OOPs Interview Questions

OOPs FAQs : Object Oriented Interview Questions

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

• Struct are Value type and are stored on stack, while Class are Reference type and are stored on heap.
• Struct “do not support” inheritance, while class supports inheritance. However struct can implements interface.
• 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.

Two uses of this keyword.

o Can be used to refer to the current object.

o It can also be used by one constructor to explicitly invoke another constructor of the same class.

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

• A constructor is a special method whose task is to initialize the object of its class.
• It is special because its name is the same as the class name.
• They do not have return types, not even void and therefore they cannot return values.
• They cannot be inherited, though a derived class can call the base class constructor.
• Constructor is invoked whenever an object of its associated class is created.
• 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().

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

Methods that you can call directly without first creating an instance of a class. Eg: Main() Method, Console.WriteLine()

You can use static fields, methods, properties and even constructors which will be called before any instance of the class is created.

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.

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 classEach 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

A destructor is just opposite to constructor.

It has same as the class name, but with prefix ~ (tilde).

They do not have return types, not even void and therefore they cannot return values.

destructor is invoked whenever an object is about to be garbage collected

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

• Garbage collection is the mechanism that reclaims the memory resources of an object when it is no longer referenced by a variable.
• .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().
• Advantage of Garbage collection : It prevents programming error that could otherwise occur by incorrectly deleting or failing to delete objects.

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.

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

As name suggest Value Type stores “value” directly.

For eg:

//I and J are both of type int

I = 20;

J = I;

int is a value type, which means that the above statements will results in two locations in memory.

For each instance of value type separate memory is allocated.

Stored in a Stack.

It Provides Quick Access, because of value located on stack.

Reference Type

As name suggest Reference Type stores “reference” to the value.

For eg:

Vector X, Y; //Object is defined. (No memory is allocated.)

X = new Vector(); //Memory is allocated to Object. //(new is responsible for allocating memory.)

X.value = 30; //Initialising value field in a vector class.

Y = X; //Both X and Y points to same memory location. //No memory is created for Y.

Console.writeline(Y.value); //displays 30, as both points to same memory

Y.value = 50;

Console.writeline(X.value); //displays 50.

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;

Reference type are stored on Heap.

It provides comparatively slower access, as value located on heap.

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

Multiple inheritance of classes is not allowed in C#.

In C# you can implements more than one interface, thus multiple inheritance is achieved through interface.

The Object class defined in the System namespace is implicitly the ultimate base class of all the classes in C# (and the .NET framework)

Structures (struct) in C# does not support inheritance, it can only implements interfaces.

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.

1) Public – Allows the members to be globally accessible.

2) Private – Limits the member’s access to only the containing type.

3) Protected – Limits the member’s access to the containing type and all classes derived from the containing type.

4) Internal – Limits the member’s access to within the current project.

Method Overloading

Method with same name but with different arguments is called method overloading.

Method Overloading forms compile-time polymorphism.

Eg:

class A1

{

void hello()

{ Console.WriteLine(“Hello”); }

void hello(string s)

{ Console.WriteLine(“Hello {0}”,s); }

}

Method Overriding

Method overriding occurs when child class declares a method that has the same type arguments as a method declared by one of its superclass.

Method overriding forms Run-time polymorphism.

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.

Eg1:

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.