Java Hashmap: How to get key from value?

Question

If I have the value \"foo\", and a HashMap ftw for which ftw.containsValue(\"foo\") returns true, how can I

Answer 1

If your data structure has many-to-one mapping between keys and values you should iterate over entries and pick all suitable keys:

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

In case of one-to-one relationship, you can return the first matched key:

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

In Java 8:

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

Also, for Guava users, BiMap may be useful. For example:

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