Session and Cookies

Cookies

Definition: – A cookie is information that a Website puts on your hard disk so that it can remember something about you at a later time. (More technically, it is information for future use that is stored by the server on the client side of a client/server communication.)

In short, Cookie is client’s information for particular site stored on client’s PC.

Typically, a cookie records your preferences when using a particular site. Using the Web’s Hypertext Transfer Protocol (HTTP), each request for a Web page is independent of all other requests. For this reason, the Web page server has no memory of what pages it has sent to a user previously or anything about your previous visits. A cookie is a mechanism that allows the server to store its own information about a user on the user’s own computer.

Procedure:-  following are steps show how cookies works

1. When you visit some site, server for that site stores information (user name or simply physical address of user’s pc) in 1 text file. For that, that site may ask you to fill details about yourself  or  it can simple fetch physical address of your computer.
2. Server sends this file to client with web page and that file is saved in client’s pc.
3. Now when user visits that website again, that cookie file is also sent to server with the web page request.
4. From that file, website’s sever can identify that particular user and do further procedures (Ex.  Prepare customized webpage for that user) using that information.

Example:-  Best example for working of cookies is “iGoogle” website. Follow these steps to see it:

1. Visit www.google.com.
2. Click on link written ‘iGoogle’ on top-right side of the page. You will see customized page of google. (When you click on this link, Physical address (Mind you…physical address  which always remains unique. While IPaddress of your pc may change) of your pc is stored in google’s server)
3. You can customize that page according to your need and that page information is saved in one text file and sent back to client’s pc.
4. Close that window.
5. Again go to IE and visit www.google.com. You will see customized page you have created before. This is because when you visit same page again, your pc send that cookie also with webpage request. Google server resolve that file and come to know that that client is having customized page and send that customized page instead of normal page

(Clear cookies from your pc and visit the same page and see what happens!!!!!!)

  Types of Cookies: session cookie
Also called a transient cookie, a cookie that is erased when you close the Web browser. The session cookie is stored in temporary memory and is not retained after the browser is closed. Session cookies do not collect information from yourcomputer. They typically will store information in the form of a session identification that does not personally identify the user.persistent cookie
Also called a permanent cookie, or a stored cookie, a cookie that is stored on your hard drive until it expires (persistent cookies are set with expiration dates) or until you delete the cookie. Persistent cookies are used to collect identifying information about the user, such as Web surfing behavior or user preferences for a specific Web site.  

Session

Definition: – A session is information of user that a Website puts on its temporary memory so that it can remember something about you when you are visiting that site. (More technically, it is information for future use that is stored by the server on the server side of a client/server communication.)

 In short, Session is client’s information for particular site stored on Server’s temporary memory. The period of time a user interfaces with an application. The user session begins when the user accesses the application and ends when the user quits the application The number of user sessions on a site is used in measuring the amount of traffic a website gets. The site administrator determines what the time frame of a user session will be (e.g., 30 minutes). If the visitor comes back to the site within that time period, it is still considered one user session because any number of visits within those 30 minutes will only count as one session. If the visitor returns to the site after the allotted time period has expired, say an hour from the initial visit, then it is counted as a separate user session. Contrast with unique visitor, hit, click-through and page view, which are all other ways that site administrators measure the amount of traffic a Web site gets. Example:- When you visit www.gmail.com , you have to enter your account ID and password first. That information is verified on server and stored on server in session until that user is logged on. 

What is the difference between session and cookie?If you set the variable to “cookies”, then your users will not have to log in each time they enter your community.

The cookie will stay in place within the user’s browser until the user deletes it.

But Sessions are popularly used, as the there is a chance of your cookies getting blocked if the user browser security setting is set high.

If you set the variable to “sessions”, then user activity will be tracked using browser sessions, and your users will have to log in each time they re-open their browser. Additionally, if you are using the “sessions” variable, you need to secure the “sessions” directory, either by placing it above the web root or by requesting that your web host make it a non-browsable directory.

The Key difference would be cookies are stored in your hard disk whereas a session aren’t stored in your hard disk. Sessions are basically like tokens, which are generated at authentication. A session is available as long as the browser is opened.

  1) session should work regardless of the settings on the client browser. even if users decide to forbid the cookie (through browser settings) session still works. there is no way to disable sessions from the client browser.
2) session and cookies differ in type and amount of information they are capable of storing.
Javax.servlet.http.Cookie class has a setValue() method that accepts Strings. javax.servlet.http.HttpSession has a setAttribute() method which takes a String to denote the name and java.lang.Object which means that HttpSession is capable of storing any java object. Cookie can only store String objects

OOPS Interview Questions and Answers (part 1)

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1) What is meant by Object Oriented Programming?
     OOP is a method of programming in which programs are organised as cooperative collections of objects. Each object is an instance of a class and each class belong to a hierarchy.

2) What is a Class?
     Class is a template for a set of objects that share a common structure and a common behaviour.

3) What is an Object?
     Object is an instance of a class. It has state,behaviour and identity. It is also called as an instance of a class.

4) What is an Instance?
     An instance has state, behaviour and identity. The structure and behaviour of similar classes are defined in their common class. An instance is also called as an object.

5) What are the core OOP’s concepts?
     Abstraction, Encapsulation,Inheritance and Polymorphism are the core OOP’s concepts.

6) What is meant by abstraction?
     Abstraction defines the essential characteristics of an object that distinguish it from all other kinds of objects. Abstraction provides crisply-defined conceptual boundaries relative to the perspective of the viewer. Its the process of focussing on the essential characteristics of an object. Abstraction is one of the fundamental elements of the object model.

7) What is meant by Encapsulation?
     Encapsulation is the process of compartmentalising the elements of an abtraction that defines the structure and behaviour. Encapsulation helps to separate the contractual interface of an abstraction and implementation.

8) What is meant by Inheritance?
     Inheritance is a relationship among classes, wherein one class shares the structure or behaviour defined in another class. This is called Single Inheritance. If a class shares the structure or behaviour from multiple classes, then it is called Multiple Inheritance. Inheritance defines “is-a” hierarchy among classes in which one subclass inherits from one or more generalised superclasses.

9) What is meant by Polymorphism?
     Polymorphism literally means taking more than one form. Polymorphism is a characteristic of being able to assign a different behavior or value in a subclass, to something that was declared in a parent class.

10) What is an Abstract Class?
     Abstract class is a class that has no instances. An abstract class is written with the expectation that its concrete subclasses will add to its structure and behaviour, typically by implementing its abstract operations.

11) What is an Interface?
     Interface is an outside view of a class or object which emphaizes its abstraction while hiding its structure and secrets of its behaviour.

12) What is a base class?
     Base class is the most generalised class in a class structure. Most applications have such root classes. In Java, Object is the base class for all classes.

13) What is a subclass?
     Subclass is a class that inherits from one or more classes

14) What is a superclass?
     superclass is a class from which another class inherits.

15) What is a constructor?
     Constructor is an operation that creates an object and/or initialises its state.

16) What is a destructor?
     Destructor is an operation that frees the state of an object and/or destroys the object itself. In Java, there is no concept of destructors. Its taken care by the JVM.

17) What is meant by Binding?
     Binding denotes association of a name with a class.

18) What is meant by static binding?
     Static binding is a binding in which the class association is made during compile time. This is also called as Early binding.

19) What is meant by Dynamic binding?
     Dynamic binding is a binding in which the class association is not made until the object is created at execution time. It is also called as Late binding.

20) Define Modularity?
     Modularity is the property of a system that has been decomposed into a set of cohesive and loosely coupled modules.

21) What is meant by Persistence?
     Persistence is the property of an object by which its existence transcends space and time.

22) What is colloboration?
     Colloboration is a process whereby several objects cooperate to provide some higher level behaviour.

23) In Java, How to make an object completely encapsulated?
     All the instance variables should be declared as private and public getter and setter methods should be provided for accessing the instance variables.

24) How is polymorphism acheived in java?
     Inheritance, Overloading and Overriding are used to acheive Polymorphism in java.