Reverse HashMap keys and values in Java

Question

It\'s a simple question, I have a simple HashMap of which i want to reverse the keys and values.

HashMap myHashMap = new HashMap<         


        

Answer 1

If the values are not unique, the safe way to inverse the map is by using java 8's groupingBy function

Map map = new HashMap<>();
map.put("a",1);
map.put("b",2);

Map> mapInversed = 
map.entrySet()
   .stream()
   .collect(Collectors.groupingBy(Map.Entry::getValue, Collectors.mapping(Map.Entry::getKey, Collectors.toList())))