ThreesAndFivesSets

import java.util.TreeSet;

public class ThreesAndFivesSets {

    private static final int LIMIT = 10_000_000;   // = 10^7

    public static void main(String[] args) {

        final TreeSet<Integer> integers = new TreeSet<>();
        for (int i = 3; i <= LIMIT; i+= 3) {
            integers.add(i);
        }
        for (int i = 5; i <= LIMIT; i+= 5) {
            integers.add(i);
        }
        System.out.println(integers);
    }

}