Birthday Problem, Java

main.java

package com.github.thanthese;

import java.util.HashSet;
import java.util.Set;
import java.util.Random;

public class Main {

    private static final int days_in_year = 365;
    private static final int trials_size = 10000;

    private static final Random random = new Random();

    private static boolean is_match_in_trial(int group_size) {
        Set<Integer> bdays = new HashSet<>();
        for (int i = 0; i < group_size; ++i) {
            bdays.add(random.nextInt(days_in_year));
        }
        return bdays.size() != group_size;
    }

    private static void run_trials(int group_size, int number_of_trials) {
        int number_of_matches = 0;
        for (int i = 0; i < number_of_trials; ++i) {
            if (is_match_in_trial(group_size)) {
                number_of_matches++;
            }
        }
        int percent = (int) (100.0 * number_of_matches / number_of_trials);
        System.out.println(String.format(
                "Found matching birthdays amongst %d people in %d out of %d trials, or %d%% of the time.",
                group_size, number_of_matches, number_of_trials, percent));
    }

    public static void main(String[] args) {
        for (int group_size = 10; group_size < 30; ++group_size) {
            run_trials(group_size, trials_size);
        }
    }
}

//        Found matching birthdays amongst 10 people in 1202 out of 10000 trials, or 12% of the time.
//        Found matching birthdays amongst 11 people in 1434 out of 10000 trials, or 14% of the time.
//        Found matching birthdays amongst 12 people in 1669 out of 10000 trials, or 16% of the time.
//        Found matching birthdays amongst 13 people in 1989 out of 10000 trials, or 19% of the time.
//        Found matching birthdays amongst 14 people in 2276 out of 10000 trials, or 22% of the time.
//        Found matching birthdays amongst 15 people in 2539 out of 10000 trials, or 25% of the time.
//        Found matching birthdays amongst 16 people in 2798 out of 10000 trials, or 27% of the time.
//        Found matching birthdays amongst 17 people in 3166 out of 10000 trials, or 31% of the time.
//        Found matching birthdays amongst 18 people in 3400 out of 10000 trials, or 34% of the time.
//        Found matching birthdays amongst 19 people in 3761 out of 10000 trials, or 37% of the time.
//        Found matching birthdays amongst 20 people in 4035 out of 10000 trials, or 40% of the time.
//        Found matching birthdays amongst 21 people in 4401 out of 10000 trials, or 44% of the time.
//        Found matching birthdays amongst 22 people in 4832 out of 10000 trials, or 48% of the time.
//        Found matching birthdays amongst 23 people in 4923 out of 10000 trials, or 49% of the time.
//        Found matching birthdays amongst 24 people in 5406 out of 10000 trials, or 54% of the time.
//        Found matching birthdays amongst 25 people in 5746 out of 10000 trials, or 57% of the time.
//        Found matching birthdays amongst 26 people in 5968 out of 10000 trials, or 59% of the time.
//        Found matching birthdays amongst 27 people in 6212 out of 10000 trials, or 62% of the time.
//        Found matching birthdays amongst 28 people in 6585 out of 10000 trials, or 65% of the time.
//        Found matching birthdays amongst 29 people in 6782 out of 10000 trials, or 67% of the time.