Chapter 12 OOP: Polymorphism, Interfaces and Operator Overloading

12.2 Polymorphism Examples

By example: Polymorphic Employee Inheritance Hierarchy Simple payroll application that polymorphically and dynamically calculates the weekly pay of several different types of employees using each employee’s Earnings method. Though the earnings of each type of employee are calculated in a specific way, polymorphism allows us to process the employees “in the general.” Two new classes — SalariedEmployee (for people paid a fixed weekly salary) and HourlyEmployee (for people paid an hourly salary and “time-and-a-half” for overtime).

By example: Polymorphic Employee Inheritance Hierarchy cont common set of functionality for all the classes in the updated hierarchy in an “abstract” class, Employee, from which classes SalariedEmployee, HourlyEmployee and CommissionEmployee inherit directly and class BasePlusCommissionEmployee inherits indirectly. => invoke each employee’s Earnings method off a base class Employee reference, the correct earnings calculation is performed due to C#’s polymorphic capabilities.

12.3 Demonstrating Polymorphic Behavior

Base class Derived class

Same

12.4 Abstract Classes and Methods

Determining the Type of an Object at Execution Time Occasionally, when performing polymorphic processing, we need to program “in the specific.” Employee case study demonstrates that an application can determine the type of an object at execution time and act on that object accordingly. In the case study, we use these capabilities to determine whether a particular employee object is a BasePlusCommissionEmployee. As the result employee’s base salary is increased by 10%.

12.5 Case Study: Payroll System Using Polymorphism

override/virtual method1_derived method2_derived method1_base public class Base {public class Base { public string method1() { return "method1_base"; } public virtual string method2() { return "method2_base"; } } public class Derived : Base public static void Main(string[] args) method1_derived Derived derived = new Derived(); Console.WriteLine(derived.method1()); Console.WriteLine(derived.method2()); Console.WriteLine(derived.method3()); Console.WriteLine(derived.method4()); public string method1() { return "method1_derived"; } //hiding public override string method2() { return "method2_derived"; } public string method3() { return base.method1(); } public string method4() { return base.method2(); } method1_derived method2_derived method1_base method2_base

sealed Methods When an instance method declaration includes a sealed modifier, that method is said to be a sealed method. If an instance method declaration includes the sealed modifier, it must also include the override modifier. Use of the sealed modifier prevents a derived class from further overriding the method. using System; class A { public virtual void F() { Console.WriteLine("A.F"); } public virtual void G() { Console.WriteLine("A.G"); class B: A sealed override public void F() { Console.WriteLine("B.F"); override public void G() { Console.WriteLine("B.G"); class C: B Console.WriteLine("C.G");

12.6 sealed Methods and Classes Sealed method in a base class cannot be overridden in a derived class. private methods implicitly sealed static methods implicitly sealed Class sealed cannot be a base class

12.7 Case Study: Creating and Using Interfaces

implements

12.8 Operator Overloading