Map<String, Object> myData = new HashMap<String, Object>();
myData.put("A", 1);
myData.put("B", 2);
for(String key: myData.keySet())
myData.remove(key);

This will throw an exception ConcurrentModificationException at runtime

Map<String, Object> myData = new ConcurrentHashMap<String, Object>();
    myData.put("A", 1);
    myData.put("B", 2);
    for(String key: myData.keySet())
    myData.remove(key);