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'.

> 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 [tag: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();

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) 

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

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

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

I wrote a simpler loop that works too (note that all my values are unique):

HashMap<Character, String> myHashMap = new HashMap<Character, String>();
HashMap<String, Character> reversedHashMap = new HashMap<String, Character>();

for (char i : myHashMap.keySet()) {
    reversedHashMap.put(myHashMap.get(i), i);
}

Iterate through the list of keys and values, then add them.

HashMap<String, Character> reversedHashMap = new HashMap<String, Character>();
for (String key : myHashMap.keySet()){
    reversedHashMap.put(myHashMap.get(key), key);
}

To answer your question on how you can do it, you could get the entrySet from your map and then just put into the new map by using getValue as key and getKey as value.

But remember that keys in a Map are unique, which means if you have one value with two different key in your original map, only the second key (in iteration order) will be kep as value in the new map.

private <A, B> Map<B, A> invertMap(Map<A, B> map) {
    Map<B, A> reverseMap = new HashMap<>();
    for (Map.Entry<A, B> entry : map.entrySet()) {
        reverseMap.put(entry.getValue(), entry.getKey());
    }
    return reverseMap;
}

It's important to remember that put replaces the value when called with the same key. So if you map has two keys with the same value only one of them will exist in the inverted map.

Tested with below sample snippet, tried with MapUtils, and Java8 Stream feature. It worked with both cases.

public static void main(String[] args) {
	Map<String, String> test = new HashMap<String, String>();
	test.put("a", "1");
	test.put("d", "1");
	test.put("b", "2");
	test.put("c", "3");
	test.put("d", "4");
	test.put("d", "41");
	
	System.out.println(test);
	
	Map<String, String> test1 = MapUtils.invertMap(test);
	
	System.out.println(test1);
	
	Map<String, String> mapInversed = 
		    test.entrySet()
		       .stream()
		       .collect(Collectors.toMap(Map.Entry::getValue, Map.Entry::getKey));
	
	System.out.println(mapInversed);
}

Output:
{a=1, b=2, c=3, d=41}
{1=a, 2=b, 3=c, 41=d}
{1=a, 2=b, 3=c, 41=d}

Use forEach introduced in Java 8

Map<Short, String> regularMap = new HashMap<>();
Map<String, Short> inversedMap = new HashMap<>();

regularMap.forEach((key, value) -> inversedMap.put(value, key));

For Reversing the Array of Dictionary. (If values are Unique)

private void reverseArrayMap(List<Map<String, String>> list) {
    // reversing the array of dictionary
    List<Map<String, String>> newList = new ArrayList<>();
    Map<String, String> resDic = new HashMap<>();

    for (Map<String, String> map : list) {
        map.forEach((key, value) -> resDic.put(value, key));
        newList.add(resDic);
    }

    System.out.println("Original Array of Dictionary" + list);
    System.out.println("Reversed Array of Dictionary" + newList);
}      

for reverting the map, in your case:

private void reverseMap(Map<Character, String> map) {
    Map<String, Character> newList = new HashMap<>();
    map.forEach((key, value) -> newList.put(value, key));
    System.out.println(newList);
}

or you can traverse the old hashmap

HashMap<String, Character> newList = new HashMap<String, Character>();
for (String key : list.keySet()){
   newList.put(list.get(key), key);
}