Double-Checked Locking with a Map

Main.java

class Main {
  private final Map<Integer, Object> map = new ConcurrentHashMap<>();

  public Object getOnce(int idx) {
    Object obj = map.get(idx);
    if (obj == null) {
      synchronized(this) {
        obj = map.get(idx);
        if (obj == null) {
          obj = new Object();
          map.put(idx, obj);
        }
      }
    }
    return obj;
  }

}