public interface MultimapCache<K, V> {
   CompletableFuture<Void> put(K key, V value);
     - Put wrong class of key/value/both
     - Put duplicates, confirm duplicates in fact not supported
   CompletableFuture<Collection<V>> get(K key);    
     - Get invalid key*
   CompletableFuture<Optional<CacheEntry<K, Collection<V>>>> getEntry(K key);    
     - Get invalid key*
   CompletableFuture<Boolean> remove(K key);    
     - Remove invalid key*
   CompletableFuture<Boolean> remove(K key, V value);    
     - Remove invalid key/value*
   CompletableFuture<Void> remove(Predicate<? super V> p);    
     - Invalid predicate
   CompletableFuture<Boolean> containsKey(K key); 
     - contains invalid key class
   CompletableFuture<Boolean> containsValue(V value); 
     - contains invalid value class
   CompletableFuture<Boolean> containsEntry(K key, V value);
     - contains invalid key*/value class
   CompletableFuture<Long> size();    
     - size with duplicates and removes
   boolean supportsDuplicates();
}