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<Character, String> myHashMap = new HashMap<Character, String>(); myHashMap.put('a', "test one"); myHashMap.put('b', "test two"); 

and I want to create a new HashMap in which i put the opposites.

HashMap<String, Character> reversedHashMap = new HashMap<String, Character>(); e.g. Keys "test one" & "test two" and values 'a' & 'b'. 

Answer 1

They all are unique, yes

If you're sure that your values are unique you can iterate over the entries of your old map .

Map<String, Character> myNewHashMap = new HashMap<>(); for(Map.Entry<Character, String> entry : myHashMap.entrySet()){     myNewHashMap.put(entry.getValue(), entry.getKey()); } 

Alternatively, you can use a Bi-Directional map like Guava provides and use the inverse() method :

BiMap<Character, String> myBiMap = HashBiMap.create(); myBiMap.put('a', "test one"); myBiMap.put('b', "test two");  BiMap<String, Character> myBiMapInversed = myBiMap.inverse(); 

As java-8 is out, you can also do it this way :

Map<String, Integer> map = new HashMap<>(); map.put("a",1); map.put("b",2);  Map<Integer, String> mapInversed =      map.entrySet()        .stream()        .collect(Collectors.toMap(Map.Entry::getValue, Map.Entry::getKey)) 

Finally, I added my contribution to the proton pack library, which contains utility methods for the Stream API. With that you could do it like this:

Map<Character, String> mapInversed = MapStream.of(map).inverseMapping().collect(); 

Answer 2

Apache commons collections library provides a utility method for inversing the map. You can use this if you are sure that the values of myHashMap are unique

org.apache.commons.collections.MapUtils.invertMap(java.util.Map map) 

Sample code

HashMap<String, Character> reversedHashMap = MapUtils.invertMap(myHashMap)