How to swap keys and values in a Map elegantly

The standard API / Java runtime doesn't offer a bi-directional map, so the only solution is to iterate over all entries and swap them manually.

What you can do is create a wrapper class which contains two maps and which does a dual put() internally so you have fast two views on the data.

[EDIT] Also, thanks to open source, you don't have to include a third party library, you can simply copy the classes you need into your own project.

If you don't have a choice to use a third party library, I don't consider the following code so ugly (though some scripting languages do have elegant ways of doing it):

//map must be a bijection in order for this to work properly
public static <K,V> HashMap<V,K> reverse(Map<K,V> map) {
    HashMap<V,K> rev = new HashMap<V, K>();
    for(Map.Entry<K,V> entry : map.entrySet())
        rev.put(entry.getValue(), entry.getKey());
    return rev;
}

Map<String, Integer> map = new HashMap<>();
Map<Integer, String> swapped = map.entrySet().stream().collect(Collectors.toMap(Map.Entry::getValue, Map.Entry::getKey));

Maps are not like lists, which can be reversed by swapping head with tail.

Objects in maps have a computed position, and using the value as key and the key as value would requiere to re-compute the storage place, essentialy building another map. There is no elegant way.

There are, however, bidirectional maps. Those may suit your needs. I'd reconsider using third-party libraries.