Interview Questions on OOPS

20 Core OOPS Questions and Answers

  • What is an Object ?
  • What is Class ?
  • What is a method In object-oriented programming?
  • What are different oops concept in java?
  • What is Polymorphism? What is Inheritance?
  • What is Abstraction? What is Encapsulation?
  • What is the difference between abstraction and encapsulation?
  • Different forms of Polymorphism?
  • How does Java implement polymorphism?
  • What is runtime polymorphism or dynamic method dispatch?
  • What is Dynamic Binding? What is method overloading?
  • What is method overriding?
  • Can overloaded methods be override too?
  • How to invoke a superclass version of an Overridden method?
  • What is super?
  • How do you prevent a method from being overridden?
  • Difference between Inheritance and Composition in Java OOPS?
What is an Object ?
Well It is the most basic concept and a key to understand the Object-Oriented programming . Object is an entity that has two characteristics , State and Behavior . Some examples of real world object can be : Bike , Chair , Dog etc. Lets take an example of a Bike . Bike has some state ( current gear , current speed ) and behavior ( change gear , apply brake ) . One way to begin thinking in an object-oriented way is to identify the state and behavior of real world objects . Software objects are also similar to the real world objects. They too contains State and Behavior . An Object stores its state in fields and exposes the behavior through methods.

What is Class ?
Class is a blueprint from which objects of same type can be created . Lets take an example of a Bike again . There are thousands of bikes with the same characteristics i.e having same make and model . They are created from the same prototype / blueprint called class.

What is a Method In object-oriented programming?
In object-oriented programming, a method is a programmed procedure that is defined as part of a class and included in any object of that class.

What are different oops concept in java?
OOPs stands for Object Oriented Programming. The concepts in oops are similar to any other programming languages. Basically, it is program agnostic.
The different OOps concepts are :
Polymorphism
Inheritance
Abstraction
Encapsulation

What is Polymorphism?
Polymorphism is briefly described as “one interface, many implementations.” Polymorphism is a characteristic of being able to assign a different meaning or usage to something in different contexts – specifically, to allow an entity such as a variable, a function, or an object to have more than one form.

What is Inheritance?
Inheritance is the process by which objects of one class acquire the properties of objects of another class.
A class that is inherited is called a superclass.
The class that does the inheriting is called a subclass.
Inheritance is done by using the keyword extends.
The two most common reasons to use inheritance are:
To promote code reuse
To use polymorphism

What is Abstraction?
Abstraction refers to the act of representing essential features without including the background details or explanations.

What is Encapsulation?
Encapsulation is a technique used for hiding the properties and behaviors of an object and allowing outside access only as appropriate. It prevents other objects from directly altering or accessing the properties or methods of the encapsulated object.

What is the difference between abstraction and encapsulation?
Abstraction focuses on the outside view of an object (i.e. the interface) Encapsulation (information hiding) prevents clients from seeing it’s inside view, where the behavior of the abstraction is implemented.
Abstraction solves the problem in the design side while Encapsulation is the Implementation.
Encapsulation is the deliverables of Abstraction. Encapsulation barely talks about grouping up your abstraction to suit the developer needs

Different forms of Polymorphism.
There are two types of polymorphism one is Compile time polymorphism and the other is run time polymorphism. Compile time polymorphism is method overloading. Runtime time polymorphism is done using inheritance and interface.
Note: From a practical programming viewpoint, polymorphism manifests itself in three distinct forms in Java:
Method overloading
Method overriding through inheritance
Method overriding through the Java interface

How does Java implement polymorphism?
(Inheritance, Overloading and Overriding are used to achieve Polymorphism in java).
Polymorphism manifests itself in Java in the form of multiple methods having the same name.
In some cases, multiple methods have the same name, but different formal argument lists (overloaded methods).
In other cases, multiple methods have the same name, same return type, and same formal argument list (overridden methods).

What is runtime polymorphism or dynamic method dispatch?
In Java, runtime polymorphism or dynamic method dispatch is a process in which a call to an overridden method is resolved at runtime rather than at compile-time. In this process, an overridden method is called through the reference variable of a superclass. The determination of the method to be called is based on the object being referred to by the reference variable.

What is Dynamic Binding?
Binding refers to the linking of a procedure call to the code to be executed in response to the call. Dynamic binding (also known as late binding) means that the code associated with a given procedure call is not known until the time of the call at run-time. It is associated with polymorphism and inheritance.

What is method overloading?
Method Overloading means to have two or more methods with same name in the same class with different arguments. The benefit of method overloading is that it allows you to implement methods that support the same semantic operation but differ by argument number or type.
Note:
Overloaded methods MUST change the argument list
Overloaded methods CAN change the return type
Overloaded methods CAN change the access modifier
Overloaded methods CAN declare new or broader checked exceptions
A method can be overloaded in the same class or in a subclass

What is method overriding?
Method overriding occurs when sub class declares a method that has the same type arguments as a method declared by one of its superclass. The key benefit of overriding is the ability to define behavior that’s specific to a particular subclass type.
Note:
The overriding method cannot have a more restrictive access modifier than the method being overridden (Ex: You can’t override a method marked public and make it protected).
You cannot override a method marked final
You cannot override a method marked static
Can overloaded methods be override too?
Yes, derived classes still can override the overloaded methods. Polymorphism can still happen. Compiler will not binding the method calls since it is overloaded, because it might be overridden now or in the future.

Can we override the main method?
NO, because main is a static method. A static method can’t be overridden in Java.

How to invoke a superclass version of an Overridden method?
To invoke a superclass method that has been overridden in a subclass, you must either call the method directly through a superclass instance, or use the super prefix in the subclass itself. From the point of the view of the subclass, the super prefix provides an explicit reference to the superclass’ implementation of the method.
// From subclass
super.overriddenMethod();

Use of  super?
super is a keyword which is used to access the method or member variables from the superclass. If a method hides one of the member variables in its superclass, the method can refer to the hidden variable through the use of the super keyword. In the same way, if a method overrides one of the methods in its superclass, the method can invoke the overridden method through the use of the super keyword.
Note:
You can only go back one level.
In the constructor, if you use super(), it must be the very first code, and you cannot access any this.xxx variables or methods to compute its parameters.

How do you prevent a method from being overridden?
To prevent a specific method from being overridden in a subclass, use the final modifier on the method declaration, which means “this is the final implementation of this method”, the end of its inheritance hierarchy.
public final void exampleMethod() {
// Method statements
}

Difference between Inheritance and Composition in Java OOPS
Both Inheritance and Composition provides code reusablility,below are the difference

Inheritance Composition
Inheritance you must extend the class for any reuse of code or functionality Composition allows reuse of code without extending it
For Final class Inheritance cannot reuse code By using Composition you can reuse code for even final class which is not extensible
Inheritance you can reuse code form just one class because in Java you can only extend one class, because multiple Inheritance is not supported in Java Composition you can reuse code from many classes as they are declared as just a member variable
   

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