Putting JsonValue in a JsonObject, I'm receiving a NullPointerException [duplicate]

I'm trying for the first time to enter some JsonValue in a JsonObject. The main focus is to create in an output file an array containing "x" number of json object. When I try to do this, I receive a NullPointerException. Someone could help me?

            JsonObject jObj = new JsonObject(); 
            JsonValue nome = "hey";
            JsonValue cognome = "hoy";
            jObj.put("nome", nome);
            jObj.put("cognome", cognome);


            FileOutputStream fileOutput = new FileOutputStream("C:\\Users\\francesco.campanile\\Desktop\\Eclipse workspace\\CreazionePropedeutiche\\src\\Prop1.json");  
            JsonWriter jWriter = Json.createWriter(fileOutput);
            jWriter.writeObject(jObj);
            jWriter.close();