Question

I was testing out covariant return types and came across this problem.

class Vehicle {

    int i = 3;
}
class Car extends Vehicle{

    int i = 5;

    public Car returningCar(){
        System.out.println("Returning Car");
        return new Car();
    }

    public Vehicle returningCarInVehicle(){
        System.out.println("Returning CarInVehicle");
        return new Car();
    }
}

public class ScjpTest{

    public static void main(String[] args){

        Car car = new Car();
        Vehicle vehicleCar = car.returningCar();
        Vehicle vehicleCar2 = car.returningCarInVehicle();

        System.out.println("vehicleCar " + vehicleCar.i);
        System.out.println("vehicleCar2 " + vehicleCar2.i);

    }
}

The output to the above is Returning Car

   Returning 
   CarInVehicle
   vehicleCar 3
   vehicleCar2 3

I dont understand why the output is 3. I was expecting the output to be 5 in both instances because at runtime the JVM uses the actual object not the reference.

Thanks

Answer 1

Fields aren’t virtual/overrideable/etc. They will be resolved according to the compile-time type of the reference, which in this case is Vehicle.

This code would print “vehicleCar2 5”:

System.out.println("vehicleCar2 " + ((Car)vehicleCar2).i);

since the cast makes the expression of compile-time type Car.