Visula C# Programming Lecture 7


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Lecture 7

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Visula C# Programming Lecture 7

  1. 1. Visual Programming OO Programming and Inheritance
  2. 2. Inheritance Inheritance allows a software developer to derive a new class from an existing one The existing class is called the parent class, or superclass, or base class The derived class is called the child class, or subclass, or derived class. As the name implies, the child inherits characteristics of the parent That is, the child class inherits the methods and data defined for the parent class 2
  3. 3. Inheritance Inheritance relationships are often shown graphically in a class diagram, with the arrow pointing to the parent class Animal # weight : int Animal Bird + GetWeight() : int Bird Inheritance should create an is-a relationship, meaning the child is a more specific version of the parent + Fly() : void 3
  4. 4. Examples: Base Classes and Derived Classes Base class Derived classes Student GraduateStudent UndergraduateStudent Shape Circle Triangle Rectangle Loan CarLoan HomeImprovementLoan MortgageLoan Employee FacultyMember StaffMember Account CheckingAccount SavingsAccount Fig. 9.1 Inheritance examples. 4
  5. 5. Declaring a Derived Class Define a new class DerivedClass which extends BaseClass class BaseClass { // class contents } class DerivedClass : BaseClass { // class contents } Base class: the “parent” class; if omitted the parent is Object See Book.cs, Dictionary.cs, BookInheritance.cs 5
  6. 6. Controlling Inheritance A child class inherits the methods and data defined for the parent class; however, whether a data or method member of a parent class is accessible in the child class depends on the visibility modifier of a member Variables and methods declared with private visibility are not accessible in the child class a private data member defined in the parent class is still part of the state of a derived class Variables and methods declared with public visibility are accessible; but public variables violate our goal of encapsulation There is a third visibility modifier that helps in inheritance situations: protected 6
  7. 7. The protected Modifier  Variables and methods declared with protected visibility in a parent class are only accessible by a child class or any class derived from that class  The details of each modifier are linked on the schedule page Book Example Book.cs Dictionary.cs BookInheritance.cs + public - private # protected # pages : int + GetNumberOfPages() : void Dictionary - definition : int + PrintDefinitionMessage() : void 7
  8. 8. Book.cs using System; namespace BookInheritance { /// <summary> /// Summary description for Book. /// </summary> public class Book { protected int pages; //---------------------------------------------------------------// Constructors //---------------------------------------------------------------- public Book() { pages = 1500; } public Book( int pages ) { this.pages = pages; } //---------------------------------------------------------------// Get the number of pages of this book. //---------------------------------------------------------------- public int GetNumberOfPages () { return pages; } public void PrintNumberOfPages () { Console.WriteLine ("Number of pages is: " + pages); } } // end of class Book } // end of namespace 8
  9. 9. Dictionary.cs using System; namespace BookInheritance { /// <summary> /// Summary description for Dictionary. /// </summary> public class Dictionary : Book { private int definitions; // number of definitions public Dictionary() { definitions = 52500; } public Dictionary(int definitions) { this.definitions = definitions; } //----------------------------------------------------------------// Prints a message using both local and inherited values. //----------------------------------------------------------------public void PrintDefinitionMessage () { Console.WriteLine ( "Number of definitions: " + definitions ); Console.WriteLine ( "Average definitions per page: " + definitions / pages ); } // end of PrintDefinitionMessages } // end of class Dictionary } // end of namespace 9
  10. 10. Book.cs using System; namespace BookInheritance{ class BookInheritance { //----------------------------------------------------------------// Instantiates a derived class and invokes its inherited and // local methods. //----------------------------------------------------------------public static void Main (String[] args) { Dictionary webster = new Dictionary (); int pages = webster.GetNumberOfPages(); // Console.WriteLine( webster.pages ); Console.WriteLine( "Number of pages is " + pages ); webster.PrintDefinitionMessage(); } } // end of class BookInheritance } // end of namespace 10
  11. 11. Calling Parent’s Constructor in a Child’s Constructor: the base Reference Constructors are not inherited, even though they have public visibility The first thing a derived class does is to call its base class’ constructor, either explicitly or implicitly implicitly it is the default constructor yet we often want to use a specific constructor of the parent to set up the "parent's part" of the object Syntax: use base public DerivedClass : BaseClass { public DerivedClass(…) : base(…) { // … } } 11
  12. 12. Defining Methods in the Child Class: Overriding Methods A child class can override the definition of an inherited method in favor of its own That is, a child can redefine a method that it inherits from its parent The new method must have the same signature as the parent's method, but can have different code in the body The type of the object executing the method determines which version of the method is invoked 12
  13. 13. Overriding Methods: Syntax override keyword is needed if a derivedclass method overrides a base-class method If a base class method is going to be overridden it should be declared virtual Example Thought.cs Advice.cs ThoughtAndAdvice.cs 13
  14. 14. Thought.cs using System; using System.Collections.Generic; using System.Text; namespace ConsoleApplication3 { class Thought { //---------------------------------------------------// Prints a message. //---------------------------------------------------public virtual void Message() { Console.WriteLine("I feel like I'm in space "); Console.WriteLine(); } // end of message } } 14
  15. 15. Advice.cs using System; using System.Collections.Generic; using System.Text; namespace ConsoleApplication3 { class Advice:Thought { //--------------------------------------------------------------// Prints a message. This method overrides the parent's version. // It also invokes the parent's version explicitly using super. //----------------------------------------------------------------public override void Message() { Console.WriteLine("Yor are on earth"); Console.WriteLine(); base.Message(); } // end of message } } 15
  16. 16. ThoughtandAdvice.cs using System; using System.Collections.Generic; using System.Text; namespace ConsoleApplication3{ class Program { //----------------------------------------------------------------// Instatiates two objects a invokes the message method in each. //----------------------------------------------------------------static void Main( string[] args ) { Thought t = new Thought(); Advice a = new Advice(); Console.WriteLine( "parked speak: "); t.Message(); Console.WriteLine( "dates speak: "); a.Message(); Console.ReadLine(); }}} 16
  17. 17. Overloading vs. Overriding Overloading deals with multiple methods in the same class with the same name but different signatures Overriding deals with two methods, one in a parent class and one in a child class, that have the same signature Overloading lets you define a similar operation in different ways for different data Overriding lets you define a similar operation in different ways for different object types 17
  18. 18. Single vs. Multiple Inheritance Some languages, e.g., C++, allow Multiple inheritance, which allows a class to be derived from two or more classes, inheriting the members of all parents collisions, such as the same variable name in two parents, are hard to resolve Thus, C# and Java support single inheritance, meaning that a derived class can have only one parent class 18
  19. 19. Class Hierarchies A child class of one parent can be the parent of another child, forming a class hierarchy Animal Reptile Snake Lizard Bird Parrot Mammal Horse Bat 19
  20. 20. Another Example: Base Classes and Derived Classes CommunityMemeber Employee Faculty Professor Fig. 9.2 Student Staff Under Alumnus Graduate Instructor Inheritance hierarchy for university CommunityMember. 20
  21. 21. Yet Another Example Shape TwoDimensionalShape Circle Fig. 9.3 Square Triangle ThreeDimensionalShape Sphere Cube Cylinder Portion of a Shape class hierarchy. 21
  22. 22. Class Hierarchies An inherited member is continually passed down the line—inheritance is transitive Good class design puts all common features as high in the hierarchy as is reasonable 22
  23. 23. The Object Class All classes in C# are derived from the Object class if a class is not explicitly defined to be the child of an existing class, it is assumed to be the child of the Object class The Object class is therefore the ultimate root of all class hierarchies The Object class defines methods that will be shared by all objects in C#, e.g., ToString: converts an object to a string representation Equals: checks if two objects are the same GetType: returns the type of a type of object A class can override a method defined in Object to have a different behavior, e.g., String class overrides the Equals method to compare the content of two strings See Student.cs GradStudent.cs and Academia.cs 23
  24. 24. References and Inheritance An object reference can refer to an object of its class, or to an object of any class derived from it by inheritance For example, if the Holiday class is used to derive a child class called Christmas, then a Holiday reference can be used to point to a Christmas object Holiday Christmas Holiday day; day = new Holiday(); … day = new Christmas(); 24
  25. 25. References and Inheritance Assigning an object to an ancestor reference is considered to be a widening conversion, and can be performed by simple assignment Holiday day = new Christmas(); Assigning an ancestor object to a reference can also be done, but it is considered to be a narrowing conversion and must be done with a cast Christmas christ = new Christmas(); Holiday day = christ; Christmas christ2 = (Christmas)day; The widening conversion is the most useful for implementing polymorphism 25
  26. 26. Polymorphism via Inheritance A polymorphic reference is one which can refer to different types of objects at different times An object reference can refer to one object at one time, then it can be changed to refer to another object (related by inheritance) at another time it is the type of the object being referenced, not the reference type, that determines which method is invoked polymorphic references are therefore resolved at run-time, not during compilation; this is called dynamic binding Careful use of polymorphic references can lead to elegant, robust software designs 26
  27. 27. Polymorphism via Inheritance Suppose the Holiday class has a method called Celebrate, and the Christmas class redfines it Now consider the following invocation: day.Celebrate(); If day refers to a Holiday object, it invokes the Holiday version of Celebrate; if it refers to a Christmas object, it invokes the Christmas version 27