How to swap keys and values in a Map elegantly

Question

I already know how to do it the hard way and got it working - iterating over entries and swapping \"manually\". But i wonder if, like so many tasks, this one can be solved

Answer 1

As a hint to answer https://stackoverflow.com/a/42091477/8594421

This only works, if the map is not a HashMap and does not contain duplicate values.

Map newMap = oldMap.entrySet().stream().collect(Collectors.toMap(Map.Entry::getValue, Map.Entry::getKey));

throws an exception

java.lang.IllegalStateException: Duplicate key

if there are values more than once.

The solution:

HashMap newMap = new HashMap<>();

for(Map.Entry entry : oldMap.entrySet())
        newMap.put(entry.getValue(), entry.getKey());

// Add inverse to old one
oldMap.putAll(newMap);