java - is a prime number ?

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import java.util.stream.IntStream; public class _Math { static int[] prime_generator(int size){ // return IntStream.iterate(2, a -> a+1).filter(InterviewQuestions::is_prime_method_1).limit(size).toArray(); int array[] = new int[size]; if(size == 0) return array; array[0] = 2; if(size == 1) return array; array[1] = 3; if(size == 2) return array; // all other than 2, 3 can is in form of 6k ± 1 for (int i = 0, index = 2; index < array.length; i++) { int n1 = 6 * i; if(is_prime(n1 - 1)) array[index++] = n1 - 1; if(is_prime(n1 + 1)) array[index++] = n1 + 1; } return array; } /** * https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes * @param upto * @return */ static int[] prime_generator_by_sieve_of_eratosthenes(int upto){ int[] base = IntStream.range(0, upto+1).toArray(); base[0] = base[1] = 0; for (int i = 2; i < base.length; i++) { if(base[i] == 0) continue; for (int j = 2; i*j < base.length; j++) { base[i*j] = 0; } } return IntStream.of(base).filter(i -> i != 0).toArray(); } /** * https://en.wikipedia.org/wiki/Primality_test * @param num * @return */ static boolean is_prime(int num){ if(num < 2) return false; if(num == 2 || num == 3) return true; if(num%2 == 0) return false; // max number which is divide a number is its sqrt, after that is can only divided by itself int max = (int) Math.sqrt(num); for (int i = 3; i <= max; i+=2) if(num%i == 0) return false; return true; } }

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