Oops Concept

1.What are the principle concepts of OOPS?

There are four principle concepts upon which object oriented design and programming rest. They are:

  • Abstraction
  • Polymorphism
  • Inheritance
  • Encapsulation
  • (i.e. easily remembered as A-PIE).

2.What is Abstraction?

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

3.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.

4.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.


5.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

6.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.

7.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).

8.Explain the 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


What are the differences between method overloading and method overriding?

  Overloaded Method Overridden Method
Arguments

Must change

Must not change

Return type

Can change

Can’t change except for covariant returns

Exceptions

Can change

Can reduce or eliminate. Must not throw new or broader checked exceptions

Access

Can change

Must not make more restrictive (can be less restrictive)

Invocation

Reference type determines which overloaded version is selected. Happens at compile time.

Object type determines which method is selected. Happens at runtime.

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