Object Oriented Programming

6. OOPS
(B) What is Object Oriented Programming ?
It is a problem solving technique to develop software systems.It’s a technique to think
real world in terms of objects.Object maps the software model to real world concept.These
objects have responsibilities and provide services to application or other objects.
(B) What’s a Class ?
A class describes all the attributes of objects , as well as the methods that implement the
behavior of member objects.Its a comprehensive data type which represent a blue print
of objects.It’s a template of object.
(B) What’s a Object ?
It’s a basic unit of a system.An object is an entity that has attributes, behavior, and
identity. Objects are members of a class.Attributes and behavior of an object are defined
by the class definition.
(A) What’s the relation between Classes and Objects ?
They look very much same but are not same.Class is a definition , while object is a
instance of the class created.Class is a blue print while objects are actual objects existing
in real world.Example we have class CAR which has attributes and methods like
Speed,Brakes,Type of Car etc.Class CAR is just a prototype , now we can create real time
objects which can be used to provide functionality . Example we can create a Maruti car
object with 100 km speed and urgent brakes.
(B) What are different properties provided by Objectoriented
systems ?
Twist :- Can you explain different properties of Object Oriented Systems?
Note:- Difference between abstraction and encapsulation is one of the favorite interview
question and quiet confusing as both the terminology look alike.Best is if you can
brainstorm with your friends or do a little reading.
Following are characteristic’s of Object Oriented System’s :-
* Who motivates you ?
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Abstraction
It allows complex real world to be represented in simplified manner.Example color is
abstracted to RGB.By just making the combination of these three colors we can achieve
any color in world.It’s a model of real world or concept.
Encapsulation
The process of hiding all the internal details of an object from the outside world.
Communication using messages
When application wants to achieve certain task it can only be done using combination of
objects.A single object can not do all the task.Example if we want to make order processing
form. We will use Customer object , Order object , Product object and Payment object to
achieve this functionality.In short these objects should communicate with each other.This
is achieved when objects send messages to each other.
Object lifetime
All objects have life time.Objects are created , initialized , necessary functionalities are
done and later the object is destroyed.Every object have there own state and identity ,
which differ from instance to instance.
Class hierarchies (Inheritance and aggregation)
Twist :- What’s difference between Association , Aggregation and Inheritance relationships?
In object oriented world objects have relation and hierarchies in between them.There are
basically three kind of relationship in Object Oriented world :-
Association
This is the simplest relationship between objects.Example every customer has sales.So
Customer object and sales object have a association relation between them.
Aggregation
This is also called as composition model.Example in order to make a “Accounts” class it
has use other objects example “Voucher”,”Journal” and “Cash” objects.So accounts class
is aggregation of these three objects.
*Which university are your from ?
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Inheritance
Hierarchy is used to define more specialized classes based on a preexisting generalized
class.Example we have VEHICLE class and we can inherit this class make more specialized
class like CAR, which will add new attributes and use some existing qualities of the
parent class.Its shows more of a parent-child relationship .This kind of hierarchy is called
inheritance.
Polymorphism
When inheritance is used to extend a generalized class to a more specialized class,it includes
behavior of the top clas(Generalized class).The inheriting class often implement a behavior
that can be somewhat different than the generalized class, but the name of the behavior
can be same.It is important that a given instance of an object use the correct behavior,
and the property of polymorphism allows this to happen automatically.
(B) How can we acheive inheritance in VB.NET ?
Note:- The following explanation is for VB.NET
Inheritance is achieved by using “Inherits” keyword in VB.NET (For C# it is “:”).Simple
Sample is provided in CD for understanding inheritance in folder
“WindowsApplicationInheritance”.There are two classes one is the parent “ClsParent”
and second is the child “ClsChild”.Parent class has a string which has to parsed for junk
data “@” and “/”.ClsParent has the functionality which parses only cleans up
“@”.”ClsChild” then inherits from parent and adds extra functionality by parsing “/”.
Public Class ClsParent
Protected strData As String = “jksdhkj@dadad///ajkdhsjakd”
Public Function Parse() As String
Dim PstrData As String
PstrData = strData
PstrData = Replace(PstrData, “@”, “”)
Return PstrData
End Function
Public Function GetActualString() As String
Return strData
End Function
End Class
Above is the source which parses only “@” of strData variable.
* What gives your greatest satisfaction in software profession ?
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Public Class ClsChild
Inherits ClsParent
‘ this is child and a special parse function is added which will
also parse “/”
Public Function ParseBackSlash()
Dim PstrData As String
PstrData = Me.Parse()
PstrData = Replace(PstrData, “/”, “”)
Return PstrData
End Function
End Class
Above is the source code for “ClsChild” which does the remaining work.It adds extra
functionality by parsing “/” junk character’s of the data.
Note:- Strdata was accessible only because it was defined as protected in the parent class.
Figure :- 6.1 Inheritance in action
(I) What are abstract classes ?
Following are features of a abstract class :-
√ You can not create a object of abstract class
* How would you describe yourself ?
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√ Abstract class is designed to act as a base class (to be inherited by other classes).
Abstract class is a design concept in program development and provides a
base upon which other classes are built.
√ Abstract classes are similar to interfaces. After declaring an abstract class, it
cannot be instantiated on it's own, it must be inherited.
√ In VB.NET abstract classes are created using “MustInherit” keyword.In C#
we have “Abstract” keyword.
√ Abstract classes can have implementation or pure abstract methods which
should be implemented in the child class.
Note:- In order to understand the concept simple sample of add and multiply functionality
is implemented in “WindowsAbstract” folder in CD.
From interview point of view just saying using “MustInherit” keyword is more than enough
to convince that you have used abstract classes.But to clear simple fundamental let’s try
to understand the sample code.There are two classes one is “ClsAbstract” class and other
is “ClsChild” class.”ClsAbstract” class is a abstract class as you can see the mustinherit
keyword.It has one implemented method “Add” and other is abstract method which has
to be implemented by child class “MultiplyNumber”.In the child class we inherit the
abstract class and implement the multiplynumber function.
Definitely this sample does not take out actually how things are implemented in live
projects.Basically you put all your common functionalities or half implemented
functionality in parent abstract class and later let child class define the full functionality
of the abstract class.Example i always use abstract class with all my SET GET properties
of object in abstract class and later make specialize classes for insert,update,delete for
the corresponding entity object.
Public MustInherit Class ClsAbstract
‘ use the mustinherit class to declare the class as abstract
Public Function Add(ByVal intnum1 As Integer, ByVal intnum2 As
Integer) As Integer
Return intnum1 + intnum2
End Function
‘ left this seconf function to be completed by the inheriting
class
Public MustOverride Function MultiplyNumber(ByVal intnum1 As
Integer, ByVal intnum2 As Integer) As Integer
End Class
* What type of environment you are looking for ?
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Public Class ClsChild
Inherits ClsAbstract
‘ class child overrides the Multiplynumber function
Public Overrides Function MultiplyNumber(ByVal intnum1 As
Integer, ByVal intnum2 As Integer) As Integer
Return intnum1 * intnum2
End Function
End Class
Figure :- 6.2 Abstract classes in action
My attitude towards abstract class has been that i put all my common functionality in
abstract class.
(B) What’s a Interface ?
Interface is a contract that defines the signature of the functionality.So if a class is
implementing a interface it says to the outer world , that it provides specific behavior .
Example if a class is implementing Idisposable interface that means it has a functionality
to release unmanaged resources . Now external objects using this class knows that it has
contract by which it can dispose unused unmanaged objects.
√ Single Class can implement multiple interfaces.
√ If a class implements a interface then it has to provide implementation to all
its methods.
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Note:- In CD sample “WindowsInterFace” is provided , which has a simple interface
implemented.
In sample there are two files.One has the interface definition and other class implements
the interface.Below is the source code “IInterface” is the interface and “ClsDosomething”
implements the “IInterface”.This sample just displays a simple message box.
Public Interface IInterFace
Sub DoSomething()
End Interface
Public Class ClsDoSomething
Implements IInterFace
Public Sub DoSomething() Implements
WindowsInterFace.IInterFace.DoSomething
MsgBox(“Interface implemented”)
End Sub
End Class
Figure:- 6.3 Interface in action
* Do you have any experience in creating Use cases , requirement documents etc ?
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(A) What is difference between abstract classes and
interfaces?
Following are the differences between abstract and interfaces :-
√ Abstract classes can have concrete methods while interfaces have no methods
implemented.
√ Interfaces do not come in inheriting chain , while abstract classes come in
inheritance.
(B) What is a delegate ?
Delegate is a class that can hold a reference to a method or a function.Delegate class has
a signature and it can only reference those methods whose signature is compliant with the
class.Delegates are type-safe functions pointers or callbacks.
Below is a sample code which shows a example of how to implement delegates.
Public Class FrmDelegates
Inherits System.Windows.Forms.Form
Public Delegate Sub DelegateAddString()
Private Sub FrmDelegates_Load(ByVal sender As System.Object,
ByVal e As System.EventArgs) Handles MyBase.Load
End Sub
Private Sub AddString()
lstDelegates.Items.Add(“Running AddString() method”)
End Sub
Private Sub cmdDelegates_Click(ByVal sender As System.Object,
ByVal e As System.EventArgs) Handles cmdDelegates.Click
Dim objDelegateAddString As DelegateAddString
objDelegateAddString = AddressOf AddString
objDelegateAddString.Invoke()
End Sub
End Class
In the above there is a method called “AddString()” which adds a string to a listbox.You
can also see a delegate declared as :-
Public Delegate Sub DelegateAddString()
This delegate signature is compatible with the “AddString” method.When i mean
compatibility that means that there return types and passing parameter types are same.Later
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in command click of the button object of the Delegate is created and the method pointer
is received from “AddressOf ” keyword.Then by using the “Invoke” method the method
is invoked.
Figure :- 6.4 Delegate in Action
(B) What are event’s ?
As compares to delegates events works with source and listener methodology . So listener’s
who are interested in receiving some events they subscribe to the source.Once this
subscription is done the source raises events to all of it’s listener when needed.One source
can have multiple listeners.
In example sample given below class “ClsWithEvents” is a event source class , which has
a event “EventAddString()”.Now the listener’s who are interested in receiving this event’s
they can subscribe to this event.In class “FrmWithEvents” you can see the handles clause
which is associated with the “mobjClsWithEvents” objects.
Public Class ClsWithEvents
Event EventAddString(ByVal Value As String)
Public Sub AddString()
RaiseEvent EventAddString(“String added by Event”)
End Sub
End Class
*How well do you work with people? Do you prefer working alone or in teams?
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Public Class FrmWithEvents
Inherits System.Windows.Forms.Form
Private WithEvents mobjClsWithEvents As New ClsWithEvents()
Private Sub FrmWithEvents_Load(ByVal sender As System.Object,
ByVal e As System.EventArgs) Handles MyBase.Load
End Sub
Private Sub mobjClsWithEvents_EventAddString(ByVal Value As
String) Handles mobjClsWithEvents.EventAddString
LstData.Items.Add(Value)
End Sub
Private Sub CmdRunEvents_Click(ByVal sender As System.Object,
ByVal e As System.EventArgs) Handles CmdRunEvents.Click
mobjClsWithEvents.AddString()
End Sub
End Class
Figure :- 6.5 Events in action
* Describe the best project manager you've ever had ?
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(I) Do events have return type ?
No events do not have return type.
(A) Can event’s have access modifiers ?
Event’s are always public as they are meant to serve every one registering to it.But you
can access modifiers in events.You can have events with protected keyword which will
be accessible only to inherited classes.You can have private events only for object in that
class.
(A) Can we have shared events ?
Yes you can have shared event’s note only shared methods can raise shared events.
(I) What is shadowing ?
When two elements in a program have same name , one of them can hide and shadow the
other one.So in such cases the element which shadowed the main element is referenced.
Below is a sample code , there are two classes “ClsParent” and “ClsShadowedParent”.In
“ClsParent” there is a variable “x” which is a integer.”ClsShadowedParent” overrides
“ClsParent” and shadows the “x” variable to a string.
Note:- In Sample CD “WindowsShadowing” is folder which has the sample code.If you
run the program you can have two output’s one which shows a integer and other which shows
a string.
Public Class ClsParent
Public x As Integer
End Class
Public Class ClsShadowedParent
Inherits ClsParent
Public Shadows x As String
End Class
* Why should i hire you ?
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Figure :- 6.6 Shadowing in Action
(A) What’s difference between Shadowing and Overriding ?
Following are the differences between shadowing and overriding :-
√ Overriding redefines only the implementation while shadowing redefines the
whole element.
√ In overriding derived classes can refer the parent class element by using “ME”
keyword , but in shadowing you can access it by “MYBASE”.
(I) What’s difference between delegate and events?
√ Actually events use delegates in bottom. But they add an extra layer on the
delegates, thus forming the publisher and subscriber model.
√ As delegates are function to pointers they can move across any clients. So any
of the clients can add or remove events , which can be pretty confusing. But
events give the extra protection by adding the layer and making it a publisher
and subscriber model.
Just imagine one of your clients doing this
*Have you ever been fired or forced to resign?
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c.XyzCallback = null
This will reset all your delegates to nothing and you have to keep figuring where the error
is.
(B) If we inherit a class do the private variables also get
inherited ?
Yes the variables are inherited but can not be accessed directly by the class interface.
(B) What are different accessibility levels defined in .NET ?
Following are the five levels of access modifiers :-
√ Private : Only members of class have access.
√ Protected :-All members in current class and in derived classes can access the
variables.
√ Friend (internal in C#) :- Only members in current project have access to the
elements.
√ Protected friend (protected internal in C#) :- All members in current project
and all members in derived class can access the variables.
√ Public :- All members have access in all classes and projects.
(I) Can you prevent a class from overriding ?
If you define a class as “Sealed” in C# and “NotInheritable” in VB.NET you can inherit
the class any further.
(I) What’s the use of “MustInherit” keyword in VB.NET ?
If you want to create a abstract class in VB.NET it’s done by using “MustInherit”
keyword.This acts only as base type and can not be inherited any further.You can not
create a object of a class which is marked as “MustInherit”.
*What have your learnt from your past project experiences ?
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(I) Why can not you specify accessibility modifier in
Interface ?
All elements in Interface should be public.So by default all interface elements are public
by default.
(A) What are similarities between Class and structure ?
Following are the similarities between classes and structures :-
√ Both can have constructors, methods, properties, fields, constants,
enumerations, events, and event handlers.
√ Structures and classes can implement interface.
√ Both of them can have constructors without parameter and with parameter.
√ Both can have delegates and events.
(A) What’s the difference between Class and structure’s ?
Following are the key differences between them :-
√ Structure are value types and classes are reference types.So structures use
stack and classes use heap.
√ Structures members can not be declared as protected , but class members can
be.You can not do inheritance in structures.
√ Structures do not require constructors while classes require.
√ Objects created from classes are terminated using Garbage collector.Structures
are not destroyed using GC.
(B) What does virtual keyword mean ?
That method and property can be overridden.
(B) What are shared (VB.NET)/Static(C#) variables?
Static/Shared classes are used when a class provides functionality which is not specific to
any instance.In short if you want a object to be shared between multiple instances you
will use a static/Shared class.
*What's the logic of link list ?
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Following are features of Static/Shared classes :-
√ They can not be instantiated.By default a object is created on the first method
call to that object.
√ Static/Shared classes can not be inherited.
√ Static/Shared classes can have only static members.
√ Static/Shared classes can have only static constructor.
Note :- In CD there is a folder “WindowsShared” which has a sample code for shared
variables.Below is a snippet.It has a “AddCount” function which increments a static
“intCount” variable.In form there are two buttons which creates a new object and displays
the count of the static variable.Even though the object is created and destroyed , the variable
values does not change.It retains its old value.
Public Class ClsShared
Shared intCount As Integer
Public Function AddCount() As Integer
intCount = intCount + 1
Return intCount
End Function
End Class
Public Class FrmSharedClasses
Inherits System.Windows.Forms.Form
Private Sub CmdInstance1_Click(ByVal sender As System.Object, ByVal
e As System.EventArgs) Handles CmdInstance1.Click
Dim pobjClsShared As New ClsShared()
MessageBox.Show(“The count at this moment is” &
pobjClsShared.AddCount.ToString())
End Sub
Private Sub CmdInstance2_Click(ByVal sender As System.Object, ByVal
e As System.EventArgs) Handles CmdInstance2.Click
Dim pobjClsShared As New ClsShared()
MessageBox.Show(“The count at this moment is” &
pobjClsShared.AddCount.ToString())
End Sub
End Class
* Can you explain logic of binary sort ?
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Figure :- 6.7 Shared/Static In Action
(B) What is Dispose method in .NET ?
.NET provides “Finalize” method in which we can clean up our resources.But relying on
this is not always good so the best is to implement “Idisposable” interface and implement
the “Dispose” method where you can put your clean up routines.
(B) Whats the use of “OverRides” and “Overridable”
keywords ?
Overridable is used in parent class to indicate that a method can be overridden.Overrides
is used in the child class to indicate that you are overriding a method
(A) Where are all .NET Collection classes located ?
System.Collection namespace has all the collection classes available in .NET.
(A) What is ArrayList ?
*Whats difference between procedural and object oriented programming ?
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Array whose size can increase and decrease dynamically.Arraylist can hold item of different
types.As Arraylist can increase and decrease size dynamically you do not have to use the
REDIM keyword.You can access any item in array using the INDEX value of the array
position.
(A) What’s a HashTable ?
Twist :- What’s difference between HashTable and ArrayList ?
You can access array using INDEX value of array , but how many times you know the
real value of index.Hashtable provides way of accessing the index using a user identified
KEY value , thus removing the INDEX problem.
(A) What are queues and stacks ?
Queue is for first-in, first-out (FIFO) structures. Stack is for last-in, first-out (LIFO)
structures.
(B) What is ENUM ?
It’s used to define constants.
(A) What is nested Classes ?
Nested classes are classes with in classes.In sample below “ClsNested” class has a
“ChildNested” class nested inside it.
Public Class ClsNested
Public Class ChildNested
Public Sub ShowMessage()
MessageBox.Show(“Hi this is nested class”)
End Sub
End Class
End Class
This is the way we can instantiate the nested class and make the method call.
Dim pobjChildNested As New ClsNested.ChildNested()
pobjChildNested.ShowMessage()
Note:-In CD the above sample is provided in “WindowsNestedClasses”.
(B)What’s Operator Overloading in .NET?
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It provides a way to define and use operators such as +, -, and / for user-defined classes
or structs. It allows us to define/redefine the way operators work with our classes and
structs. This allows programmers to make their custom types look and feel like simple
types such as int and string.
VB.NET till now does not support operator overloading. Operator overloading is done
by using the “Operator” keyword.
Note:- Operator overloading is supported in VB.NET 2005
(I) In below sample code if we create a object of class2
which constructor will fire first ?
Public Class Class1
Sub New()
End Sub
End Class
Public Class class2
Inherits Class1
Sub New()
End Sub
End Class
* I leave this to the readers......
(B)What’s the significance of Finalize method in .NET?
.NET Garbage collector does almost all clean up activity for your objects. But unmanaged
resources (ex: - Windows API created objects, File, Database connection objects, COM
objects etc) is outside the scope of .NET framework we have to explicitly clean our
resources. For these types of objects .NET framework provides Object.Finalize method
which can be overridden and clean up code for unmanaged resources can be put in this
section.
(A)Why is it preferred to not use finalize for clean up?
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Problem with finalize is that garbage collection has to make two rounds in order to remove
objects which have finalize methods.
Below figure will make things clear regarding the two rounds of garbage collection rounds
performed for the objects having finalized methods.
In this scenario there are three objects Object1, Object2 and Object3. Object2 has the
finalize method overridden and remaining objects do not have the finalize method
overridden.
Now when garbage collector runs for the first time it searches for objects whose memory
has to freed. He sees three objects but only cleans the memory for Object1 and Object3.
Object2 it pushes to the finalization queue.
Now garbage collector runs for the second time. He see’s there are no objects to be freed
and then checks for the finalization queue and at this moment it clears object2 from the
memory.
So if you notice that object2 was freed from memory in the second round and not first.
That’s why the best practice is not to write clean up Non.NET resources in Finalize
method rather use the DISPOSE.
* Where you a part of some unsuccessful projects , then why was the project unsucessful ?
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Figure :- 6.8 Garbage collection in actions
(I)How can we suppress a finalize method?
GC.SuppressFinalize ()
(B)What’s the use of DISPOSE method?
Dispose method belongs to IDisposable interface. We had seen in the previous section
how bad it can be to override the finalize method for writing the cleaning of unmanaged
resources. So if any object wants to release its unmanaged code best is to implement
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IDisposable and override the Dispose method of IDisposable interface. Now once your
class has exposed the Dispose method it’s the responsibility of the client to call the
Dispose method to do the cleanup.
(A)How do I force the Dispose method to be called
automatically, as clients can forget to call Dispose method?
Note :- I admire this question.
Call the Dispose method in Finalize method and in Dispose method suppress the finalize
method using GC.SuppressFinalize. Below is the sample code of the pattern. This is the
best way we do clean our unallocated resources and yes not to forget we do not get the hit
of running the Garbage collector twice.
Note:- It will suppress the finalize method thus avoiding the two trip.
Public Class ClsTesting
Implements IDisposable
Public Overloads Sub Dispose()Implements IDisposable.Dispose
' write ytour clean up code here
GC.SuppressFinalize(Me)
End Sub
Protected Overrides Sub Finalize()
Dispose()
End Sub
End Class
(I)In what instances you will declare a constructor to be
private?
When we create a private constructor, we can not create object of the class directly from
a client. So you will use private constructors when you do not want instances of the class
to be created by any external client. Example UTILITY functions in project will have no
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instance and be used with out creating instance, as creating instances of the class would
be waste of memory.
(I)Can we have different access modifiers on get/set
methods of a property ?
No we can not have different modifiers same property. The access modifier on a property
applies to both its get and set accessors.
(I)If we write a goto or a return statement in try and catch
block will the finally block execute ?
The code in the finally always runs even if there are statements like goto or a return
statements.
(A)What is Indexer ?
An indexer is a member that enables an object to be indexed in the same way as an array.
(A)Can we have static indexer in C# ?
No.
(A)In a program there are multiple catch blocks so can it
happen that two catch blocks are executed ?
No once the proper catch section is executed the control goes to finally block.So there
will not be any scenarios in which multiple catch blocks will be executed.
(A) What is the difference between System.String and
System.StringBuilder classes?
System.String is immutable; System.StringBuilder can have mutable string where a variety
of operations can be performed.