Most Elegant Way to write isPrime in java

Posted by Anantha Kumaran on Stack Overflow See other posts from Stack Overflow or by Anantha Kumaran
Published on 2010-03-05T10:12:06Z Indexed on 2010/05/06 6:28 UTC
Read the original article Hit count: 265

Filed under:
|
    public class Prime {

        public static boolean isPrime1(int n) {
            if (n <= 1) {
                return false;
            }
            if (n == 2) {
                return true;
            }
            for (int i = 2; i <= Math.sqrt(n) + 1; i++) {
                if (n % i == 0) {
                    return false;
                }
            }
            return true;
        }
        public static boolean isPrime2(int n) {
            if (n <= 1) {
                return false;
            }
            if (n == 2) {
                return true;
            }
            if (n % 2 == 0) {
                return false;
            }
            for (int i = 3; i <= Math.sqrt(n) + 1; i = i + 2) {
                if (n % i == 0) {
                    return false;
                }
            }
            return true;
        }
    }



public class PrimeTest {

    public PrimeTest() {
    }

    @Test
    public void testIsPrime() throws IllegalArgumentException, IllegalAccessException, InvocationTargetException {

        Prime prime = new Prime();
        TreeMap<Long, String> methodMap = new TreeMap<Long, String>();


        for (Method method : Prime.class.getDeclaredMethods()) {

            long startTime = System.currentTimeMillis();

            int primeCount = 0;
            for (int i = 0; i < 1000000; i++) {
                if ((Boolean) method.invoke(prime, i)) {
                    primeCount++;
                }
            }

            long endTime = System.currentTimeMillis();

            Assert.assertEquals(method.getName() + " failed ", 78498, primeCount);
            methodMap.put(endTime - startTime, method.getName());
        }


        for (Entry<Long, String> entry : methodMap.entrySet()) {
            System.out.println(entry.getValue() + " " + entry.getKey() + " Milli seconds ");
        }
    }
}

I am trying to find the fastest way to check whether the given number is prime or not. This is what is finally came up with. Is there any better way than the second implementation(isPrime2).

© Stack Overflow or respective owner

Related posts about java

Related posts about primes