Overview of Inheritance in Object-Oriented Programming

1. CIS162AD Inheritance Part 3 09_inheritance.ppt

2. Overview of Topics • Inheritance • Virtual Methods used for Overriding • Abstract Classes and Methods • Interface Classes • Delegates and Events • Constructors & Inheritance

3. 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.

4. 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.

5. Base Class Parent Class Super Class Derived Class Child Class Sub Class Inheritance Terminology 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.

6. 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.

7. 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.).

8. 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); } }

9. 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.

10. 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.

11. 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;}

12. Overriding calcExtendedPrice • In the derived class, clsOrderPreferred we override the method as follows:public override void calcExtendedPrice ( ){ cdecExtendedPrice = cintQuantity * (cdecPrice - cdecPrice * cdecDISCOUNT_RATE);}

13. 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.

14. Abstract Methods • A virtual method in the base class has some actual code that can be executed. • An abstract method in the base class does NOT have any code – empty method. • Classes with abstract methods are designed to inherited. • Abstract methods must be overridden and implemented in the derived class. • Classes with abstract methods can not be used to instantiate an object.

15. Interface Classes • A interface class is similar to an abstract class. • Use keyword interface to declare class:interface IEmployee { … } • All methods in an interface are abstract methods. • When an interface is inherited, it is referred to implementing because all members must be implemented in the derived class. • Multiple interfaces can be inherited: public class HourlyEmp : IEmployee, IUser { … }

16. Delegates and Events • Coding delegates and events is an advanced topic, so the terms are just introduce here. • A delegate can be used to specify the signature of a method that can handle an event. • Signature is the number of and the data types of the parameters; not the name or return type. • An event is a signal that an action has occurred on an object. • An event declaration specifies a delegate that will handle the event. • Raise the event and pass the arguments required by the delegate.

17. 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.

18. Call Base Constructors public class clsOrderPreferred : clsOrder {//The descr, qty, price and all the Get and Put methods are inherited//Only add the new properties and/or methodspublic 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. } }

19. Using Inherited Classes private clsOrder cobjOrder;private clsOrderPreferred cobjOrderPreferred;if (preferredCheckBox.Checked){ cobjOrderPreferred = new clsOrderPreferred; cobjOrderPreferred.calcExtendedPrice( ); cobjOrderPreferred.accumulateTotals( );}else{ cobjOrder = new clsOrder; cobjOrder.calcExtendedPrice( ); cobjOrder.accumulateTotals( );}

20. 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.

21. Summary • Inheritance • Virtual Methods used for Overriding • Abstract Classes and Methods • Interface Classes • Delegates and Events • Constructors & Inheritance