how to get prime number in java

1public class Prime {
2    public static void main(String[] args) {
3
4        int num = 29;
5        boolean flag = false;
6        for(int i = 2; i <= num/2; ++i){
7            if(num % i == 0){
8                flag = true;
9                break;
10            }
11        }
12
13        if (!flag)
14            System.out.println(num + " is a prime number.");
15        else
16            System.out.println(num + " is not a prime number.");
17}}