Retrieve all Map keys having given value in Java

This post will discuss how to retrieve all map keys having given value in Java.

1. Create Reverse Map

The idea is to decorate the map with our own implementation by extending the HashMap class and overloading its put() method such that for every call to the put method, it not only inserts the key-value pair in the original map but also inserts the value-key pair into a reverse map.

import java.util.HashMap;

import java.util.HashSet;

import java.util.Map;

import java.util.Set;

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

{

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

@Override

public V put(K key, V value)

{

if (reverseMap.get(value) == null) {

reverseMap.put(value, new HashSet<K>());

}

reverseMap.get(value).add(key);

return super.put(key, value);

}

public Set<K> getKeys(V value) {

return reverseMap.get(value);

}

// Program to retrieve all map keys having given value in Java

class Main

{

public static void main(String[] args)

{

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

hashMap.put("A", 1);

hashMap.put("B", 1);

hashMap.put("C", 2);

System.out.println(hashMap.getKeys(1));

}

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2. Using `entrySet()` method

We can also iterate over all entries present in the map using the entrySet() method and insert all keys that satisfy the criteria into a set.

import java.util.HashMap;

import java.util.HashSet;

import java.util.Map;

import java.util.Set;

// Program to retrieve all map keys having given value in Java

class Main

{

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

{

Set<K> keys = new HashSet<>();

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

{

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

keys.add(entry.getKey());

}

return keys;

}

public static void main(String[] args)

{

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

hashMap.put("A", 1);

hashMap.put("B", 1);

hashMap.put("C", 2);

System.out.println(getKeys(hashMap, 1));

}

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In Java 8 and above, we can do something like:

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

{

return map.entrySet().stream()

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

.map(Map.Entry::getKey)

.collect(Collectors.toSet());

}

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3. Using `keySet()` method

Like the entrySet() 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.

import java.util.HashMap;

import java.util.HashSet;

import java.util.Map;

import java.util.Set;

// Program to retrieve all map keys having given value in Java

class Main

{

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

{

Set<K> keys = new HashSet<>();

for (K key: map.keySet())

{

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

keys.add(key);

}

return keys;

}

public static void main(String[] args)

{

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

hashMap.put("A", 1);

hashMap.put("B", 1);

hashMap.put("C", 2);

System.out.println(getKeys(hashMap, 1));

}

Download Run Code

In Java 8 and above, we can do something like:

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

{

return map.keySet().stream()

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

.collect(Collectors.toSet());

}

Download Run Code

That’s all about retrieving all Map keys having a given value in Java.

Retrieve all Map keys having given value in Java

1. Create Reverse Map

2. Using entrySet() method

3. Using keySet() method

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2. Using `entrySet()` method

3. Using `keySet()` method