public interface Listener {
    void onEvent();
}

public class EventMaker {
    private List<Listener> listeners = new ArrayList<>();
    public void addListener(Listener listener) { listeners.add(listener); }
    public void removeListener(Listener listener) { listeners.remove(listener); }
    public void doSomething() {
        for (Listener l : listeners) listener.onEvent();
    }
}

public class Main {
    public static void main(String[] args) {
        EventMaker em = new EventMaker();
        em.addListener(() -> System.out.println("First listener got an event!"));
        em.addListener(() -> System.out.println("Second listener got an event!"));

        em.doSomething();
    }
}