singletonClass.java

public class CustomListenerService {


    public interface MyCustomListener{
        public void onSetText(String text);
    }

    private List<MyCustomListener> myCustomListeners = new ArrayList<>();

    public static volatile CustomListenerService customListenerService;

    private CustomListenerService(){
        if (customListenerService != null){
            throw new RuntimeException("use getInstance() to get the sigle instance of this class");
        }
    }

    public static CustomListenerService getInstance(){


        if (customListenerService == null){
            synchronized (CustomListenerService.class){
                customListenerService = new CustomListenerService();
            }
        }

        return customListenerService;
    }

    public void addCustomListener(MyCustomListener customListener){

        this.myCustomListeners.add(customListener);

    }

    public void removeCustomListener(MyCustomListener listener){

        this.myCustomListeners.remove(listener);
    }

    private void updateViewOnCustomEventDone(String text){

        for (int i = 0; i < myCustomListeners.size(); i++ ){
            this.myCustomListeners.get(i).onSetText(text);
        }
    }

    public void changeValue(String text){

        if (text != null){
            updateViewOnCustomEventDone(text);
        }
    }
}