Wednesday, 7 September 2011

Object Oriented Programming Concepts


OOP Basics
Visual Basic was Object-Based, Visual Basic .NET is Object-Oriented, which means that it's a true Object-Oriented Programming Language. Visual Basic .NET supports all the key OOP features like Polymorphism, Inheritance, Abstraction and Encapsulation. It's worth having a brief overview of OOP before starting OOP with VB.
Why Object Oriented approach?

A major factor in the invention of Object-Oriented approach is to remove some of the flaws encountered with the procedural approach. In OOP, data is treated as a critical element and does not allow it to flow freely. It bounds data closely to the functions that operate on it and protects it from accidental modification from outside functions. OOP allows decomposition of a problem into a number of entities called objects and then builds data and functions around these objects. A major advantage of OOP is code reusability.

Some important features of Object Oriented programming are as follows:
  • Emphasis on data rather than procedure
  • Programs are divided into Objects
  • Data is hidden and cannot be accessed by external functions
  • Objects can communicate with each other through functions
  • New data and functions can be easily added whenever necessary
  • Follows bottom-up approach
Concepts of OOP:
  • Objects
  • Classes
  • Data Abstraction and Encapsulation
  • Inheritance
  • Polymorphism
Briefly on Concepts:

Objects

Objects are the basic run-time entities in an object-oriented
system. Programming problem is analyzed in terms of objects and nature of communication between them. When a program is executed, objects interact with each other by sending messages. Different objects can also interact with each other without knowing the details of their data or code.

Classes

A class is a collection of objects of similar type. Once a class is defined, any number of objects can be created which belong to that class.

Data Abstraction and Encapsulation

Abstraction refers to the act of representing essential features without including the background details or explanations. Classes use the concept of abstraction and are defined as a list of abstract attributes.

Storing data and functions in a single unit (class) is encapsulation. Data cannot be accessible to the outside world and only those functions which are stored in the class can access it.

Inheritance

Inheritance is the process by which objects can acquire the properties of objects of other class. In OOP, inheritance provides reusability, like, adding
additional features to an existing class without modifying it. This is achieved by deriving a new class from the existing one. The new class will have combined features of both the classes.

Polymorphism

Polymorphism means the ability to take more than one form. An operation may exhibit different behaviors in different instances. The behavior depends on the data types used in the operation. Polymorphism is extensively used in implementing Inheritance.

Advantages of OOP

Object-Oriented Programming has the following advantages over conventional approaches:
  • OOP provides a clear modular structure for programs which makes it good for defining abstract datatypes where implementation details are hidden and the unit has a clearly defined interface.
  • OOP makes it easy to maintain and modify existing code as new objects can be created with small differences to existing ones.
  • OOP provides a good framework for code libraries where supplied software components can be easily adapted and modified by the programmer. This is particularly useful for developing graphical user interfaces.
Class
A user-defined data structure that groups properties and methods. Class doesn’t occupies memory.

Object

Instance of Class is called object. An object is created in memory using keyword “new”.

Difference between Struct and Class

·         <!--[if !supportLists]--> Struct are Value type and are stored on stack, while Class are Reference type and are stored on heap.<!--[endif]-->
·         <!--[if !supportLists]--> Struct “do not support” inheritance, while class supports inheritance. However struct can implements interface.<!--[endif]-->
·         <!--[if !supportLists]--> Struct should be used when you want to use a small data structure, while Class is better choice for complex data structure.

What is the difference between instantiating structures with and without using the new keyword?
When a structure is instantiated using the new keyword, a constructor (no-argument or custom, if provided) is called which initializes the fields in the structure. When a structure is instantiated without using the new keyword, no constructor is called. Hence, one has to explicitly initialize all the fields of the structure before using it when instantiated without the new keyword.

Encapsulation

Wrapping up of data and function into a single unit is known as Encapsulation.

Properties

Attribute of object is called properties. Eg1:- A car has color as property.
Eg2:
private string m_Color;;

public string Color
{
get
{
return m_Color;
}
set
{
m_Color = value;
}
}
Car Maruti = new Car();
Maruti.Color= “White”;
Console.Write(Maruti.Color);
Isn't it better to make a field public than providing its property with both set { } and get { } block? After all the property will allow the user to both read and modify the field so why not use public field instead? Motivate your answer



Not always! Properties are not just to provide access to the fields; rather, they are supposed to provide controlled access to the fields of our class. As the state of the class depends upon the values of its fields, using properties we can assure that no invalid (or unacceptable) value is assigned to the fields.
Eg:
private int age;
public int Age
{
get
{
return age;
}
set
{
if(value <> 100)
//throw exception
else
age = value;
}
}
 
this Keyword
Each object has a reference “this” which points to itself.
<!--[if !supportLists]-->Two uses of this keyword.<!--[endif]-->
<!--[if !supportLists]-->o Can be used to refer to the current object.<!--[endif]-->
<!--[if !supportLists]-->o It can also be used by one constructor to explicitly invoke another constructor of the same class.<!--[endif]-->
Eg1:
class Student
{
private string name;
private int age;
Student(string name, int age)
{
this.name = name;
this.age = age;
}
}
Eg2:
class Circle
{
double x,y,radius;
Circle(double x){
this(x,0,1);
}
Circle(double x, double y){
this(x,y,1);
}
Circle(double x, double y, double radius){
this.x = x;
this.y = y;
this.radius = radius;
}
}

Constructor

·         <!--[if !supportLists]--> A constructor is a special method whose task is to initialize the object of its class.<!--[endif]-->
·         <!--[if !supportLists]--> It is special because its name is the same as the class name.<!--[endif]-->
·         <!--[if !supportLists]--> They do not have return types, not even void and therefore they cannot return values.<!--[endif]-->
·         <!--[if !supportLists]--> They cannot be inherited, though a derived class can call the base class constructor.<!--[endif]-->
·         <!--[if !supportLists]--> Constructor is invoked whenever an object of its associated class is created.<!--[endif]-->
·         Note: There is always atleast one constructor in every class. If you do not write a constructor, C# automatically provides one for you, this is called default constructor. Eg: class A, default constructor is A().
<!--[if !supportLists]--><!--[endif]-->

Static Members of the class

Static members belong to the whole class rather than to individual object

Static members are accessed with the name of class rather than reference to objects.
Eg:
class Test
{
public int rollNo;
public int mathsMarks;
public static int totalMathMarks;
}
class TestDemo
{
public static void main()
{
Test stud1 = new Test();
stud1.rollNo = 1;
stud1.mathsMarks = 40;
stud2.rollNo = 2;
stud2.mathsMarks = 43;
Test.totalMathsMarks = stud1.mathsMarks + stud2.mathsMarks;
}
}

Static Method of the class

<!--[if !supportLists]--> Methods that you can call directly without first creating an instance of a class. Eg: Main() Method, Console.WriteLine()<!--[endif]-->
<!--[if !supportLists]--> You can use static fields, methods, properties and even constructors which will be called before any instance of the class is created. <!--[endif]-->
<!--[if !supportLists]--> As static methods may be called without any reference to object, you can not use instance members inside static methods or properties, while you may call a static member from a non-static context. The reason for being able to call static members from non-static context is that static members belong to the class and are present irrespective of the existence of even a single object.<!--[endif]-->

Static Constructor

In C# it is possible to write a static no-parameter constructor for a class. Such a class is executed once, when first object of class is created.
One reason for writing a static constructor would be if your class has some static fields or properties that need to be initialized from an external source before the class is first used.
Eg:
Class MyClass
{
static MyClass()
{
//Initialization Code for static fields and properties.
}
}
Finalize() Method of Object class
Each class in C# is automatically (implicitly) inherited from the Object class which contains a method Finalize(). This method is guaranteed to be called when your object is garbage collected (removed from memory). You can override this method and put here code for freeing resources that you reserved when using the object.
For example
Protected override void Finalize()
{
try
{
Console.WriteLine(“Destructing Object….”);
//put some code here.
}
finally
{
base.Finalize();
}
}

Destructor

<!--[if !supportLists]--> A destructor is just opposite to constructor.<!--[endif]-->
<!--[if !supportLists]--> It has same as the class name, but with prefix ~ (tilde).<!--[endif]-->
<!--[if !supportLists]--> They do not have return types, not even void and therefore they cannot return values.<!--[endif]-->
<!--[if !supportLists]--> destructor is invoked whenever an object is about to be garbage collected<!--[endif]-->
Eg:
class person
{
//constructor
person()
{
}
//destructor
~person()
{
//put resource freeing code here.
}
}
What is the difference between the destructor and the Finalize() method? When does the Finalize() method get called?
Finalize() corresponds to the .Net Framework and is part of the
System.Object class. Destructors are C#'s implementation of the Finalize() method. The functionality of both Finalize() and the destructor is the same, i.e., they contain code for freeing the resources when the object is about to be garbage collected. In C#, destructors are converted to the Finalize() method when the program is compiled. The Finalize() method is called by the .Net Runtime and we can not predict when it will be called. It is guaranteed to be called when there is no reference pointing to the object and the object is about to be garbage collected.

Garbage Collection

·         <!--[if !supportLists]--> Garbage collection is the mechanism that reclaims the memory resources of an object when it is no longer referenced by a variable.<!--[endif]-->
·         <!--[if !supportLists]--> .Net Runtime performs automatically performs garbage collection, however you can force the garbage collection to run at a certain point in your code by calling System.GC.Collect().<!--[endif]-->
·         <!--[if !supportLists]--> Advantage of Garbage collection : It prevents programming error that could otherwise occur by incorrectly deleting or failing to delete objects.<!--[endif]-->

Enumeration

Enumeration improves code readability. It also helps in avoiding typing mistake.


Concept of Heap and Stack

Local Variables

Stack

Free Memory
(Larger Memory Area than Stack).
Heap
Global Variables
Permanent Storage area
Program Instruction
The Program Instruction and Global and Static variables are stored in a region known as permanent storage area and the local variables are stored in another area called stack. The memory space located between these two regions is available for dynamic memory allocation during execution of program. This free memory region is called heap. The size of heap keeps on changing when program is executed due to creation and death of variables that are local to functions and blocks. Therefore, it is possible to encounter memory “overflow” during dynamic allocation process.


<!--[if !supportLineBreakNewLine]--> <!--[endif]-->
Value Type and Reference Type
A variable is value type or reference type is solely determined by its data type.
Eg: int, float, char, decimal, bool, decimal, struct, etc are value types, while object type such as class, String, Array, etc are reference type.

Value Type

<!--[if !supportLists]--> As name suggest Value Type stores “value” directly.<!--[endif]-->
<!--[if !supportLists]--> For eg: <!--[endif]-->
<!--[if !supportLists]--> //I and J are both of type int<!--[endif]-->
<!--[if !supportLists]--> I = 20;<!--[endif]-->
<!--[if !supportLists]--> J = I;<!--[endif]-->
<!--[if !supportLists]--> int is a value type, which means that the above statements will results in two locations in memory.<!--[endif]-->
<!--[if !supportLists]--> For each instance of value type separate memory is allocated.<!--[endif]-->
<!--[if !supportLists]--> Stored in a Stack.<!--[endif]-->
<!--[if !supportLists]--> It Provides Quick Access, because of value located on stack.<!--[endif]-->

Reference Type

<!--[if !supportLists]--> As name suggest Reference Type stores “reference” to the value.<!--[endif]-->
<!--[if !supportLists]--> For eg: <!--[endif]-->
<!--[if !supportLists]--> Vector X, Y; //Object is defined. (No memory is allocated.)<!--[endif]-->
<!--[if !supportLists]--> X = new Vector(); //Memory is allocated to Object. //(new is responsible for allocating memory.)<!--[endif]-->
<!--[if !supportLists]--> X.value = 30; //Initialising value field in a vector class.<!--[endif]-->
<!--[if !supportLists]--> Y = X; //Both X and Y points to same memory location. //No memory is created for Y.<!--[endif]-->
<!--[if !supportLists]--> Console.writeline(Y.value); //displays 30, as both points to same memory<!--[endif]-->
<!--[if !supportLists]--> Y.value = 50;<!--[endif]-->
<!--[if !supportLists]--> Console.writeline(X.value); //displays 50.<!--[endif]-->
<!--[if !supportLists]--> Note: If a variable is reference it is possible to indicate that it does not refer to any object by setting its value to null;<!--[endif]-->
<!--[if !supportLists]--> Reference type are stored on Heap.<!--[endif]-->
<!--[if !supportLists]--> It provides comparatively slower access, as value located on heap.<!--[endif]-->
ref keyword
Passing variables by value is the default. However, we can force the value parameter to be passed by reference. Note: variable “must” be initialized before it is passed into a method.
out keyword
out keyword is used for passing a variable for output purpose. It has same concept as ref keyword, but passing a ref parameter needs variable to be initialized while out parameter is passed without initialized.
It is useful when we want to return more than one value from the method.
Note: You must assigned value to out parameter in method body, otherwise the method won’t compiled.

Boxing and Un-Boxing

Boxing: means converting value-type to reference-type.
Eg:
int I = 20;
string s = I.ToSting();
UnBoxing: means converting reference-type to value-type.
Eg:
int I = 20;
string s = I.ToString(); //Box the int
int J = Convert.ToInt32(s); //UnBox it back to an int.
Note: Performance Overheads due to boxing and unboxing as the boxing makes a copy of value type from stack and place it inside an object of type System.Object in the heap.

Inheritance

The process of sub-classing a class to extend its functionality is called Inheritance.
It provides idea of reusability.
Order of Constructor execution in Inheritance
constructors are called in the order from the top to the bottom (parent to child class) in inheritance hierarchy.
Order of Destructor execution in Inheritance
The destructors are called in the reverse order, i.e., from the bottom to the top (child to parent class) in the inheritance hierarchy.
What are Sealed Classes in C#?
The sealed modifier is used to prevent derivation from a class. A compile-time error occurs if a sealed class is specified as the base class of another class. (A sealed class cannot also be an abstract class)

Can you prevent your class from being inherited by another class?
Yes. The keyword “sealed” will prevent the class from being inherited.

Can you allow a class to be inherited, but prevent the method from being over-ridden?
Yes. Just leave the class public and make the method sealed.

Fast Facts of Inheritance

<!--[if !supportLists]--> Multiple inheritance of classes is not allowed in C#.<!--[endif]-->
<!--[if !supportLists]--> In C# you can implements more than one interface, thus multiple inheritance is achieved through interface.<!--[endif]-->
<!--[if !supportLists]--> The Object class defined in the System namespace is implicitly the ultimate base class of all the classes in C# (and the .NET framework) <!--[endif]-->
<!--[if !supportLists]--> Structures (struct) in C# does not support inheritance, it can only implements interfaces.<!--[endif]-->

Polymorphism

Polymorphism means same operation may behave differently on different classes.
Eg:
Method Overloading is an example of Compile Time Polymorphism.
Method Overriding is an example of Run Time Polymorphism
Does C#.net supports multiple inheritance?
No. A class can inherit from only one base class, however a class can implements many interface, which servers some of the same purpose without increasing complexity.
How many types of Access Modifiers.
<!--[if !supportLists]-->1) Public – Allows the members to be globally accessible.<!--[endif]-->
<!--[if !supportLists]-->2) Private – Limits the member’s access to only the containing type.<!--[endif]-->
<!--[if !supportLists]-->3) Protected – Limits the member’s access to the containing type and all classes derived from the containing type.<!--[endif]-->
<!--[if !supportLists]-->4) Internal – Limits the member’s access to within the current project.<!--[endif]-->

Method Overloading

<!--[if !supportLists]--> Method with same name but with different arguments is called method overloading.<!--[endif]-->
<!--[if !supportLists]--> Method Overloading forms compile-time polymorphism.<!--[endif]-->
<!--[if !supportLists]--> Eg:<!--[endif]-->
class A1
{
void hello()
{ Console.WriteLine(“Hello”); }
void hello(string s)
{ Console.WriteLine(“Hello {0}”,s); }
}

Method Overriding

<!--[if !supportLists]--> Method overriding occurs when child class declares a method that has the same type arguments as a method declared by one of its superclass.<!--[endif]-->
<!--[if !supportLists]--> Method overriding forms Run-time polymorphism.<!--[endif]-->
<!--[if !supportLists]--> Note: By default functions are not virtual in C# and so you need to write “virtual” explicitly. While by default in Java each function are virtual.<!--[endif]-->
<!--[if !supportLists]--> Eg1:<!--[endif]-->
Class parent
{
virtual void hello()
{ Console.WriteLine(“Hello from Parent”); }
}
Class child : parent
{
override void hello()
{ Console.WriteLine(“Hello from Child”); }
}
static void main()
{
parent objParent = new child();
objParent.hello();
}
//Output
Hello from Child.

Virtual Method

By declaring base class function as virtual, we allow the function to be overridden in any of derived class.
Eg:
Class parent
{
virtual void hello()
{ Console.WriteLine(“Hello from Parent”); }
}
Class child : parent
{
override void hello()
{ Console.WriteLine(“Hello from Child”); }
}
static void main()
{
parent objParent = new child();
objParent.hello();
}
//Output
Hello from Child.


Making choice between Interface and Abstract Class

In which Scenario you will go for Interface or Abstract Class?



Interfaces, like classes, define a set of properties, methods, and events. But unlike classes, interfaces

do not provide implementation. They are implemented by classes, and defined as separate entities from

classes. Even though class inheritance allows your classes to inherit implementation from a base class, it

also forces you to make most of your design decisions when the class is first published.



Abstract classes are useful when creating components because they allow you specify an invariant level

of functionality in some methods, but leave the implementation of other methods until a specific

implementation of that class is needed. They also version well, because if additional functionality is

needed in derived classes, it can be added to the base class without breaking code.

Readonly
he readonly keyword is different from the const keyword. A const field can only be initialized at the declaration of the field. A readonly field can be initialized either at the declaration or in a constructor. Therefore, readonly fields can have different values depending on the constructor used. Also, while a const field is a compile-time constant, the readonly field can be used for runtime constants as in the following example:
public static readonly uint l1 = (uint) DateTime.Now.Ticks;




No comments:

Post a Comment