Largest prime factor program takes aaaages - Java

Question

So this is problem 3 from project Euler. For those who don't know, I have to find out the largest prime factor of 600851475143. I have the below code:

import java.lang.Math;
// 600851475143
public class LargestPrimeFactor {
    public static void main(String[] stuff) {
        long num = getLong("What number do you want to analyse? ");
        long[] primes = primeGenerator(num);
        long result = 0;
        for(int i = 0; i < primes.length; i++) {
            boolean modulo2 = num % primes[i] == 0;
            if(modulo2) {
                result = primes[i];
            }
        }
        System.out.println(result);
    }
    public static long[] primeGenerator(long limit) {
        int aindex = 0;
        long[] ps = new long[primeCount(limit)];
        for(long i = 2; i < limit + 1; i++) {
            if(primeCheck(i)) {
                ps[aindex] = i;
                aindex++;
            }
        }
        return ps;
    }

    public static boolean primeCheck(long num) {
        boolean r = false;
        if(num == 2 || num == 3) {
            return true;
        }
        else if(num == 1) {
            return false;
        }
        for(long i = 2; i < Math.sqrt(num); i++) {
            boolean modulo = num % i == 0;
            if(modulo) {
                r = false;
                break;
            }
            else if(Math.sqrt(num) < i + 1 && !modulo) {
                r = true;
                break;
            }
        }
        return r;
    }
    public static int primeCount(long limit) {
        int count = 0;
        if(limit == 1 || limit == 2) {
            return 0;
        }
        for(long i = 2; i <= limit; i++) {
            if(primeCheck(i)) {
                count++;
            }
        }
        return count;
    }
public static long getLong(String prompt) {
    System.out.print(prompt + " ");
    long mrlong = input.nextLong();
    input.nextLine();
    return mrlong;
}
}

But when I test the program with something (a lot) smaller than 600851475143, like 100000000, then the program takes its time - in fact, 100000000 has taken 20 minutes so far and is still going. I've obviously got the wrong approach here (and yes, the program does work, I tried it out with smaller numbers). Can anyone suggest a less exhaustive way?

Answer 1

try this ..

public class LargestPrimeFactor{
public static int largestPrimeFactor(long number) {
    int i;
    for (i = 2; i <= number; i++) {
        if (number % i == 0) {
            number /= i;
            i--;
        }
    }
    return i;
}

/*  change according to ur requirement. 
public static long getLong(String prompt) {
    System.out.print(prompt + " ");
    long mrlong = input.nextLong();
    input.nextLine();
    return mrlong;
}
 */

public static void main(String[] args) {
    //long num = getLong("What number do you want to analyse? ");
    System.out.println(largestPrimeFactor(600851475143l));
}
}