Hello, Recently I had the need of using a MultiMap in one of my projects. I found that commons collection already has a MultiMap interface and an implementation.
While using the same, I found that the MultiMap interface has methods that are not strongly typed even though the interface supports generics. For example if I have a MultiMap like so MultiMap<String, User> multiMap = new MultiValueMap<String, User>(); where User is a custom Class, then the get(key) method would return me an Object which I would need to cast to a Collection like so Collection<User> userCol = (Collection<User>) multiMap.get(key); I understand that this limitation comes from that fact that the MultiMap extends IterableMap which in turn extends Map and other interfaces. Hence the MultiMap cannot have a get method which returns a Collection instead of Object as that would mean extending IterableMap with the Generics set to be <K,Collection<V>>. In that case the put method's signature would become public Collection<V> put(K key, Collection<V> value); which we do not want.The same problem would arise with other methods as well, ex: containsValue method. My proposal is why carry on the signatures of a Map and put it on MultiMap. Where as I do agree that it is a Map after all and has very similar implementation and functionality, it is very different at other levels. And even though the MultiMap interface supports generics, the methods are not strongly typed, which defeats the purpose of having generics. So why can't we have a separate set of interfaces for MultiMap which do not extend Map. That way we can have strongly typed methods on the MultiMap. Please let me know your thoughts on this. I can submit a patch for these changes based on your feedback. One more thing, I also am in need of a MultiTrie which is currently not there. I am implementing the same by wrapping PatriciaTrie. Now I am a bit confused here as, if I make the MultiTrie interface on the lines of MultiMap, it would have the same limitations. In that case I was planning to have a separate set of interfaces for MultiTrie which does not extend any other interface. And in case, we do change the MultiMap interface to be independent of Map, then MultiTrie can extend MultiMap. Please let me know your thoughts on this as well as I am implementing the same for my project right now and would like to contribute it back to the commons collection. Regards Dipanjan
