OOPS Concept

Brief Introduction to OOP
Object Oriented Programming or OOP is the technique to create programs based on the real world. Unlike procedural programming, here in the OOP programming model programs are organized around objects and data rather than actions and logic. Objects represent some concepts or things and like any other objects in the real Objects in programming language have certain behavior, properties, type, and identity. In OOP based language the principal aim is to find out the objects to manipulate and their relation between each other. OOP offers greater flexibility and compatibility and is popular in developing larger application. Another important work in OOP is to classify objects into different types according to their properties and behavior. So OOP based software application development includes the analysis of the problem, preparing a solution, coding and finally its maintenance.

Java is a object oriented programming  and to understand the functionality of OOP in Java, we first need to understand several fundamentals related to objects. These include class, method, inheritance, encapsulation, abstraction, polymorphism etc.

Class

It is the central point of OOP and that contains data and codes with behavior. In Java everything happens within class and it describes a set of objects with common behavior. The class definition describes all the properties, behavior, and identity of objects present within that class. As far as types of classes are concerned, there are predefined classes in languages like C++ and Pascal. But in Java one can define his/her own types with data and code.  

Object 

 Objects are the basic unit of object orientation with behavior, identity. As we mentioned above, these are part of a class but are not the same. An object is expressed by the variable and methods within the objects. Again these variables and methods are distinguished from each other as instant variables, instant methods and class variable and class methods. 

Methods 

  We know that a class can define both attributes and behaviors. Again attributes are defined by variables and behaviors are represented by methods. In other words, methods define the abilities of an object. 

Inheritance

This is the mechanism of organizing and structuring software program. Though objects are distinguished from each other by some additional features but there are objects that share certain things common. In object oriented programming classes can inherit some common behavior and state from others. Inheritance in OOP allows to define a general class and later to organize some other classes simply adding some details with the old class definition. This saves work as the special class inherits all the properties of the old general class and as a programmer you only require the new features. This helps in a better data analysis, accurate coding and reduces development time. 

Abstraction 

The process of abstraction in Java is used to hide certain details and only show the essential features of the object. In other words, it deals with the outside view of an object (interface). 

Encapsulation 

This is an important programming concept that assists in separating an object's state from its behavior. This helps in hiding an object's data describing its state from any further modification by external component. In Java there are four different terms used for hiding data constructs and these are public, private, protected and package. As we know an object can associated with data with predefined classes and in any application an object can know about the data it needs to know about. So any unnecessary data are not required by an object can be hidden by this process. It can also be termed as information hiding that prohibits outsiders in seeing the inside of an object in which abstraction is implemented.  

Polymorphism 

It describes the ability of the object in belonging to different types with specific behavior of each type. So by using this, one object can be treated like another and in this way it can create and define multiple level of interface. Here the programmers need not have to know the exact type of object in advance and this is being implemented at runtime.

Q1) What is polymorphism? Ans) Polymorphism gives us the ultimate flexibility in extensibility. The abiltiy to define more than one function with the same name is called Polymorphism. In java,c++ there are two type of polymorphism: compile time polymorphism (overloading) and runtime polymorphism (overriding). When you override methods, JVM determines the proper methods to call at the program’s run time, not at the compile time. Overriding occurs when a class method has the same name and signature as a method in parent class. Overloading occurs when several methods have same names with Overloading is determined at the compile time. Different method signature and different number or type of parameters. Same method signature but different number of parameters. Same method signature and same number of parameters but of different type Example of Overloading      int add(int a,int b)      float add(float a,int b)      float add(int a ,float b)      void add(float a)      int add(int a)      void add(int a)                 //error conflict with the method int add(int a) Example: Overloading Class BookDetails{             String title;             String publisher;             float price; setBook(String title){ } setBook(String title, String publisher){ } setBook(String title, String publisher,float price){ } } Example: Overriding class BookDetails{             String title; setBook(String title){ } } class ScienceBook extends BookDetails{             setBook(String title){}                                             //overriding setBook(String title, String publisher,float price){ }  //overloading }

Q2) What is inheritance? Ans) Inheritance is the property which allows a Child class to inherit some properties from its parent class. In Java this is achieved by using extends keyword. Only properties with access modifier public and protected can be accessed in child class. public class Parent{ public String parentName; public int parentage; public String familyName; } public class Child extends Parent{ public String childName; public int childAge; public void printMyName(){ System.out.println(“ My name is “+ chidName+” “ +familyName) } } In above example the child has inherit its family name from the parent class just by inheriting the class.

Q3) What is multiple inheritance and does java support? Ans) If a child class inherits the property from multiple classes is known as multiple inheritance. Java does not allow to extend multiple classes but to overcome this problem it allows to implement multiple Interfaces.

Q4) What is abstraction? Ans) Abstraction is way of converting real world objects in terms of class. For example creating a class Vehicle and injecting properties into it. E.g public class Vehicle { public String colour; public String model; }

Q5) What is encapsulation? Ans) The encapsulation is achieved by combining the methods and attribute into a class. The class acts like a container encapsulating the properties. The users are exposed mainly public methods.The idea behind is to hide how thinigs work and just exposing the requests a user can do.

Q6) What is Association? Ans) Association is a relationship between two classes. In this relationship the object of one instance perform an action on behalf of the other class. The typical behaviour can be invoking the method of other class and using the member of the other class. public class MyMainClass{ public void init(){ new OtherClass.init(); } }

Q7) What is Aggregation? Ans) Aggregation has a relationship between two classes. In this relationship the object of one class is a member of the other class. Aggregation always insists for a direction. public class MyMainClass{ OtherClass otherClassObj = new OtherClass(); }

Q8) What is Composition? Ans) Composition is a special type of aggregation relationship with a difference that its the compulsion for the OtherClass object (in previous example) to exist for the existence of MyMainClass.