gen-class.java

import demo.api.Store;
import demo.api.StoreFactory;

public class Main {
    public static void main(String[] args) {
        Store s = StoreFactory.makeStore();
        System.out.println("qty = " + s.getQty());

        s.buy().buy().buy().sell().buy();
        System.out.println("qty = " + s.getQty());

        s.buy(100);
        System.out.println("qty = " + s.getQty());
    }
}