Object Oriented Concepts

Classes II

Object Oriented concepts

EncapsulationCompositionInheritancePolymorphism

Encapsulation• Classes normally hide the details of their implementation

from their clients. This is called information hiding.• The client cares about what functionality a class offers, not

about how that functionality is implemented. This concept is referred to as data abstraction.

• C# provides this object oriented concept with properties.

CompositionA class can have references to objects of other classes as

members. This is called composition and is sometimes referred to as a has-a relationship

One form of software reuse is compositionExample: Employee object using Date object for hire date

See composition.cs

InheritanceAnother form of software reuse is inheritanceInheritance allows a new class to absorb an existing class’s

members.A derived class normally adds its own fields and methods to

represent a more specialized group of objects.The is-a relationship represents inheritance.Examples:

Base class Derived classes

Student GraduateStudent, UndergraduateStudentShape Circle, Triangle, RectangleLoan CarLoan, HomeImprovementLoan, MortgageLoanEmployee Faculty, Staff, HourlyWorker, CommissionWorkerBankAccount CheckingAccount, SavingsAccount

Base and Derived Classes in C#Objects of all classes that extend a common base class can be

treated as objects of that base class.A base class’s private members are not directly accessible by

derived-class methods and properties.A base class’s protected members can be accessed by

members of both that base class and of its derived classes.A base class’s protected internal members can be accessed by

members of a base class, the derived classes and by any class in the same assembly.

If a class does not specify that it inherits from another class,it implicitly inherits from object.public class CommissionEmployee : object

Base and Derived Classes in C# The derived class can override the base-class method. To override a

base-class method, a derived class must declare a method with keyword override . public override string ToString()

A constructor initializer with keyword base invokes the base-class constructor. See example BasePlusCommissionEmployee

The virtual and abstract keywords are required in a base-class method so that the derived classes can override it. See example BasePlusCommissionEmployee

Using protected instance variables creates several potential problems. The derived-class object can set an inherited variable’s value directly

without validity checking. Derived-class methods would need to be written to depend on the base

class’s data implementation. You should be able to change the base-class implementation while still

providing the same services to the derived classes.

Base and Derived Classes in C#• The derived-class constructor, before performing its own

tasks, invokes its direct base class’s constructor.• This is done either explicitly or implicitly.

• When a base-class method is overridden in a derived class, if the derived-class version wants to call the base-class version, it uses base keyword.public override decimal Earnings()

{

return BaseSalary + base.Earnings();

} // end method Earnings

System.Object

Method DescriptionEquals This method compares two objects for equality

and returns true if they are equal and false otherwise.

Finalize Finalize is called by the garbage collector before it reclaims an object’s memory.

GetHashCode The hashcode value returned can be used by a hashtable to determine the location at which to insert the corresponding value.

GetType Returns an object of class Type that contains information about the object’s type.

MemberwiseClone This protected method makes a copy of the object on which it is called. Instance-variable values in one object are copied into another object of the same type. For reference types, only the references are copied.

ReferenceEquals This static method returns true if twoobjects are the same instance or if they are null references.

ToString Returns a string representation of an object. The default implementation returns the namespace and class name.

• All classes inherit directly or indirectly from the object class.• Figure below summarizes object’s methods:

PolymorphismPolymorphism enables you to write applications that

process objects that share the same base class in a class hierarchy as if they were all objects of the base class.

If class Rectangle is derived from class Quadrilateral, then a Rectangle is a more specific version of a Quadrilateral.

Any operation that can be performed on a Quadrilateral object can also be performed on a Rectangle object.

These operations also can be performed on other Quadrilaterals, such as Squares, Parallelograms and Trapezoids.

The polymorphism occurs when an application invokes a method through a base-class variable.

PolymorphismAn object of a derived class can be treated as an object of its

base class.When the compiler encounters a method call made through a

variable, it determines if the method can be called by checking the variable’s class type.

At execution time, the type of the object to which the variable refers determines the actual method to use.

See example: polymorphismvirtual, override and new keywords

http://msdn.microsoft.com/en-us/library/6fawty39(VS.80).aspx

as / is operatorsWhen downcasting an object, a System.InvalidCastException 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.

You can avoid a potential InvalidCastException by using the as operator to do a downcast rather than a cast operator.If the downcast is invalid, the expression will be null instead of throwing an

exception.Before performing a cast from a base-class object to a derived-

class object, use the is operator to ensure that the object is indeed an object of an appropriate derived-class type.

Abstract ClassesAbstract classes, or abstract base classes cannot be used to

instantiate objects.Abstract base classes are too general to create real objects—

they specify only what is common among derived classes.Classes that can be used to instantiate objects are called

concrete classes. Concrete classes provide the specifics that make it reasonable to

instantiate objects.An abstract class normally contains one or more abstract

methods, which have the keyword abstract in their declaration.A class that contains abstract methods must be declared as an

abstract class even if it contains concrete (nonabstract) methods.

Abstract methods do not provide implementations.Constructors and static methods cannot be declared abstract .

Abstract Classes

abstract property declarations have the form:public abstract PropertyType MyProperty{ get; set;

} // end abstract propertyAn abstract property may omit implementations for the get

accessor, the set accessor or both.Concrete derived classes must provide implementations for

every accessor declared in the abstract property.

Case Study: Payroll System Using Polymorphism In this example, we create an enhanced employee hierarchy to solve the following problem:A company pays its employees on a weekly basis. The employees are of four types: Salaried employees are paid a fixed weekly salary regardless of the number of hours worked, hourly employees are paid by the hour and receive overtime pay for all hours worked in excess of 40 hours, commission employees are paid a percentage of their sales, and salaried-commission employees receive a base salary plus a percentage of their sales. For the current pay period, the company has decided to reward salaried-commission employees by adding 10% to their base salaries.

Payroll System Using PolymorphismWe use abstract class Employee to represent the general

concept of an employee.SalariedEmployee, CommissionEmployee and HourlyEmployee extend Employee.

Class BasePlusCommissionEmployee—which extends CommissionEmployee—represents the last employee type.

Let’s look at the example code: PayrollSystem solution

Figure | Employee hierarchy UML class diagram

InterfacesInterfaces define and standardize the ways in which people

and systems can interact with one another. A C# interface describes a set of methods that can be called

on an object—to tell it, for example, to perform some task or return some piece of information.

An interface declaration begins with the keyword interface and can contain only abstract methods, properties, indexers and events.

All interface members are implicitly declared both public and abstract.

An interface can extend one or more other interfaces to create a more elaborate interface that other classes can implement.

InterfacesAn interface is typically used when disparate (i.e., unrelated)

classes need to share common methods so that they can be processed polymorphically

A programmer can create an interface that describes the desired functionality, then implement this interface in any classes requiring that functionality.

An interface often is used in place of an abstract class when there is no default implementation to inherit—that is, no fields and no default method implementations.

Like abstract classes, interfaces are typically public types, so they are normally declared in files by themselves with the same name as the interface and the .cs file-name extension.

Interface Example: IPayable• To build an application that can determine payments for

employees and invoices alike, we first create an interface named IPayable.

• Interface IPayable contains method GetPaymentAmount that returns a decimal amount to be paid for an object of any class that implements the interface.

• By convention, the name of an interface begins with "I". This helps distinguish interfaces from classes, improvingcode readability.

• When a class implements an interface, the same is-a relationship provided by inheritance applies.

• Example: IPayable solution

Multiple InheritanceNot supported in classes, but supported via interfacesTo implement more than one interface, use a comma-

separated list of interface names after the colon (:) in the class declaration.

Example: public abstract class Array :

ICloneable, IList, ICollection, IEnumerable

Common Interfaces of the .NET Framework Class Library

Interface Description

IComparable Objects of a class that implements the interface can be compared to one another.

IComponent Implemented by any class that represents a component, including Graphical User Interface (GUI) controls.

IDisposable Implemented by classes that must provide an explicit mechanism for releasing resources.

IEnumerator Used for iterating through the elements of a collection (such as an array) one element at a time.

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sealed Methods and ClassesA method declared sealed in a base class cannot be

overridden in a derived class.Methods that are declared private are implicitly sealed.Methods that are declared static also are implicitly sealed, because static methods cannot be overridden either.

A derived-class method declared both override and sealed can override a base-class method, but cannot be overridden in classes further down the inheritance hierarchy.

Operator Overloading

Just like methods, we can override operators to give them new meanings (e.g. + addition)

We use the operator keywordOperator methods need to be staticOperator methods need to take at least one parameterOperator methods could be overloaded too (multiple

operator methods can exist)Let’s see an example: ComplexNumber

Extension MethodsUsed to add functionality to an existing class without modifying

the class’s source code.See example ExtensionMethods.slnThe this keyword before a method’s first parameter notifies the

compiler that the method extends an existing class.An extension method is called on an object of the class that it

extends as if it were a members of the class. The compiler implicitly passes the object that is used to call the method as the extension method’s first argument.

The type of an extension method’s first parameter specifies the class that is being extended—extension methods must define at least one parameter.

Extension methods must be defined as static methods in a static top-level class.

struct (structure)

struct is very similar to a classLike classes, struct’s can have methods and properties

with access modifiers.Struct members are accessed via (.) operator similarly.So they are almost the same with one difference:

Class is a reference type whereas struct is a value type.Example: mystruct.cs