Set operation

Set operation

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

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