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

While this does not directly answer the question, it is related.

This way you don't need to keep creating/iterating. Just create a reverse map once and get what you need.

/**
 * Both key and value types must define equals() and hashCode() for this to work.
 * This takes into account that all keys are unique but all values may not be.
 *
 * @param map
 * @param 
 * @param 
 * @return
 */
public static  Map> reverseMap(Map map) {
    if(map == null) return null;

    Map> reverseMap = new ArrayMap<>();

    for(Map.Entry entry : map.entrySet()) {
        appendValueToMapList(reverseMap, entry.getValue(), entry.getKey());
    }

    return reverseMap;
}


/**
 * Takes into account that the list may already have values.
 * 
 * @param map
 * @param key
 * @param value
 * @param 
 * @param 
 * @return
 */
public static  Map> appendValueToMapList(Map> map, K key, V value) {
    if(map == null || key == null || value == null) return map;

    List list = map.get(key);

    if(list == null) {
        List newList = new ArrayList<>();
        newList.add(value);
        map.put(key, newList);
    }
    else {
        list.add(value);
    }

    return map;
}