Object Oriented Programming oops principles interview questions and answers

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1. What are the OOPS concepts?

1) Encapsulation: It is the mechanism that binds together code and data in manipulates, and keeps both safe from outside interference and misuse. In short it isolates a particular code and data from all other codes and data. A well-defined interface controls the access to that particular code and data.

Inheritance: It is the process by which one object acquires the properties of another object. This supports the hierarchical classification. Without the use of hierarchies, each object would need to define all its characteristics explicitly.
However, by use of inheritance, an object need only define those qualities that make it unique within its class. It can inherit its general attributes from its parent. A new sub-class inherits all of the attributes of all of its ancestors.

3) Polymorphism: It is a feature that allows one interface to be used for general class of actions. The specific action is determined by the exact nature of the situation. In general polymorphism means "one interface, multiple methods", This means that it is possible to design a generic interface to a group of related activities. This helps reduce complexity by allowing the same interface to be used to specify a general class of action. It is the compiler's job to select the specific action (that is, method) as it applies to each situation. 2. What is the difference between a Struct and a Class?
* The struct type is suitable for representing lightweight objects such as Point, Rectangle, and Color. Although it is possible to represent a point as a class, a struct is more efficient in some scenarios. For example, if you declare an array of 1000 Point objects, you will allocate additional memory for referencing each object. In this case, the struct is less expensiv * When you create a struct object using the new operator, it gets created and the appropriate constructor is called. Unlike classes, structs can be instantiated without using the new operator. If you do not use new, the fields will remain unassigned and the object cannot be used until all of the fields are initialized.

* It is an error to declare a default (parameterless) constructor for a struct. A default constructor is always provided to initialize the struct members to their default values.
* It is an error to initialize an instance field in a struct.
* There is no inheritance for structs as there is for classes. A struct cannot inherit from another struct or class, and it cannot be the base of a class. Structs, however, inherit from the base class Object. A struct can implement interfaces, and it does that exactly as classes do.
* A struct is a value type, while a class is a reference type.
3. Value type & reference types difference? Example from .NET. Integer & struct are value types or reference types in .NET?
Most programming languages provide built-in data types, such as integers and floating-point numbers, that are copied when they are passed as arguments (that is, they are passed by value). In the .NET Framework, these are called value types. The runtime supports two kinds of value types: 

* Built-in value types
The .NET Framework defines built-in value types, such as System.Int32 and System.Boolean, which correspond and are identical to primitive data types used by programming languages.
* User-defined value types
Your language will provide ways to define your own value types, which derive from System.ValueType. If you want to define a type representing a value that is small, such as a complex number (using two floating-point numbers), you might choose to define it as a value type because you can pass the value type efficiently by value. If the type you are defining would be more efficiently passed by reference, you should define it as a class instead. 

Variables of reference types, referred to as objects, store references to the actual data. This following are the reference types:

* class
* interface
* delegate 

This following are the built-in reference types:

* object
* string 
3. What is Inheritance, Multiple Inheritance, Shared and Repeatable Inheritance?
4. What is Method overloading?
Method overloading occurs when a class contains two methods with the same name, but different signatures.
5. What is Method Overriding? How to override a function in C#?
Use the override modifier to modify a method, a property, an indexer, or an event. An override method provides a new implementation of a member inherited from a base class. The method overridden by an override declaration is known as the overridden base method. The overridden base method must have the same signature as the override method.
You cannot override a non-virtual or static method. The overridden base method must be virtual, abstract, or override.
6. Can we call a base class method without creating instance?
Its possible If its a static method.
Its possible by inheriting from that class also.
Its possible from derived classes using base keyword.
7. You have one base class virtual function how will call that function from derived class?

9.class a

10. {

11.public virtual int m()


13.return 1;


15. }

16. class b:a

17. {

18.public int j()


20.return m();



22. In which cases you use override and new base?
Use the new modifier to explicitly hide a member inherited from a base class. To hide an inherited member, declare it in the derived class using the same name, and modify it with the new modifier. 

C# Language features

23. 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)
24. What is Polymorphism? How does VB.NET/C# achieve polymorphism?

25.class Token

26. {

27.public string Display()


29.//Implementation goes here

30.return "base";


32. }

33. class IdentifierToken:Token

34. {

35.public new string Display() //What is the use of new keyword


37.//Implementation goes here

38.return "derive";


40. }

41. static void Method(Token t)

42. {


44. }

45. public static void Main()

46. {

47.IdentifierToken Variable=new IdentifierToken();

48.Method(Variable); //Which Class Method is called here


50. }

51. For the above code What is the "new" keyword and Which Class Method is

52. called here

A: it will call base class Display method

53.class Token

54. {

55.public virtual string Display()


57.//Implementation goes here

58.return "base";


60. }

61. class IdentifierToken:Token

62. {

63.public override string Display() //What is the use of new keyword


65.//Implementation goes here

66.return "derive";


68. }

69. static void Method(Token t)

70. {


72. }

73. public static void Main()

74. {

75.IdentifierToken Variable=new IdentifierToken();

76.Method(Variable); //Which Class Method is called here


78. }

79. A: Derive
80. 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. 

Interfaces vs. Abstract Classes



Abstract class

Multiple inheritance

A class may implement several interfaces.

A class may extend only one abstract class.

Default implementation

An interface cannot provide any code at all, much less default code.

An abstract class can provide complete code, default code, and/or just stubs that have to be overridden.


Static final constants only, can use them without qualification in classes that implement the interface. On the other paw, these unqualified names pollute the namespace. You can use them and it is not obvious where they are coming from since the qualification is optional.

Both instance and static constants are possible. Both static and instance intialiser code are also possible to compute the constants.

Third party convenience

An interface implementation may be added to any existing third party class.

A third party class must be rewritten to extend only from the abstract class.

is-a vs -able or can-do

Interfaces are often used to describe the peripheral abilities of a class, not its central identity, e.g. an Automobile class might implement the Recyclable interface, which could apply to many otherwise totally unrelated objects.

An abstract class defines the core identity of its descendants. If you defined a Dog abstract class then Damamation descendants are Dogs, they are not merely dogable. Implemented interfaces enumerate the general things a class can do, not the things a class is.


You can write a new replacement module for an interface that contains not one stick of code in common with the existing implementations. When you implement the interface, you start from scratch without any default implementation. You have to obtain your tools from other classes; nothing comes with the interface other than a few constants. This gives you freedom to implement a radically different internal design.

You must use the abstract class as-is for the code base, with all its attendant baggage, good or bad. The abstract class author has imposed structure on you. Depending on the cleverness of the author of the abstract class, this may be good or bad. Another issue that's important is what I call "heterogeneous vs. homogeneous." If implementors/subclasses are homogeneous, tend towards an abstract base class. If they are heterogeneous, use an interface. (Now all I have to do is come up with a good definition of hetero/homogeneous in this context.) If the various objects are all of-a-kind, and share a common state and behavior, then tend towards a common base class. If all they share is a set of method signatures, then tend towards an interface.


If all the various implementations share is the method signatures, then an interface works best.

If the various implementations are all of a kind and share a common status and behavior, usually an abstract class works best.


If your client code talks only in terms of an interface, you can easily change the concrete implementation behind it, using a factory method.

Just like an interface, if your client code talks only in terms of an abstract class, you can easily change the concrete implementation behind it, using a factory method.


Slow, requires extra indirection to find the corresponding method in the actual class. Modern JVMs are discovering ways to reduce this speed penalty.



The constant declarations in an interface are all presumed public static final, so you may leave that part out. You can't call any methods to compute the initial values of your constants. You need not declare individual methods of an interface abstract. They are all presumed so.

You can put shared code into an abstract class, where you cannot into an interface. If interfaces want to share code, you will have to write other bubblegum to arrange that. You may use methods to compute the initial values of your constants and variables, both instance and static. You must declare all the individual methods of an abstract class abstract.

Adding functionality

If you add a new method to an interface, you must track down all implementations of that interface in the universe and provide them with a concrete implementation of that method.

If you add a new method to an abstract class, you have the option of providing a default implementation of it. Then all existing code will continue to work without change.

81. see the code

82.interface ICommon

83. {

84.int getCommon();

85. }

86. interface ICommonImplements1:ICommon

87. {

88. }

89. interface ICommonImplements2:ICommon

90. {

91. }

92. public class a:ICommonImplements1,ICommonImplements2

93. {


How to implement getCommon method in class a? Are you seeing any problem in the implementation?

public class a:ICommonImplements1,ICommonImplements2


public int getCommon()


return 1;



94.interface IWeather

95. {

96.void display();

97. }

98. public class A:IWeather

99. {

100.public void display()




104. }

105. public class B:A

106. {

107. }

108. public class C:B,IWeather

109. {

110.public void display()




114. }

115. When I instantiate C.display(), will it work? 

116.interface IPrint

117. {

118.string Display();

119. }

120. interface IWrite

121. {

122.string Display();

123. }

124. class PrintDoc:IPrint,IWrite

125. {

126.//Here is implementation

127. }

how to implement the Display in the class printDoc (How to resolve the naming Conflict) A: no naming conflicts

class PrintDoc:IPrint,IWrite


public string Display()


return "s";



128.interface IList

129. {

130. int Count { get; set; }

131. }

132. interface ICounter

133. {

134. void Count(int i);

135. }

136. interface IListCounter: IList, ICounter {}

137. class C

138. {

139. void Test(IListCounter x)

140. {

141. x.Count(1); // Error

142. x.Count = 1; // Error

143. ((IList)x).Count = 1; // Ok, invokes IList.Count.set

144. ((ICounter)x).Count(1); // Ok, invokes ICounter.Count

145. }

146. }

147. Write one code example for compile time binding and one for run time binding? What is early/late binding?
An object is early bound when it is assigned to a variable declared to be of a specific object type. Early bound objects allow the compiler to allocate memory and perform other optimizations before an application executes.
' Create a variable to hold a new object.
Dim FS As FileStream
' Assign a new object to the variable.
FS = New FileStream("C:\tmp.txt", FileMode.Open)
By contrast, an object is late bound when it is assigned to a variable declared to be of type Object. Objects of this type can hold references to any object, but lack many of the advantages of early-bound objects.
Dim xlApp As Object
xlApp = CreateObject("Excel.Application")
148. Can you explain what inheritance is and an example of when you might use it?
149. How can you write a class to restrict that only one object of this class can be created (Singleton class)?
150. What are the access-specifiers available in c#?
Private, Protected, Public, Internal, Protected Internal.
151. Explain about Protected and protected internal, “internal” access-specifier?
protected - Access is limited to the containing class or types derived from the containing class.
internal - Access is limited to the current assembly.
protected internal - Access is limited to the current assembly or types derived from the containing class.


1. Difference between type constructor and instance constructor? What is static constructor, when it will be fired? And what is its use?
(Class constructor method is also known as type constructor or type initializer)
Instance constructor is executed when a new instance of type is created and the class constructor is executed after the type is loaded and before any one of the type members is accessed. (It will get executed only 1st time, when we call any static methods/fields in the same class.) Class constructors are used for static field initialization. Only one class constructor per type is permitted, and it cannot use the vararg (variable argument) calling convention.
A static constructor is used to initialize a class. It is called automatically to initialize the class before the first instance is created or any static members are referenced.
2. What is Private Constructor? and it’s use? Can you create instance of a class which has Private Constructor?
A: When a class declares only private instance constructors, it is not possible for classes outside the program to derive from the class or to directly create instances of it. (Except Nested classes)
Make a constructor private if:
- You want it to be available only to the class itself. For example, you might have a special constructor used only in the implementation of your class' Clone method.
- You do not want instances of your component to be created. For example, you may have a class containing nothing but Shared utility functions, and no instance data. Creating instances of the class would waste memory.
3. I have 3 overloaded constructors in my class. In order to avoid making instance of the class do I need to make all constructors to private?
4. Overloaded constructor will call default constructor internally?
5. What are virtual destructors?
6. Destructor and finalize
Generally in C++ the destructor is called when objects gets destroyed. And one can explicitly call the destructors in C++. And also the objects are destroyed in reverse order that they are created in. So in C++ you have control over the destructors.
In C# you can never call them, the reason is one cannot destroy an object. So who has the control over the destructor (in C#)? it's the .Net frameworks Garbage Collector (GC). GC destroys the objects only when necessary. Some situations of necessity are memory is exhausted or user explicitly calls System.GC.Collect() method.
Points to remember:
1. Destructors are invoked automatically, and cannot be invoked explicitly.
2. Destructors cannot be overloaded. Thus, a class can have, at most, one destructor.
3. Destructors are not inherited. Thus, a class has no destructors other than the one, which may be declared in it.
4. Destructors cannot be used with structs. They are only used with classes.
5. An instance becomes eligible for destruction when it is no longer possible for any code to use the instance.
6. Execution of the destructor for the instance may occur at any time after the instance becomes eligible for destruction.
7. When an instance is destructed, the destructors in its inheritance chain are called, in order, from most derived to least derived.
7. What is the difference between Finalize and Dispose (Garbage collection)
Class instances often encapsulate control over resources that are not managed by the runtime, such as window handles (HWND), database connections, and so on. Therefore, you should provide both an explicit and an implicit way to free those resources. Provide implicit control by implementing the protected Finalize Method on an object (destructor syntax in C# and the Managed Extensions for C++). The garbage collector calls this method at some point after there are no longer any valid references to the object.
In some cases, you might want to provide programmers using an object with the ability to explicitly release these external resources before the garbage collector frees the object. If an external resource is scarce or expensive, better performance can be achieved if the programmer explicitly releases resources when they are no longer being used. To provide explicit control, implement the Dispose method provided by the IDisposable Interface. The consumer of the object should call this method when it is done using the object. Dispose can be called even if other references to the object are alive.
Note that even when you provide explicit control by way of Dispose, you should provide implicit cleanup using the Finalize method. Finalize provides a backup to prevent resources from permanently leaking if the programmer fails to call Dispose.
8. What is close method? How its different from Finalize & Dispose?
9. What is boxing & unboxing?
10. What is check/uncheck?
11. What is the use of base keyword? Tell me a practical example for base keyword’s usage?
12. What are the different .net tools which u used in projects? 

13.What will do to avoid prior case?

14. Will it go to finally block if there is no exception happened?
Ans: Yes. The finally block is useful for cleaning up any resources allocated in the try block. Control is always passed to the finally block regardless of how the try block exits.
15. Is goto statement supported in C#? How about Java?
Gotos are supported in C#to the fullest. In Java goto is a reserved keyword that provides absolutely no functionality.
16. What’s different about switch statements in C#?
No fall-throughs allowed. Unlike the C++ switch statement, C# does not support an explicit fall through from one case label to another. If you want, you can use goto a switch-case, or gotodefault.
case 1:
cost += 25;
case 2:
cost += 25;
goto case 1;