An useful util to handle nested collection in Java

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import java.util.HashMap; import java.util.HashSet; import java.util.Map; import java.util.Set;   public class CommonUtil {       public static <K, V, O extends V> V getAndCreateIfNotExist(K key, Map<K, V> map, Class<O> c) {         V result = map.get(key);         if (result == null) {             synchronized (map) {                 result = map.get(key);                 if (result == null) {                     try {                         result = c.newInstance();                     } catch (Exception e) {                         throw new RuntimeException("Error in getAndCreateIfNotExist", e);                     }                       map.put(key, result);                 }             }         }         return result;     }     // Test     public static void main(String[] args) {         Map<String, Set<String>> phoneNumbersPerName = new HashMap<>();         String personName = "Alex";         Set<String> phoneNumbers = getAndCreateIfNotExist(personName, phoneNumbersPerName, HashSet.class);         phoneNumbers.add("01234567891");         phoneNumbers.add("09090909090");         System.out.println(phoneNumbersPerName);         // {Alex=[09090909090, 01234567891]}     } }