Why can't I use List as Iterable?

Question

Hi I have that error:

incompatibles types: List<Car> cannot be converted to Iterable<Iterator>

incompatibles types: List<Truck> cannot be converted to Iterable<Iterator>

The class Car extends the class Vehicle. The Truck also extends Vehicle. I have to create the Vehicle class iterable??

public static void print(Iterable<Vehicle> it){
    for(Vehicle v: it) System.out.println(v);
}

public static void main(String[] args) { 
    List<Car> lcotxe = new LinkedList<Car>();
    List<Truck> lcamio = new LinkedList<Truck>();

    print(lcotxe);//ERROR
    print(lcamio);//ERROR


}

You can use generics for that; see https://ideone.com/kTB71Y — , May 22 '16 at 17:52
see also http://stackoverflow.com/questions/933447/how-do-you-cast-a-list-of-supertypes-to-a-list-of-subtypes — , May 22 '16 at 17:53

score 1 · Answer 1 · answered May 22 '16 at 17:51

1

That doesn't compile because List<Car> is not a subtype of Iterable<Vehicle>.

It is, however, a subtype of Iterable<? extends Vehicle>. This is called covariance.

public static void print(Iterable<? extends Vehicle> it){
    for(Vehicle v: it) System.out.println(v);
}

You could also choose to make the method generic.

public static <A extends Vehicle> void print(Iterable<A> it){
    for(Vehicle v: it) System.out.println(v);
}

answered May 22 '16 at 17:51

Chris Martin

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this was new to me , thank you @chris-martin ! – Rahmat Waisi May 22 '16 at 18:03

Why can't I use List as Iterable?

1 Answers1