Question

Is there any function in a Util libary (commonsXXX or something else) that can rotate a Map.

By rotating I mean I have a Map<User,Set<Permission>> that should be “rotated” to Map<Permission,Set<User>>?

I know it is not hard to implement, but using a common existing library seems to be better in terms of “don’t invent the wheel twice”.

Answer 1

I needed to do the same thing as what you describe, I used a multimap from commons-collections and wrote my own inverse function. (That was at least 3 years ago, using jdk1.4, so guava wasn’t an option.)

But if you changed your original map to a SetMultimap (I prefer multimaps anyway when mapping keys to collections of values, since they make inserting values easier), then you could use com.google.common.collect.Multimaps.invertFrom:

public static <K,V,M extends Multimap<K,V>> M invertFrom(Multimap<? extends V,? extends K> source,
                                         M dest)

Copies each key-value mapping in source into dest, with its key and value reversed.

Parameters:
    source - any multimap
    dest - the multimap to copy into; usually empty
Returns:
    dest