Objectives

• Exploring Inheritance and Overriding Methods
• Be able to design inheritance class relationships
• Be able to use an abstract class
• Be able to override a virtual methods and implement an abstract method

Activities:

File Dog contains a declaration for a Dog class. Save this file to your directory and study itnotice what instance variables and methods are provided. Files Labrador and Yorkshirea contain declarations for classes that extend Dog. Save and study these files as well.

File DogTest contains a simple driver program that creates a dog and makes it speak. Study DogTest, save it to your directory, and compile and run it to see what it does. Now modify these files as follows:

1. Add statements in DogTest after you create and print the dog to create and print a Yorkshire and a Labrador. Note that the Labrador constructor takes two parameters: the name and color of the labrador, both strings. Don't change any files besides DogTest. Now recompile DogTest; you should get an error saying something like

./Labrador 18: Dog(string) in Dog cannot be applied to ()

{

^

1 error

If you look at line 18 of Labrador it's just a {, and the constructor the compiler can't find (Dog()) isn't called anywhere in this file.

a. What's going on? (Hint: What call must be made in the constructor of a subclass?)

=>

b. Fix the problem (which really is in Labrador) so that DogTest creates and makes the Dog, Labrador, and Yorkshire all speak.

2. Add code to DogTest to print the average breed weight for both your Labrador and your Yorkshire. Use the avgBreedWeight() method for both. What error do you get? Why?

=>

Fix the problem by adding the needed code to the Yorkshire class.

3. Add an abstract int avgBreedWeight() method to the Dog class. Remember that this means that the word abstract appears in the method header after public, and that the method does not have a body (just a semicolon after the parameter list). It makes sense for this to be abstract, since Dog has no idea what breed it is. Now any subclass of Dog must have an avgBreedWeight method; since both Yorkshire and Laborador do, you should be all set.

Save these changes and recompile DogTest. You should get an error in Dog (unless you made more changes than described above). Figure out what's wrong and fix this error, then recompile DogTest. You should get another error, this time in DogTest. Read the error message carefully; it tells you exactly what the problem is. Fix this by changing DogTest (which will mean taking some things out).

// ****************************************************************

// Dog

//

// A class that holds a dog's name and can make it speak.

//

// ****************************************************************

public class Dog

{

protected internal string name;

// ------------------------------------------------------------

// Constructor -- store name

// ------------------------------------------------------------

public Dog(string name)

{

this.name = name;

}

// ------------------------------------------------------------

// Returns the dog's name

// ------------------------------------------------------------

public string getName

{

return name;

}

// ------------------------------------------------------------

// Returns a string with the dog's comments

// ------------------------------------------------------------

public virtual string speak()

{

return "Woof";

}

}

// ****************************************************************

// Labrador

//

// A class derived from Dog that holds information about

// a labrador retriever. Overrides Dog speak method and includes

// information about avg weight for this breed.

//

// ****************************************************************

public class Labrador : Dog

{

private string color; //black, yellow, or chocolate?

private static int breedWeight = 75;

public Labrador(string name, string color)

{

this.color = color;

}

// ------------------------------------------------------------

// Big bark -- overrides speak method in Dog

// ------------------------------------------------------------

public override string speak()

{

return "WOOF";

}

// ------------------------------------------------------------

// Returns weight

// ------------------------------------------------------------

public static int avgBreedWeight()

{

return breedWeight;

}

}

// ****************************************************************

// Yorkshire

//

// A class derived from Dog that holds information about

// a Yorkshire terrier. Overrides Dog speak method.

//

// ****************************************************************

public class Yorkshire : Dog

{

public Yorkshire(string name) : base(name)

{

}

// ------------------------------------------------------------

// Small bark -- overrides speak method in Dog

// ------------------------------------------------------------

public override string speak()

{

return "woof";

}

}

using System;

// ****************************************************************

// DogTest

//

// A simple test class that creates a Dog and makes it speak.

//

// ****************************************************************

public class DogTest

{

public static void Main(string[] args)

{

Dog dog = new Dog("Spike");

Console.WriteLine(dog.Name + " says " + dog.speak());

}

}