How to turn this string to double without error i want to get double number

Question

Here is the code bellow is where is the error thrown here is the error: java.lang.Integer cannot be cast to java.lang.Double

package swing3;

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

public class rob {
public static void main(String[] args) {
    ScriptEngineManager mgr = new ScriptEngineManager();
     ScriptEngine engine = mgr.getEngineByName("JavaScript");

Here i got an error:

     String a = "3+5";
     try {
        double b = (Double)engine.eval(a);
        System.out.println(b);
    } catch (ScriptException e) {
        e.printStackTrace();
    }

}

  }