How to set enum DTO object using enum VO object?

How to convert one object to another? For example, I have 3 classes Car (equals to CarVO), CarDTO and CarFactory. The issue in that I can not set an object of CarDTO by an object of Car. Should I convert it to string or what? I would appreciate any help!

public class Car {
    private Long id;
    private String model;
    private Engine engine;
    private Integer manufacturedYear;

    private enum Engine {Petrol, Diesel, Gas}

    // Constructor, setters & getters

}

public class CarDTO {
    private Long id;
    private String model;
    private Engine engine;
    private Integer manufacturedYear;

    private enum Engine {Pertol, Diesel, Gas}

    // Constructor, setters & getters

}

public enum CarFactory {
    INSTANCE;

    CarFactory() {}

    public static CarFactory getInstance() {return INSTANCE;}

    public CarDTO createCarDTO(Car car) {
        CarDTO carDTO = new CarDTO();
        carDTO.setId(car.getId());
        carDTO.setModel(car.getModel());
        carDTO.setEngine(car.getEngine()); // Here is an issue 
        carDTO.setManufacturedYear(car.getManufacturedYear());
        return carDTO;
    }
}

You can just do the mapping between the different Engine enums in the factory method:

public CarDTO createCarDTO(Car car) {
    CarDTO carDTO = new CarDTO();
    carDTO.setId(car.getId());
    carDTO.setModel(car.getModel());
    switch (car.getEngine()) {
        case Diesel:
            carDTO.setEngine(CarDTO.Engine.Diesel);
        case Gas:
            carDTO.setEngine(CarDTO.Engine.Gas);
        case Petrol:
            carDTO.setEngine(CarDTO.Engine.Pertol);
    }
    carDTO.setManufacturedYear(car.getManufacturedYear());
    return carDTO;
}

Note that I had to change the visibility of the two Engine enums to package scope to do this.

A cleaner, more reusable way would be to write a separate method that converted the one Engine type to the other, and then use that in the factory method.

...and sure, you can certainly go the way of the other answers of bringing in a mapper library or something else that is more robust. I just wanted to show you that what you were trying to do directly was doable.

CarDTO and Car look like data transfer object and model, e.g. with Hybernate annotations. These classes lools mostly similar but cannot be uses one for the other.
To convert one to another you have several options:

• Create your own converters with copy all fields from one class to another
• MapStruct is your friend! - use MapStruct that does the same but with much less code.