 2006 Pearson Education, Inc. All rights reserved. 1 11 Polymorphism, Interfaces & Operator Overloading.

 2006 Pearson Education, Inc. All rights reserved. 2 One Ring to rule them all, One Ring to find them, One Ring to bring them all and in the darkness bind them. — John Ronald Reuel Tolkien General propositions do not decide concrete cases. — Oliver Wendell Holmes

 2006 Pearson Education, Inc. All rights reserved. 3 Why art thou cast down, O my soul? — Psalms 42:5 A philosopher of imposing stature doesn’t think in a vacuum. Even his most abstract ideas are, to some extent, conditioned by what is or is not known in the time when he lives. — Alfred North Whitehead

 2006 Pearson Education, Inc. All rights reserved. 4 OBJECTIVES In this chapter you will learn:  The concept of polymorphism and how it enables you to "program in the general."  To use overridden methods to effect polymorphism.  To distinguish between abstract and concrete classes.  To declare abstract methods to create abstract classes.  How polymorphism makes systems extensible and maintainable.  To determine an object's type at execution time.  To create sealed methods and classes.  To declare and implement interfaces.  To overload operators to enable them to manipulate objects.

 2006 Pearson Education, Inc. All rights reserved. 5 11.1 Introduction 11.2 Polymorphism Examples 11.3 Demonstrating Polymorphic Behavior 11.4 Abstract Classes and Methods 11.5 Case Study: Payroll System Using Polymorphism 11.5.1 Creating Abstract Base Class Employee 11.5.2 Creating Concrete Derived Class SalariedEmployee 11.5.3 Creating Concrete Derived Class HourlyEmployee 11.5.4 Creating Concrete Derived Class CommissionEmployee 11.5.5 Creating Indirect Concrete Derived Class BasePlusCommissionEmployee 11.5.6 Polymorphic Processing, Operator is and Downcasting 11.5.7 Summary of the Allowed Assignments Between Base Class and Derived Class Variables

 2006 Pearson Education, Inc. All rights reserved. 6 11.6 sealed Methods and Classes 11.7 Case Study: Creating and Using Interfaces 11.7.1 Developing an IPayable Hierarchy 11.7.2 Declaring Interface IPayable 11.7.3 Creating Class Invoice 11.7.4 Modifying Class Employee to Implement Interface IPayable 11.7.5 Modifying Class SalariedEmployee for Use in the IPayable Hierarchy 11.7.6 Using Interface IPayable to Process Invoice s and Employee s Polymorphically 11.7.7 Common Interfaces of the.NET Framework Class Library 11.8 Operator Overloading 11.9 (Optional) Software Engineering Case Study: Incorporating Inheritance and Polymorphism into the ATM System 11.10 Wrap-Up

 2006 Pearson Education, Inc. All rights reserved. 7 11.1 Introduction Polymorphism – Enables “programming in the general” – The same invocation can produce “many forms” of results Interfaces – Implemented by classes to assign common functionality to possibly unrelated classes

 2006 Pearson Education, Inc. All rights reserved. 8 11.2 Polymorphism Examples Polymorphism – When a program invokes a method through a base class variable, the correct derived class version of the method is called Based on the type of the reference stored in the base class variable – The same method name and signature can cause different actions to occur Depends on the type of object on which the method is invoked – Facilitates adding new classes to a system with minimal modifications to the system’s code

 2006 Pearson Education, Inc. All rights reserved. 9 Software Engineering Observation 11.1 Polymorphism promotes extensibility: Software that invokes polymorphic behavior is independent of the object types to which messages are sent. New object types that can respond to existing method calls can be incorporated into a system without requiring modification of the base system. Only client code that instantiates new objects must be modified to accommodate new types.

 2006 Pearson Education, Inc. All rights reserved. 10 11.3 Demonstrating Polymorphic Behavior Downcasting – A base class reference can be aimed at a derived class object This is possible because a derived class object is a base class object as well When invoking a method from that reference, the type of the actual referenced object, not the type of the reference, determines which method is called Cannot treat a base class object as a derived class object

 2006 Pearson Education, Inc. All rights reserved. 12 Outline PolymorphismTest.cs (2 of 3) Assign a reference to a BasePlusCommissionEmployee4 object to a CommissionEmployee3 variable Polymorphically call basePlusCommissionEmployee ’s ToString method

 2006 Pearson Education, Inc. All rights reserved. 14 11.4 Abstract Classes and Methods Abstract classes – Classes that are too general to create real objects – Used only as abstract base classes for concrete derived classes and to declare reference variables – Many inheritance hierarchies have abstract base classes occupying the top few levels – Constructors and static methods cannot be abstract – Keyword abstract Use to declare a class abstract Also use to declare a method abstract – Abstract classes normally contain one or more abstract methods – All concrete derived classes must override all inherited abstract methods

 2006 Pearson Education, Inc. All rights reserved. 15 11.4 Abstract Classes and Methods (Cont.) Iterator class – Traverses all the objects in a collection, such as an array – Often used in polymorphic programming to traverse a collection that contains references to objects from various levels of a hierarchy

 2006 Pearson Education, Inc. All rights reserved. 16 Software Engineering Observation 11.2 An abstract class declares common attributes and behaviors of the various classes that inherit from it, either directly or indirectly, in a class hierarchy. An abstract class typically contains one or more abstract methods or properties that concrete derived classes must override. The instance variables, concrete methods and concrete properties of an abstract class are subject to the normal rules of inheritance.

 2006 Pearson Education, Inc. All rights reserved. 18 Common Programming Error 11.2 Failure to implement a base class’s abstract methods and properties in a derived class is a compilation error unless the derived class is also declared abstract.

 2006 Pearson Education, Inc. All rights reserved. 19 Software Engineering Observation 11.3 A derived class can inherit “interface” or “implementation” from a base class. Hierarchies designed for implementation inheritance tend to have their functionality high in the hierarchy—each new derived class inherits one or more methods that were implemented in a base class, and the derived class uses the base class implementations. Hierarchies designed for interface inheritance tend to have their functionality lower in the hierarchy—a base class specifies one or more abstract methods that must be declared for each concrete class in the hierarchy, and the individual derived classes override these methods to provide derived-class-specific implementations.

 2006 Pearson Education, Inc. All rights reserved. 20 11.5.1 Creating Abstract Base Class Employee abstract base class Employee – Earnings() is declared abstract No implementation can be given for Earnings() in the Employee abstract class – An array of Employee variables will store references to derived class objects Earnings method calls from these variables will call the appropriate version of the Earnings method

 2006 Pearson Education, Inc. All rights reserved. 25 11.5.2 Creating Concrete Derived Class SalariedEmployee SalariedEmployee – Concrete class Inherits from Abstract Employee class Override Earnings – weeklySalary

 2006 Pearson Education, Inc. All rights reserved. 26 Outline SalariedEmployee.cs (1 of 2) Class SalariedEmployee extends class Employee Call base class constructor Validate and set weekly salary value

 2006 Pearson Education, Inc. All rights reserved. 27 Outline SalariedEmployee.cs (2 of 2) Override Earnings method so SalariedEmployee can be concrete Override ToString method Call base class’s version of ToString

 2006 Pearson Education, Inc. All rights reserved. 28 11.5.3 Creating Concrete Derived Class HourlyEmployee HourlyEmployee – Concrete class Inherits from Abstrict Employee class Overrides Earnings – wage and hours

 2006 Pearson Education, Inc. All rights reserved. 32 11.5.4 Creating Concrete Derived Class CommisionEmployee CommisionEmployee – Concrete class Inherits from Abstract Employee class Override Earnings – grossSales and commissionRate

 2006 Pearson Education, Inc. All rights reserved. 33 Outline CommissionEmployee.cs (1 of 2) Class CommissionEmployee extends class Employee Call base class constructor Validate and set commission rate value

 2006 Pearson Education, Inc. All rights reserved. 34 Outline CommissionEmployee.cs (2 of 2) Validate and set the gross sales value Override Earnings method so CommissionEmployee can be concrete Override ToString method Call base class’s ToString method

 2006 Pearson Education, Inc. All rights reserved. 35 11.5.5 Creating Indirect Concrete Derived Class BasePlusCommisionEmployee BasePlusCommisionEmployee – Inherits from class CommissionEmployee Therefore is an indirect derived class of class Employee

 2006 Pearson Education, Inc. All rights reserved. 36 Outline BasePlusCommission Employee.cs (1 of 2) Class BasePlusCommissionEmployee extends class CommissionEmployee Call base class constructor Validate and set base salary value

 2006 Pearson Education, Inc. All rights reserved. 37 Outline BasePlusCommission Employee.cs (2 of 2) Override Earnings method Call base class’s Earnings method Override ToString method Call base class’s ToString method

 2006 Pearson Education, Inc. All rights reserved. 38 11.5.6 Polymorphic Processing, Operator is and Downcasting Dynamic binding – Also known as late binding – Calls to overridden methods are resolved at execution time, based on the type of object referenced is operator – Determines if a particular object is of a certain type or a derived class of that type

 2006 Pearson Education, Inc. All rights reserved. 41 Outline PayrollSystemTest. cs (3 of 5) If the currentEmployee variable points to a BasePlusCommissionEmployee object Downcast currentEmployee to a BasePlusCommissionEmployee reference Give BasePlusCommissionEmployee s a 10% base salary bonus Polymorphically call Earnings method Call GetType method to display each Employee derived class object’s class name

 2006 Pearson Education, Inc. All rights reserved. 45 Software Engineering Observation 11.4 If at execution time the reference to a derived class object has been assigned to a variable of one of its direct or indirect base classes, it is acceptable to cast the reference stored in that base class variable back to a reference of the derived class type. Before performing such a cast, use the is operator to ensure that the object is indeed an object of an appropriate derived class type.

 2006 Pearson Education, Inc. All rights reserved. 46 Common Programming Error 11.4 When downcasting an object, an InvalidCastException (of namespace System ) occurs if at execution time the object does not have an is-a relationship with the type specified in the cast operator. An object can be cast only to its own type or to the type of one of its base classes.

 2006 Pearson Education, Inc. All rights reserved. 47 11.5.6 Polymorphic Processing, Operator is and Downcasting (Cont.) GetType method – Return an object of class Type – Contains information on: Object’s type Class name public methods Name of its base class Downcasting – Convert a reference to a base class to a reference to a derived class – Allowed only if the object has an is-a relationship with the derived class

 2006 Pearson Education, Inc. All rights reserved. 48 11.5.7 Summary of the Allowed Assignments Between Base Class and Derived Class Variables Base class and derived class assignment rules – Assigning a base class reference to a base class variable is straightforward – Assigning a derived class reference to a derived variable is straightforward – Assigning a derived class reference to a base class variable is safe because of the is-a relationship Referring to derived class-only members through base class variables is a compilation error – Assigning a base class reference to a derived class variable is a compilation error Downcasting can get around this error The is operator can be used to ensure cast is performed only is the object is a derived class object

 2006 Pearson Education, Inc. All rights reserved. 49 11.6 sealed Methods and Classes sealed methods – Cannot be overridden in a derived class – private and static methods are implicitly sealed – sealed methods are resolved at compile time, this is known as static binding Compilers can optimize by inlining the code sealed classes – Cannot be extended by a derived class – All methods in a sealed class are implicitly sealed

 2006 Pearson Education, Inc. All rights reserved. 50 Performance Tip 11.1 The compiler can decide to inline a sealed method call and will do so for small, simple sealed methods. Inlining does not violate encapsulation or information hiding, but does improve performance because it eliminates the overhead of making a method call.

 2006 Pearson Education, Inc. All rights reserved. 52 Software Engineering Observation 11.5 In the FCL, the vast majority of classes are not declared sealed. This enables inheritance and polymorphism—the fundamental capabilities of object-oriented programming.

 2006 Pearson Education, Inc. All rights reserved. 53 11.7 Case Study: Creating and Using Interfaces Interfaces – Keyword interface – Contains only constants and abstract methods All fields are implicitly public, static and sealed All methods are implicitly public abstract methods – Classes can implement interfaces Use the colon (“:”) operator The class must declare each method in the interface using the same signature or the class must be declared abstract – Typically used when disparate classes need to share common methods and constants – Normally declared in their own files with the same names as the interfaces and with the.cs file-name extension

 2006 Pearson Education, Inc. All rights reserved. 54 Common Programming Error 11.6 It is a compilation error to declare an interface member public or abstract explicitly, because they are redundant in interface member declarations. It is also a compilation error to specify any implementation details, such as concrete method declarations, in an interface.

 2006 Pearson Education, Inc. All rights reserved. 55 Common Programming Error 11.7 Failing to declare any member of an interface in a class that implements the interface results in a compilation error.

 2006 Pearson Education, Inc. All rights reserved. 56 11.7.1 Developing a IPayable Hierarchy IPayable interface – Contains method GetPaymentAmount – Is implemented by the Invoice and Employee classes UML representation of interfaces – Interfaces are distinguished from classes by placing the word “interface” in guillemets (« and ») above the interface name – The relationship between a class and an interface is known as realization A class “realizes” the method of an interface

 2006 Pearson Education, Inc. All rights reserved. 57 Good Programming Practice 11.1 By convention, the name of an interface begins with " I ". This helps distinguish interfaces from classes, improving code readability.

 2006 Pearson Education, Inc. All rights reserved. 58 Good Programming Practice 11.2 When declaring a method in an interface, choose a method name that describes the method’s purpose in a general manner, because the method may be implemented by a broad range of unrelated classes.

 2006 Pearson Education, Inc. All rights reserved. 61 11.7.3 Creating Class Invoice A class can implement as many interfaces as it needs – Use a comma-separated list of interface names after keyword implements Example: public class ClassName : BaseClassName, FirstInterface, SecondInterface, …

 2006 Pearson Education, Inc. All rights reserved. 69 11.7.5 Modifying Class SalariedEmployee for Use in the IPayable Hierarchy Objects of any derived classes of the class that implements the interface can also be thought of as objects of the interface – A reference to a derived class object can be assigned to an interface variable if the base class implements that interface

 2006 Pearson Education, Inc. All rights reserved. 72 Software Engineering Observation 11.6 Inheritance and interfaces are similar in their implementation of the is-a relationship. An object of a class that implements an interface may be thought of as an object of that interface type. An object of any derived classes of a class that implements an interface also can be thought of as an object of the interface type.

 2006 Pearson Education, Inc. All rights reserved. 73 Software Engineering Observation 11.7 The is-a relationship that exists between base classes and derived classes, and between interfaces and the classes that implement them, holds when passing an object to a method. When a method parameter receives a variable of a base class or interface type, the method polymorphically processes the object received as an argument.

 2006 Pearson Education, Inc. All rights reserved. 74 11.7.6 Using Interface IPayable to Process Invoices and Employees Polymorphically Module PayableInterfaceTest – Array of type IPayable Able to hold any objects that implements IPayable

 2006 Pearson Education, Inc. All rights reserved. 75 Software Engineering Observation 11.8 All methods of class object can be called by using a reference of an interface type—the reference refers to an object, and all objects inherit the methods of class object.

 2006 Pearson Education, Inc. All rights reserved. 76 Outline PayableInterfaceTe st.cs (1 of 2) Declare array of IPayable variables Assigning references to Invoice objects to IPayable variables Assigning references to SalariedEmployee objects to IPayable variables

 2006 Pearson Education, Inc. All rights reserved. 78 11.7.7 Common Interfaces of the.NET Framework Class Library Interfaces in.NET FCL – These interfaces enable you to extend many important aspects of C#

 2006 Pearson Education, Inc. All rights reserved. 81 11.8 Operator Overloading Operator Overloading – Enable operators to work with class objects – C# allow overloading for most operators – Keyword operator followed by the operator Example: operator + – Must be public and static

 2006 Pearson Education, Inc. All rights reserved. 82 Software Engineering Observation 11.9 Use operator overloading when it makes an application clearer than accomplishing the same operations with explicit method calls.

 2006 Pearson Education, Inc. All rights reserved. 86 Software Engineering Observation 11.10 Overload operators to perform the same function or similar functions on class objects as the operators perform on objects of simple types. Avoid non-intuitive use of operators.

 2006 Pearson Education, Inc. All rights reserved. 87 Software Engineering Observation 11.11 At least one argument of an overloaded operator method must be a reference to an object of the class in which the operator is overloaded. This prevents programmers from changing how operators work on simple types.

 2006 Pearson Education, Inc. All rights reserved. 90 11.9 (Optional) Software Engineering Case Study: Incorporating Inheritance into the ATM System UML model for inheritance – The generalization relationship The base class is a generalization of the derived classes The derived classes are specializations of the base class Transaction base class – Contains the methods and fields BalanceInquiry, Withdrawal and Deposit have in common Execute method accountNumber field

 2006 Pearson Education, Inc. All rights reserved. 92 Fig. 11.20 | Class diagram modeling the generalization (i.e., inheritance) relationship between the base class Transaction and its derived classes BalanceInquiry, Withdrawal and Deposit.

 2006 Pearson Education, Inc. All rights reserved. 95 Software Engineering Observation 11.12 A complete class diagram shows all the associations among classes, and all the attributes and operations for each class. When the number of class attributes, operations and associations is substantial (as in Figs. 11.21 and 11.22), a good practice that promotes readability is to divide this information between two class diagrams—one focusing on associations and the other on attributes and operations. When examining classes modeled in this fashion, it is crucial to consider both class diagrams to get a complete picture of the classes. For example, one must refer to Fig. 11.21 to observe the inheritance relationship between Transaction and its derived classes; that relationship is omitted from Fig. 11.22.

 2006 Pearson Education, Inc. All rights reserved. 96 11.9 (Optional) Software Engineering Case Study: Incorporating Inheritance into the ATM System (Cont.) Incorporating inheritance into the ATM system design – If class A is a generalization of class B, then class B extends class A – If class A is an abstract class and class B is a derived class of class A, then class B must implement the abstract methods of class A if class B is to be a concrete class

 2006 Pearson Education, Inc. All rights reserved. 1 11 Polymorphism, Interfaces & Operator Overloading.

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 2006 Pearson Education, Inc. All rights reserved. 1 11 Polymorphism, Interfaces & Operator Overloading.

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