Ken Ken - 2 months ago 20
Java Question

Reverse HashMap keys and values in Java

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

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 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();
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