numbers in Java using an isPrime function for the primality check and a value comparison between the prime numbers.

On Thursday, October 2nd 2008 at 04:00 AM

By Andrew Pociu (View Profile)

(Rated 0 with 0 votes)

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class TwinPrimes
{
        public static void main(String[] args)
        {
                int LastPrime = 1;
                for (int n = 0; n < 1000; n++)
                {
                        if (isPrime(n)) // We only care about prime numbers
                        {
                                if ((n - LastPrime) == 2) // If the difference between this prime and the last prime is 2
                                {
                                        System.out.println((n - 2) + " and " + n + " are twin primes"); // We have a twin prime
                                }
                                LastPrime = n; // Store the last prime so we can compare it to the next one
                        }
                }
        }
 
        public static boolean isPrime(int n)
        {
                if (n <= 1) // Not prime
                        return false;
                if (n == 2) // Prime
                        return true;
                if (n % 2 == 0) // Divisible by 2 means it's always not a prime
                        return false;
                
                // For all other numbers, test by checking the divisibility of the square root of the number
                int m = (int)Math.round(Math.sqrt(n));
 
                for (int i = 3; i <= m; i += 2)
                {
                        if (n % i == 0)
                        {
                                return false;
                        }
                }
                return true;
        }
}