import java.util.HashSet;
import java.util.concurrent.ConcurrentSkipListSet;

public class NewClass {

    static class E implements Comparable<E> {

        @Override
        public int compareTo(E t) {
            return 0;
        }

    }

    public static void main(String[] args) {

        var set = new HashSet<E>();
        var conc_set = new ConcurrentSkipListSet<E>();

        for (var i = 0; i < 5; i++) {
            set.add(new E());
            conc_set.add(new E());
        }

        System.out.println("set size: " + set.size());
        System.out.println("concurrent set size: " + conc_set.size());

/**
 * set size: 5
 * concurrent set size: 1
 */
    }

}