Invoice.java

import java.util.HashMap;
import java.util.Map;

public class Invoice implements Subscriber {
    @Override
    public void update(Broker.Context ctx) {
        System.out.println("tax rates changed");
    }

    public static void main(String[] args) {
        // register to listen to events
        Invoice subscriber = new Invoice();
        Broker.getInstance().register(Broker.CommonTopics.ON_TAX_CHANGE,subscriber);
    }
}