我是傻逼。

加了L才是long，不加L是int。

public class test {
    public static void main(String[] args) {
        System.out.println(primeFactor(600851475143L));
    }
    
    public static long primeFactor(long source) {
        long start = (long) Math.floor(Math.sqrt(source));
        
        for (long number = start; number > 0; number--) {
            System.out.println(number);
            long module = source % number;
            if (module == 0 && isPrime(number))
                return number;
        }
        return -1;
    }
    
    public static boolean isPrime(long source) {
        long start = (long) Math.floor(source / 2);
        
        for (long number = start; number > 1; number--) {
            long module = source % number;
            if (module == 0)
                return false;
        }
        return true;
    }
}