Java cant adding object in list without generic

Question

I have those classes.

public class Flight {
    public List<FlightOffer> flightOffers;
}

public class Hotel<T> {
    public List<HotelOffer> hotelOffers;
    public T address
}

public class FlightOffer extends BaseOffer{
}

public class HotelOffer extends BaseOffer{
}

public class BaseOffer{
    public String id;
}

flightOffers and hotelOffers returns a list.

Flight flightObject = new Flight();
flightObject.flightOffers.add(new BaseOffer()); // not working 
//(add (FlightOffer) List  cannot be applied to (BaseOffer))

Hotel hotelObject = new Hotel<String>();
hotelObject.hotelOffers.add(new BaseOffer()); // working

but if i convert Flight to Flight<T> its working, why?

Thomas · Accepted Answer · 2018-01-26 14:45:21Z

9

List<FlightOffer> flightOffers defines quite clearly what you can add to the list: instances of FlightOffer. flightOffers.add(new BaseOffer()); hence can't work since a BaseOffer is not a FlightOffer.

So why does hotelOffers.add(new BaseOffer()); work? Because you are disabling generic type checks by using the raw type Hotel in Hotel hotelObject = new Hotel<String>();.

You should get a warning about that as well which conveys another lesson: don't ignore the warnings. The compiler doesn't know that something will fail but more often than not it "guesses" correctly and warns you about that.

Edit:

Note what the JLS states on raw types (in the first box of section 4.8):

Another implication of the rules above is that a generic inner class of a raw type can itself only be used as a raw type.

That means that because Hotel is a raw type, every generic type inside it can also only be a raw type, i.e. hotelOffers is treated as if it were a raw List which allows any type of object (thus even this should work in your code hotelObject.hotelOffers.add("I'm not a hotel offer");)

edited Jan 26, 2018 at 14:45

answered Jan 26, 2018 at 14:38

Thomas

88.9k13 gold badges126 silver badges162 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

marstran Over a year ago

Why does it disable the generic type check for the hotelOffers list? The list doesn't use the type parameter of the Hotel object. It will always be List<HotelOffer>.

Murat Karagöz Over a year ago

Might want to add that by using Hotel<String> = .. it will use the type check.

marstran Over a year ago

Does the compiler transitively disable the generic type check on every member of the object when using the raw type?

Thomas Over a year ago

@marstran sorry, looked up the wrong section of the JLS. I'll add it later,

Murat Karagöz Over a year ago

Here is an explanation for that behavior from Angelika Langer.

Giancarlo Romeo · Accepted Answer · 2018-01-26 14:46:13Z

1

You're trying to add a BaseOffer object (superclass) in a List of FlightOffer (subclass) objects.

https://docs.oracle.com/javase/tutorial/java/IandI/subclasses.html

Flight flightObject = new Flight();
flightObject.flightOffers.add(new FlightOffer());    // that's correct

edited Jan 26, 2018 at 14:46

answered Jan 26, 2018 at 14:35

Giancarlo Romeo

6611 gold badge10 silver badges26 bronze badges

1 Comment

marstran Over a year ago

I think the weird part here is that the second example actually compiles (although with a warning).

Collectives™ on Stack Overflow

Java cant adding object in list without generic

2 Answers 2

5 Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

5 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related