Can someone explain the benefits of polymorphism?

Question

So I understand pretty much how it works, but I just can\'t grasp what makes it useful. You still have to define all the separate functions, you still have to create an instanc

Answer 1

Reuse, generalisation and extensibility.

I may have an abstract class hierarchy like this: Vehicle > Car. I can then simply derive from Car to implement concrete types SaloonCar, CoupeCar etc. I implement common code in the abstract base classes. I may have also built some other code that is coupled with Car. My SaloonCar and CoupeCar are both Cars so I can pass them to this client code without alteration.

Now consider that I may have an interface; IInternalCombustionEngine and a class coupled with with this, say Garage (contrived I know, stay with me). I can implement this interface on classes defined in separate class hierarchies. E.G.

public abstract class Vehicle {..}

public abstract class Bus : Vehicle, IPassengerVehicle, IHydrogenPowerSource, IElectricMotor {..}

public abstract class Car : Vehicle {..}

public class FordCortina : Car, IInternalCombustionEngine, IPassengerVehicle {..}

public class FormulaOneCar : Car, IInternalCombustionEngine {..}

public abstract class PowerTool {..}

public class ChainSaw : PowerTool, IInternalCombustionEngine {..}

public class DomesticDrill : PowerTool, IElectricMotor {..}

So, I can now state that an object instance of FordCortina is a Vehicle, it's a Car, it's an IInternalCombustionEngine (ok contrived again, but you get the point) and it's also a passenger vehicle. This is a powerful construct.