How Can I Retrieve Keys from Values in a Java HashMap?

Retrieving Keys from Values in Java HashMap

HashMap is a valuable Java data structure for storing key-value pairs. While retrieving values from keys is straightforward, retrieving keys from values can be more challenging. Here are several efficient methods to achieve this:

One-to-Many Mappings

In situations where multiple keys can map to a single value, you can iterate over the HashMap's entries and collect all matching keys:

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;
}

One-to-One Mappings

When each value maps to a unique key, you can 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

Java 8 provides a convenient method for filtering and collecting matching 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

For cases where the bi-directional mapping is required, consider using Guava's BiMap:

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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