"Can't add an observer from within an observer."
"Can't add a new observer from within another " + "observer.\n" + "Trying to add: " + observer + "\n" + "Currently firing: " + m_firing + "\n" + "Current observers: " + m_observers
    public void addObserver(DataObserver observer) {        if (observer == null) {            throw new IllegalArgumentException("Can't add a null observer.");        }        ObserverEntry entry = new ObserverEntry(observer, m_observers.size());        if (!m_observers.contains(entry)) {            if (m_firing) {                throw new IllegalStateException(                    "Can't add an observer from within an observer."                    );            }            m_observers.add(entry);        }    }