How to Retrieve Keys from a HashMap Given Its Values?

Retrieving Keys from HashMaps Based on Values

Given a HashMap with values stored against keys, let's explore how to retrieve the key corresponding to a specific value.

Value-to-Multiple Keys:

If a key-value pair can map to multiple values, you'll need to iterate through the entries and filter those with the desired value:

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;
}

Value-to-Single Key:

If there's a one-to-one mapping between keys and values, you can directly return the first matching 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;
}

Using Java 8 Streams:

With Java 8 streams, you can filter and map entries more efficiently:

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 BiMap:

If you're using Guava, the BiMap collection provides a more convenient way to work with inverse mappings:

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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