Constructor, Destructor,Inheritance and Polymorphism

Constructor, Destructor & Gerbage Collection

 Constructor:

• Constructors are class methods that are executed when an object of a Class or Struct is created.
• They have the same name as the class or struct, and usually initialize the data members of the new object.
• A class or struct may have multiple constructors that take different arguments.
• Constructors enable the programmer to set default values, limit instantiation and write code that is flexible and easy to read.
• If you do not provide a constructor for your object, C# will create one by default that instantiates the object and sets member variables to the default values .
• Static classes and structs can also have constructors.

Constructor-example:

public class Time

   {

       int Year;

       int Month;

       int Date;

       int Hour;

       int Minute;

       int Second;

public void DisplayCurrentTime( )

      {

Console.WriteLine("{0}/{1}/{2} {3}:{4}:{5}“,   Month, Date, Year, Hour, Minute, Second);

      } 

 

Gerbage Collectin(GC):

GC frees up memory used by those objects which can no longer be accessed by your runtime code.

Gerbage Collection

void A()
{

    B();
    int i = 10;
 }

void B()
{
    C c = new C();
   Console.WriteLine(c.Name);
}

• In the above example once the control comes to the line int i = 10; in function A(), there is no way for the system to access the object c created in function B(). This is an object which is eligible for Garbage Collection. So the next time when the GC thread runs it can cleanup the memory allotted to the object c.
  

Destructor:

• Destructors are used to destruct instances of classes.
• Destructors cannot be defined in structs. They are only used with classes.
• A class can only have one destructor.
• Destructors cannot be inherited or overloaded.
• Destructors cannot be called. They are invoked automatically.
• A destructor does not take modifiers or have parameters.

class Car

{

    ~Car()  // destructor

  {

        // code

   }

}

• The C# compiler translates it to:

  protected override void Finalize()

{

    try

    {

        // Code

    }

    finally

    {

        base.Finalize();

    }

}

 

 

Inheritance and Polymorphism

Inheritance:

• Inheritance allows a software developer to derive a new class from an existing one.
• The existing class is called the parent, super, or base class.
• The derived class is called a child or subclass.
• The child inherits characteristics of the parent.

• Methods and data defined for the parent class.

• The child has special rights to the parents methods and data.

• Public access like any one else

• Protected access available only to child classes (and their descendants).

• The child has its own unique behaviors and data.

• Inheritance relationships are often shown graphically in a class diagram, with the arrow pointing to the parent class.
• Inheritance should create an is-a relationship, meaning the child is a more specific version of the parent.

Examples: Base Classes and Derived Classes

Declaring a Derived Class

• Define a new class DerivedClass which extends BaseClass.
  

    class BaseClass

  {

      // class contents

}

  class DerivedClass : BaseClass

  {

      // class contents

}

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

• However, 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
• There is a third visibility modifier that helps in inheritance situations: protected.

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.

Single Inheritance:

• C# supports single inheritance, meaning that a derived class can have only one parent 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 a different implementation.
• The type of the object executing the method determines which version of the method is invoked.

Class Hierarchies:

• A child class of one parent can be the parent of another child, forming a class hierarchy

• 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. Avoids redundant code.

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 day;

day = new Holiday();

…

day = new Christmas();

Dynamic Binding

• A polymorphic reference is one which can refer to different types of objects at different times.
• The type of the actual instance, not the declared type, determines which method is invoked.
• Polymorphic references are therefore resolved at run-time, not during compilation.

• This is called dynamic binding.

Dynamic Binding:

• Suppose the Holiday class has a method called Celebrate, and the Christmas class redefines it (overrides 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

Overriding Methods:

• C# requires that all class definitions communicate clearly their intentions.
• The keywords virtual, override and new provide this communication.
• If a base class method is going to be overridden it should be declared virtual.
• A derived class would then indicate that it indeed does override the method with the override keyword.

Sealed Method:

What if you don't want anyone to derive classes from your class?

Mark class as sealed.

sealed class Square : Default_Shape
{...}

A program that atttempts to define a derived class based on Square will get a compile error.

All structs are implicitly sealed.

You can also mark a method as sealed.

• This prevents any derived class from overriding that method.
• Normally a method that overrides a virtual method in a base class is itself virtual.
• The method can be re-implemented again and again in successive derived functions. “sealed” puts an end to this.

Overloading vs. Overriding:

• Overloading deals with multiple methods in the same class with the same name but different signatures
• Overloading lets you define a similar operation in different ways for different data
• Example:

int foo(string[] bar);

int foo(int bar1, float a);

• Overriding deals with two methods, one in a parent class and one in a child class, that have the same signature.
• Overriding lets you define a similar operation in different ways for different object types
  Example:

class Base {

    public virtual int foo() {} }

class Derived {

    public override int foo() {}}

 

Polymorphism via Inheritance

The System.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 a direct descendant 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