import java.io.*;
import javax.script.*;

public class Jtest{

    public static void main(String[] args){
        ScriptEngineManager factory = new ScriptEngineManager();
        ScriptEngine js = factory.getEngineByName("JavaScript");
        Bindings b = new SimpleBindings();
        try {
            js.eval("function f(){}",b);
            js.eval("");
        } catch (ScriptException e){
            System.out.println(e);
        }
        System.out.printf("visible names: %s\n",b.entrySet());

        // Bindings b contains here the list of the (predefined) names that are visible when
        // a new one is defined using eval(). This allows this a program, when it defines
        // a new function to test in advance that it does not redefine a predefined one, which
        // can easily be done getting the functions of Bindings b and saving them in a table,
        // and then testing that the new function name being defined is not one of them. 

        Class<?> funClass;
        funClass = (Class<?>)b.get("print").getClass();
        System.out.printf("function %s\n",funClass);

        // build here a table of predefined functions

        // suppose here the program wants to define a new function "print", but does not
        // want to redefine a predefined one. It tests if the name "print" is a function.

        Object obj = js.get("print");
        if (obj.getClass() == funClass){
            System.out.printf("print is a function\n");
            // check that it is not a predefined one by testing if this object is in
            // the table of predefined functions
        }

    }

/*
output rhino:

visible names: [f=sun.org.mozilla.javascript.internal.InterpretedFunction@1eef0a8, context=javax.script.SimpleScriptContext@1f9e529, print=sun.org.mozilla.javascript.internal.InterpretedFunction@133e8c9, println=sun.org.mozilla.javascript.internal.InterpretedFunction@f7e9dd]
function class sun.org.mozilla.javascript.internal.InterpretedFunction
print is a function

output nashorn:

visible names: [nashorn.global=[object global]]
Exception in thread "main" java.lang.NullPointerException
        at Jtest.main(Jtest.java:25)

*/

}