<p>Is there a reason to sort MultiMap in Java and how to do that?</p>

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.