How to Retrieve Keys from Values in a Java HashMap?

Retrieving Keys from Values in Java Hashmaps

In a Java HashMap, retrieving the key corresponding to a specific value can be useful in various scenarios. Here's a comprehensive exploration of how to accomplish this task:

Loop-Based Approach:

The traditional method for obtaining the key from a value is to iterate through the HashMap's entries. For each entry, compare the value to the desired one. If a match is found, the corresponding key can be retrieved.

public static <T, E> Set<T> getKeysByValue(Map<T, E> map, E value) {
    Set<T> keys = new HashSet<>();
    for (Entry<T, E> entry : map.entrySet()) {
        if (Objects.equals(value, entry.getValue())) {
            keys.add(entry.getKey());
        }
    }
    return keys;
}

Key Retrieval for One-to-One Mappings:

In scenarios where each value maps to only one key, the loop-based approach can be simplified to return the first matched key.

public static <T, E> T getKeyByValue(Map<T, E> map, E value) {
    for (Entry<T, E> entry : map.entrySet()) {
        if (Objects.equals(value, entry.getValue())) {
            return entry.getKey();
        }
    }
    return null;
}

Java 8 Stream Approach:

For Java 8 and above users, stream operations can provide a concise way to retrieve keys.

public static <T, E> Set<T> getKeysByValue(Map<T, E> map, E value) {
    return map.entrySet()
              .stream()
              .filter(entry -> Objects.equals(entry.getValue(), value))
              .map(Map.Entry::getKey)
              .collect(Collectors.toSet());
}

Guava's BiMap:

Alternatively, for users of the Guava library, the BiMap data structure can be particularly useful. It provides a bi-directional mapping between keys and values, allowing for efficient retrieval of both keys and values.

BiMap<Token, Character> tokenToChar = 
    ImmutableBiMap.of(Token.LEFT_BRACKET, '[', Token.LEFT_PARENTHESIS, '(');
Token token = tokenToChar.inverse().get('(');
Character c = tokenToChar.get(token);

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