Traditional Event Subscription/Listening

Traditional.java

public interface ApplicationEvent {
    public void invoke(Object value);
}

public class ApplicationEventListener implements ApplicationEvent {
    private Application application;

    public ApplicationEventListener(Application application) {
        this.application = application;
        this.application.addApplicationEventListener(this);
    }

    public void invoke(Object value) {
        // handle event
    }
}

public class Application {
    private ArrayList<ApplicationEvent> events = new ArrayList<ApplicationEvent>();

    public void addApplicationEventListener(ApplicationEvent event) {
        events.add(event);
    }

    public void postEvent() {
        for (ApplicationEvent event : events) {
            event.invoke(this);
        }
    }
}