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lang-java
Map<K,Collection<V>>to map keys to collections of values. When adding an entry, you check if the key is already associated with a collection; if it is, add the new value to the collection. If it's not create a new collection, add the value and put the collection into the map by the key. Using Java8 this all can be done simply viamap.computeIfAbsent(key, (k,v) -> new ArrayList<V>()).add(value);.