DOUBT on DOWNCASTING

[debasmita pattnayak]

Hi,
i am really confused with the concept of downcasting.can anyone explain me in context to the following question:

class Vehicle {}
class Car extends Vehicle {}

class VarArgSeven {
public static void washVehicle(Vehicle ... v) {
for(Vehicle vv : v) {
Car c = (Car)vv;
System.out.print(c);
}
}
public static void main(String[] args) {
//insert the line here
}
}

Options :

a) washVehicle(new Car[] { new Car()});
b) washVehicle(new Vehicle[]{new Car(), new Vehicle()});
c) washVehicle(new Car(), new Car());
d) washVehicle(new Vehicle(), new Vehicle());
e) none of the above.

the answer to this is a and c but why not b and d too.
please explain.
thanks

[Keith Lynn]

The reason is that even though Car extends Vehicle, if the runtime type of an object is Vehicle, you cannot refer to it as a Car.

[debasmita pattnayak]

hi keith,
thanks for the reply...
i want to know then when is downcasting needed and what is the legal way of doing that? can you please sustain the same with an example
thanks in advance!

[John Stone]

class Vehicle {}
class Car extends Vehicle { void startEngine() {} }
 
class VarArgSeven {
public static void main(String[] args) {
Vehicle v1 = new Car();
Vehicle v2 = new Vehicle();

//v1.startEngine(); // won't work, Vehicle doesn't have startEngine method
                    // I need to downcast, if I want to access startEngine

((Car)v1).startEngine(); // I know, that object stored 
                         // in Vehicle reference has Car type/class, so I can cast to Car
                         // and access its method startEngine

((Car)v2).startEngine(); // I'm saying here, that in reference v2 is Car
                         // compiler believes me, but in run-time I get exception, 
                         // because JVM finds, that v2 is not Car 
 
if (v2 instanceof Car) { // I ask, whether v2 is pointing to object of type/class Car, 
                     // so I can be sure about cast
((Car)v2).startEngine(); // this line is never reached
}
}
}

[debasmita pattnayak]

hi,
thanks a lot!
now its more clear to me than before.