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Inheritance

09_inheritance.ppt

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Overview of Topics

Inheritance Virtual Methods used for Overriding Constructors & Inheritance Abstract Classes and Methods Interface Classes Delegates and Events

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Class and Object

A class is a definition of a new data type. A class definition includes member variables

and methods. An object is a variable declared using a class as

the data type.

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Inheritance

Inheritance is the process by which a new class is created from an existing class, but the new class has additional member variables and/or methods.

This is a powerful feature of object-oriented languages.

This allows us to reuse code that has already been developed and tested.

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Inheritance Terminology

Base Class Parent Class Super Class

Derived Class

Child Class

Sub Class

A derived class would be able to do everything the base class can do, plus it would add some attributes or operations to increase its functionality.

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Inheritance Possibilities

When creating a new class, we can use– An existing class (form, button, etc.)

– A programmer defined class (clsOrder, clsCustomer)

When designing an application, we should recognize classes that lend themselves to inheritance.

When there are two or more classes with similar properties and methods, we should create a base class that contains the common items to reduce duplicate code.

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Data Inheritance In CS8 we created a class to process orders, clsOrder. In CS9 we want to create a new class,

clsOrderPreferred, to handle orders for preferred customers, which would include a discount in the calculation.

If we went through the design phase, we would see that the class would include many of the items and calculations already defined in clsOrder.

The best thing to do would be to inherit clsOrder, and then add or redefine properties and methods that are needed for a preferred order (Discount Rate, calcExtendedPrice with discount, etc.).

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clsOrderPreferred//Add only the variables and methods needed for a preferred

Order

public class clsOrderPreferred : clsOrder{

const decimal cdecDISCOUNT_RATE = 0.05M; //5%

public override void calcExtendedPrice( ){

cdecExtendedPrice = cintQuantity * (cdecPrice - cdecPrice * cdecDISCOUNT_RATE); }}

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Method Overloading - reviewed Method Overloading occurs when there are two methods with

the same name, but differ in the number or data types of the parameters.

MessageBox.Show(TextMessage);MessageBox.Show(TextMessage, TitleBar);MessageBox.Show(TextMessage, TitleBar, ButtonType);MessageBox.Show(TextMessage, TitleBar, ButtonsType, Icon);

C# determines which method is executed based on the arguments provided in the method call.

Overloading and Overriding are forms of Polymorphism. Polymorphism means the ability to take on many shapes or

forms.

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Method Overriding Method Overriding occurs when a method in the

derived class has the exact same name and parameters as a method in the base class – same signature.

C# determines which method is executed based on the class the object was created from.– If the object is created using the base class, then the base

method is called.– If the object is created using the derived class, then the

overriding method is called. We override methods because we want a standard

name for our methods across classes, such calcExtendedPrice instead of calcExtendedPrice1, calcExtendedPrice2, etc.

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Virtual calcExtendedPrice is Overridable

In clsOrder there is a calcExtendedPrice. If we wanted to create a clsOrderPreferred that would

calculated the extended amount with a discount, we can redefine calcExtendedPrice in the new class by Overriding it.

To allow the calcExtendedPrice method to be overridden, declare method as virtual n the base class, clsOrder:

public virtual void calcExtendedPrice ( ){

cdecExtendedPrice = cintQuantity * cdecPrice;}

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Overriding calcExtendedPrice

In the derived class, clsOrderPreferred we override the method as follows:

public override void calcExtendedPrice ( ){

cdecExtendedPrice = cintQuantity * (cdecPrice - cdecPrice * cdecDISCOUNT_RATE);

}

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Constructors & Inheritance Constructors are NOT inherited. Each derived class must have a constructor defined, and it

should call the constructor from the base class. The base constructor would be responsible to initialize

variables in the base class, so the derived class would only need its constructor to initialized variables defined in its class.

Do not duplicate code. Pass the parameter values to the constructor in the base class so

it can assign the parameter values to the class variables through the property methods.

Use the existing code in the base class.

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Call Base Constructorspublic class clsOrderPreferred : clsOrder{

//The descr, qty, price and all the Get and Put methods are inherited//Only add the new properties and/or methods

public clsOrderPreferred( ) : base( ){

//Call default constructor in base class}

public clsOrderPreferred (string descr, int qty, decimal price) : base (descr, qty, price)

{//Call overloaded constructor in base class//to assign parameter values to class variables. //Do not duplicate the code to assign values.

}}

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Using Inherited Classesif (preferredCheckBox.Checked){

clsOrderPreferred cobjOrder = new clsOrderPreferred;cobjOrder .calcExtendedPrice( );cobjOrder.accumulateTotals( );

}else{

clsOrder cobjOrder = new clsOrder;cobjOrder.calcExtendedPrice( );cobjOrder.accumulateTotals( );

}//each object calls a different calcExtendedPrice( )

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Inheritance and Form class In all of the programs we have written so far we have

seen the following statement:

public partial class CS8Form : Form

The form we create is based on a predefined Form class that includes such things as:– Properties: .Font, .AcceptButton, .CancelButton, – Methods: .Close( ), .Activate( ), .Hide( ).

We then add our own buttons, textboxes, etc.

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Abstract Classes

An abstract class can NOT be instantiated. It can only be used as a base class that other

classes can inherit. Use keyword abstract to declare class:

public abstract class Employee{ … }

Abstract classes may contain method implementations or abstract methods.

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Abstract Methods

A virtual method in the base class has some actual code that can be executed, so overriding is optional.

An abstract method in the base class does NOT have any code – empty method.

public abstract void printCheck( );

Abstract methods must be overridden and implemented in the derived class.

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Interface Classes A interface class is similar to an abstract class. Use keyword interface to declare class and

usually named with a capital I prefix:

public interface IEmployee{

decimal calcGross( ); //no implementation}

All methods in an interface are basically abstract methods – no implementation included.

Delegates and Events Coding delegates and events is an advanced topic,

so the terms are just introduce here. A delegate is used to specify the signature of a method

that can handle an event. EventHandler is a predefined delegate in System:

Public delegate void EventHandler(object sender, EventArgs e);

Next indicate the method that a delegate should call.New System.EventHandler(this.clickButton_Click);This code found in designer file.

The event sender controls the generation of events and calls the defined event handler.

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Summary

Inheritance Virtual Methods used for Overriding Constructors & Inheritance Abstract Classes and Methods Interface Classes Delegates and Events