GetKeysByValue.java

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 (entry.getValue().equals(value)) {
      keys.add(entry.getKey());
    }
  }
  return keys;
}