Get key from value in a Java Map

This post will discuss how to get Map’s key from the value in Java, where there is a 1:1 relationship between keys and values in the map, i.e., no two keys have the same value.

1. Using `entrySet()` method

The idea is to iterate over all mappings present in the map using the entrySet() method and compare each value with the desired value until we get the corresponding key.

import java.util.HashMap;

import java.util.Map;

class Main

{

public static <K, V> K getKey(Map<K, V> map, V value)

{

for (Map.Entry<K, V> entry: map.entrySet())

{

if (value.equals(entry.getValue())) {

return entry.getKey();

}

return null;

}

public static void main(String[] args)

{

Map<String, Integer> hashMap = new HashMap();

hashMap.put("A", 1);

hashMap.put("B", 2);

hashMap.put("C", 3);

System.out.println(getKey(hashMap, 2)); // prints `B`

}

Download Run Code

From Java 8, we can use Stream:

import java.util.HashMap;

import java.util.Map;

class Main

{

public static <K, V> K getKey(Map<K, V> map, V value)

{

return map.entrySet().stream()

.filter(entry -> value.equals(entry.getValue()))

.findFirst().map(Map.Entry::getKey)

.orElse(null);

}

public static void main(String[] args)

{

Map<String, Integer> hashMap = new HashMap();

hashMap.put("A", 1);

hashMap.put("B", 2);

hashMap.put("C", 3);

System.out.println(getKey(hashMap, 2)); // prints `B`

}

Download Run Code

2. Using `keySet()` method

We can also iterate over all keys present in the map using the keySet() method and compare each key’s value with the desired value until we get the corresponding key.

import java.util.HashMap;

import java.util.Map;

class Main

{

public static <K, V> K getKey(Map<K, V> map, V value)

{

for (K key: map.keySet())

{

if (value.equals(map.get(key))) {

return key;

}

return null;

}

public static void main(String[] args)

{

Map<String, Integer> hashMap = new HashMap();

hashMap.put("A", 1);

hashMap.put("B", 2);

hashMap.put("C", 3);

System.out.println(getKey(hashMap, 2)); // prints `B`

}

Download Run Code

From Java 8, we can use Stream:

public static <K, V> K getKey(Map<K, V> map, V value)

{

return map.keySet()

.stream()

.filter(key -> value.equals(map.get(key)))

.findFirst().get();

}

Download Run Code

3. Using Reverse Map

The idea is to extend the HashMap class and overload its put() method such that it also inserts the value-key pair into a reverse map along with the key-value pair in the original map. We also create a getKey() method that facilitates the value lookup in the reverse map.

import java.util.HashMap;

import java.util.Map;

class MyHashMap<K, V> extends HashMap<K, V>

{

Map<V, K> reverseMap = new HashMap<>();

@Override

public V put(K key, V value)

{

reverseMap.put(value, key);

return super.put(key, value);

}

public K getKey(V value) {

return reverseMap.get(value);

}

class Main

{

public static void main(String[] args)

{

MyHashMap<String, Integer> hashMap = new MyHashMap();

hashMap.put("A", 1);

hashMap.put("B", 2);

hashMap.put("C", 3);

System.out.println(hashMap.getKey(2)); // prints `B`

}

Download Run Code

4. Using Guava’s `BiMap` Class

Guava provides a BiMap class, a bidirectional map to provide an inverse view of mappings, i.e., with reversed keys and values. BiMap doesn’t allow duplicate values and throws IllegalArgumentException when multiple entries with the same value are encountered. To get the inverse view of the BiMap, we can use the inverse() method.

import com.google.common.collect.BiMap;

import com.google.common.collect.ImmutableBiMap;

class Main

{

public static <K, V> K getKey(BiMap<K, V> map, V value) {

return map.inverse().get(value);

}

public static void main(String[] args)

{

BiMap<String, Integer> bimap = ImmutableBiMap.of("A", 1, "B", 2, "C", 3);

System.out.println(getKey(bimap, 2)); // prints `B`

}

Download Code

5. Using Apache Commons Collections

Like Guava, Apache Commons also facilitates bidirectional lookup between key and values by providing a BidiMap interface. It has several implementations that allow a key to be looked up from a value using the inverseBidiMap() method.

Please note that when multiple entries with the same value are found, the last inserted key is returned.

import org.apache.commons.collections4.BidiMap;

import org.apache.commons.collections4.bidimap.DualHashBidiMap;

class Main

{

public static <K, V> K getKey(BidiMap<K, V> map, V value) {

return map.inverseBidiMap().get(value);

}

public static void main(String[] args)

{

BidiMap<String, Integer> bimap = new DualHashBidiMap<>();

bimap.put("A", 1);

bimap.put("B", 2);

bimap.put("C", 3);

System.out.println(getKey(bimap, 2)); // prints `B`

}

Download Code

That’s all about getting the map key from the value in Java.

Follow-up:

Retrieve all Map keys having given value in Java

Get key from value in a Java Map

1. Using entrySet() method

2. Using keySet() method

3. Using Reverse Map

4. Using Guava’s BiMap Class

5. Using Apache Commons Collections

Share this:

1. Using `entrySet()` method

2. Using `keySet()` method

4. Using Guava’s `BiMap` Class