Java ScriptEngine call function from Javascript object

import javax.script.Invocable;
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;

public class Main {
  public static void main(String[] args) {
    // Get the Nashorn engine
    ScriptEngineManager manager = new ScriptEngineManager();
    ScriptEngine engine = manager.getEngineByName("JavaScript");
    // Make sure the script engine implements the Invocable interface
    if (!(engine instanceof Invocable)) {
      System.out.println("Invoking methods is not supported.");
      return;/*from  w ww.j  a  v a  2s . c  om*/
    }

    // Cast the engine reference to the Invocable type
    Invocable inv = (Invocable) engine;

    try {
      // Declare a global object with an add() method
      String script = "var calculator = new Object();" + "calculator.add = function add(n1, n2){return n1 + n2;}";

      // Evaluate the script first
      engine.eval(script);

      // Get the calculator object reference created in the script
      Object calculator = engine.get("calculator");

      // Invoke the add() method on the calculator object
      Object result = inv.invokeMethod(calculator, "add", 30, 40);
      System.out.println("Result = " + result);
    } catch (ScriptException | NoSuchMethodException e) {
      e.printStackTrace();
    }
  }
}