Dynamic Method Dispatch

[rahul sengupta]

Can any of you please help me out with dynamic method dispatch. This is not very clear to me.
If someone can please elaborate on this it will be of great help.

[Campbell Ritchie]

Welcome to the Ranch

If you search the Ranch, you will find a few references, where people seem to use it similarly to polymorphism. There is something in the Java™ Language Specification, but that is by no means easy to read.

Please explain what people have told you about dynamic method dispatch, and what you think it means. Then we shall know how to answer your question.

[rahul sengupta]

First of all thanks a lot.

What I have read is that here run time polymorphism is applied. As it is during the run time that it is decided which method will get overridden and get used.

From another source what I have read is that in normal cases we can use members of the base class through subclass. But in case of dynamic method dispatch we are able to access members of subclass from base class. Is it really the case? I am a bit confused with this explanation.

Thank You in advance

Rahul

[Campbell Ritchie]

That sounds like a good description of polymorphism. I would remind you that polymorphism and overriding only apply to instance methods, not to static members or instance fields.

public abstract class Vehicle
{
   private final String make;
   private final String registrationNumber;
   private final String colour;

   public Vehicle(String make, String registrationNumber, String colour)
   {
      . . .
   }

   public abstract void pickUpPassenger(int howMany);
 . . .
}

public class Car extends Vehicle
{
 . . .
   public void pickUpPassenger(int howMany)
   { 
       System.out.println("You're going to the station? Hop aboard, I'm going into town and it's not out of my way at all.");
      . . .
   }
}

public class Bus extends Vehicle
{
 . . .
   public void pickUpPassenger(int howMany)
   { 
       System.out.println("You're going to the station? Well, we go to the town hall, that costs 75p and it's not far to walk the rest of the way.");
      . . .
   }
}

public class Taxi extends Vehicle
{
 . . .
   public void pickUpPassenger(int howMany)
   { 
       System.out.println("You're going to the station? Of course we can take you. It costs £3.40 plus 25p for your suitcase.");
      . . .
   }
}

Vehicle car = new Car("Ford", "NU09 PZW", "green");
Vehicle taxi = new Taxi("Skoda", "NV04 TXI", "yellow and black");
Vehicle bus = new Bus("Volvo", "NC03 ARR", "Turquoise and cream");
bus,pickUpPassenger(97);
taxi.pickUpPassenger(2);
car.pickUpPassenger(1);

Complete those classes, check whether I have spelt the methods the same throughout, and execute the last code snippet. Then you can see how it works.

[Javin Paul]

Just to add on Ritchie's reply , polymorphism will not allow you to override either static, private or final method.

[Darryl Burke]

Javin Paul wrote:

Just to add on Ritchie's reply , polymorphism will not allow you to override either static, private or final method.

Static methods can't be overridden at all.
Private methods aren't inherited so can't be overridden at all.
Final methods can't be overridden at all.

So what's your point re polymorphism?