Java: When to clear an Observable Object

I have and Observable object with following object being shared:-

HashMap<String,HashSet<String>> data;

I have a put function which appends entries in the above data and notify the observers after getting enough data.

put(String k, String line){
    if (!data.containsKey(key)) {
        addNewKey(k, line);
    }else{
        data.get(key).appendToList(deviceid);
    }
    if (data.get(key).size() > writeSizePerList || data.size() > writeSizeLists || System.currentTimeMillis() > lastDumpTimestamp + waitTimeMillis) {
        setChanged();
        notifyObservers();
        lastDumpTimestamp = System.currentTimeMillis();
    }
}

However I am stuck in the function. I need to clear the data object once all the observers have already processed after notify call. How would I capture this event? I can't keep adding data due to scalability issues.