why classcastException is not thrown at compile time when downcasted?

Question

Consider the example below. line 5 of main(commented) throws ClassCastException at Runtime. Line 4 is a valid cast because v1 has "knowledge" of car". In the same token, shouldn't line 5 give compile time error since it has "knowledge of v2" being a vehicle and not car and hence throw compile time error saying "hey I have no knowledge of car, I am a vehicle, you can't be cast to a car".

During compile time, Vehicle v1 = new Car(), a new Car is not created. but v1 has knowledge that it is a car correct?

class Vehicle {
}

class Bus extends Vehicle {
}

class Car extends Vehicle {
}

public class UpcastDownCast {

    public static void main(String[] args) {
        Vehicle v1 = new Car(); // line 1
        Vehicle v2 = new Vehicle();// line 2
        // compile time error. Type mis-match
        Car c0 = v1; // line 3
        // v1 has knowledge of Car due to line 1
        Car c1 = (Car) v1;//line 4
        // Runtime Exception. v2 has no knowledge of car
        Car c2 = (Car) v2;//line 5

    }
}

Answer 1

Java doesn't propagate information like that. As soon as you say Vehicle v2 = new Vehicle(), Java forgets any knowledge of what v2 is other than "it's Vehicle or some subtype of Vehicle.