OOP concepts

                                    Java is an object oriented programming language. Object-oriented programming is at the core of Java. Object Oriented Programming or OOP is the technique to create programs based on the real world. All java programs are object oriented.

          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 and polymorphism.

• Classes and Objects:

               A class is a blueprint for an object, and that nearly everything in java is an object. All java code is defined in a class. A class describes how to make an object of that class type. Class is the central point of OOP and that contains data and codes with behavior. The class definition describes all the properties, behavior, and identity of objects present within that class. In Java everything happens within class and it describes a set of objects with common behavior.

             Objects are the basic unit with identity and behavior, and object can take care of itself; you don’t have to know or care how the object does it. An object knows things and does things. Things an object knows about itself are called instance variables. They represent the state of an object Things an object does are called methods. They represent the behavior of an object.

• Abstraction:

              An abstraction is an essential element of object-oriented programming. An abstraction is the way of converting real world objects in terms of class. 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). For example creating a class Vehicle and injecting properties into it.

Example:

public class Vehicle {

public String colour;
public String model;
}

• Encapsulation:

               Encapsulation is the mechanism that binds together code and the data it manipulates, and keeps both safe from outside interference and misuse. 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. One way to think about encapsulation is as a protective wrapper that prevents the code and data from being arbitrarily accessed by other code defined outside the wrapper.

• Inheritance:

              The process by which one object acquires the properties of another object is known as inheritance. 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. 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. 

• Polymorphism:

               Polymorphism (many forms) is a feature that allows one interface to be used for a general class of actions. 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. Polymorphism gives us the ultimate flexibility in extensibility. The ability to define more than one function with the same name is called Polymorphism. In java, 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.