如何对MultiMap进行排序 <k,v> 在java中？

Question

是否有理由在Java中对MultiMap进行排序，以及如何做到这一点？

Answer 1

Assuming you're talking about org.apache.commons.collections.MultiMap , then you can't; since it returns a Collection and not a List , it doesn't support the concept of order.

If you are talking about org.apache.commons.collections.MultiHashMap , then all you need to do is iterate over the keys, take the ArrayList returned and sort it using Collections.sort() .

That's assuming you are using that implementation though .

There is nothing stopping you from implementing your own MultiMap fairly easily though, that supports sorting lists. It may be as easy as HashMap<K, Collection<V>> , I'm not familiar with how MultiMaps work.

Answer 2

Actually I don't know why you would want to sort a Map. A Map is a dictionary and you retrieve from this dictionary a (or in the case of multimaps a collection of) value(s) you're interested in.

In the case of a MultiMap you perhaps would want to sort the Collection resulting from a get. But what advantage do you have by a sorted Map since it doesn't speed up anything in finding a specific value?

Answer 3

Easiest solution is to use TreeMultimap from guava. Use either directly

TreeMultimap<...> sortedMap = TreeMultimap.create(notSortedMultiMap);

if your keys and values are sortable naturally (implement Comparable), or

TreeMultimap<...> sortedMap = TreeMultimap.create(keyComparator, valueComparator);
sortedMap.putAll(notSortedMultiMap);

if you need to provide custom comparators.

If you know you will need it sorted and not care about retrieval speed that much, you can of course use TreeMap from very start.

Then you can iterate over TreeMap or use something like values() or entries to get sorted collection.