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Frequently asked question in companies using Rest Web services.
 Q91. Different between POST and PUT in Rest Rest
Ans. PUT requests are only meant to place the object as it is on server. For example - You want a file to be uploaded and then placed in a particular folder or you want an Employee object to be persisted in the table.

POST requests are also meant to transfer information from client to server but that information once received at the server is evaluated , modified or refactored before persisting.

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   Like         Discuss         Correct / Improve     Put vs Post   Rest     Asked in 5 Companies        frequent

 Q92. What are the fail safe systems ?Operating System
Ans. Fail Safe systems are tolerant systems that continue processing even if they sense any problem. the objective here is to continue with the processing even if there are some problems instead of completely shutting it down. Example could be to catch an exception and still letting it complete with partial results.

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   Like         Discuss         Correct / Improve     fail safe systems   system design   exception handling     Asked in 3 Companies

Frequently asked. Among first few questions in the J2EE segment of interview.
  Q93. What is deployment descriptor ?

or

What is the use of deployment descriptor ?		Java EE
Ans. Deployment Descriptor which is usually web.xml is used to specify the classes, resources and configuration of the application and how the web server uses them to serve web requests.This file is usually added to WEB-INF folder and contains following

* Servlet entries and url mapping
* Plugins
* Some info regarding authentication / filters
* Landing Page
* Event Handlers

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   Like         Discuss         Correct / Improve     web.xml   servlets  deployment descriptor     Asked in 15 Companies      basic        frequent
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  Q94. What is a cookie ?Java EE
Ans. A cookie is a small piece of text stored on a user's computer by the browser for a specific domain. Commonly used for authentication, storing site preferences, and server session identification.

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   Like         Discuss         Correct / Improve     cookie   javascript   web application   session management   browser   j2ee     Asked in 16 Companies      basic        frequent

  Q95. What are the different types of inner classes ?Core Java
Ans. Simple Inner Class,
Local Inner Class,
Anonymous Inner Class,
Static Nested Inner Class.

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   Like         Discuss         Correct / Improve     java   inner classes   classes     Asked in 6 Companies      basic        frequent
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Very frequently asked. Favorite question in Walk in Drive of many Indian service companies.
  Q96. Difference between TreeMap and HashMap ?Core Java
Ans. They are different the way their elements are stored in memory. TreeMap stores the Keys in order whereas HashMap stores the key value pairs randomly.

  Sample Code for treemap

  Sample Code for hashmap

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Frequently asked in Indian Service Companies. Tech Mahindra ( Based on 3 inputs ) and Infosys ( 3 inputs )
  Q97. What is the difference between List, Set and Map ?

or

What are the different Java Collections Interfaces ?		Core Java
Ans. List - Members are stored in sequence in memory and can be accessed through index.
Set - There is no relevance of sequence and index. Sets doesn't contain duplicates whereas multiset can have duplicates.
Map - Contains Key , Value pairs.

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   Like         Discuss         Correct / Improve     java   collections   list   set   map   list vs set vs map     Asked in 8 Companies      basic        frequent
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Very Frequently asked across all type of companies and across all levels.
  Q98. Difference between Public, Private, Default and Protected ?Core Java
Ans. Private - Not accessible outside object scope.

Public - Accessible from anywhere.

Default - Accessible from anywhere within same package.

Protected - Accessible from object and the sub class objects.

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   Like         Discuss         Correct / Improve     java   oop   access specifier   public   private   default   protected   public vs private vs default vs protected     Asked in 12 Companies      basic        frequent
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Very frequently asked. Among first few questions in almost all interviews. Among Top 5 frequently asked questions. Frequently asked in Indian service companies (HCL,TCS,Infosys,Capgemini etc based on multiple feedback ) and Epam Systems
  Q99. Difference between == and .equals() ?Core Java
Ans. "equals" is the method of object class which is supposed to be overridden to check object equality, whereas "==" operator evaluate to see if the object handlers on the left and right are pointing to the same object in memory.

x.equals(y) means the references x and y are holding objects that are equal. x==y means that the references x and y have same object.

Sample code:

String x = new String("str");
String y = new String("str");

System.out.println(x == y); // prints false
System.out.println(x.equals(y)); // prints true

  Sample Code for equals

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   Like         Discuss         Correct / Improve     java   string comparison   string   object class   ==    equals   object equality  operator   == vs equals   equals vs ==     Asked in 294 Companies      basic        frequent
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Frequently asked at HCL Technologies ( Based on 3 feedback )
  Q100. Difference between Checked and Unchecked exceptions ?Core Java
Ans. Checked exceptions are the exceptions for which compiler throws an errors if they are not checked whereas unchecked exceptions are caught during run time only and hence can't be checked.

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   Like         Discuss         Correct / Improve     java   exceptions   checked exceptions   unchecked exceptions   exception handling   checked vs unchecked exceptions     Asked in 39 Companies      basic        frequent
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Very frequently asked. Favorite question in Walk in Drive of many Indian service companies.
  Q101. What is a Final Variable ?Core Java
Ans. Final variable is a variable constant that cannot be changed after initialization.

  Sample Code for final variable

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   Like         Discuss         Correct / Improve     oops   java   final   final variable   basic interview question     Asked in 12 Companies      basic        frequent
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Very frequently asked. Favorite question in Walk in Drive of many Indian service companies.
  Q102. What is a final method ?Core Java
Ans. Its a method which cannot be overridden. Compiler throws an error if we try to override a method which has been declared final in the parent class.

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   Like         Discuss         Correct / Improve     java   oops   final   final method     Asked in 30 Companies      basic        frequent
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Frequently asked.
  Q103. What is an Iterator?Core Java
Ans. Iterator is an interface that provides methods to iterate over any Collection.

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   Like         Discuss         Correct / Improve     java   collections   iterator     Asked in 11 Companies      basic        frequent

Very frequently asked across all types of companies.
  Q104. Difference between HashMap and Hashtable?Core Java
Ans. Hashtable is synchronized whereas HashMap is not.HashMap allows null values whereas Hashtable doesnt allow null values.

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   Like         Discuss         Correct / Improve     java   collections   hashmap   map   hashtable   hashmap vs hashtable     Asked in 14 Companies      basic        frequent
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 Q105. What will this code print ?

String a = new String ("TEST");
String b = new String ("TEST");
if(a == b) {
System.out.println ("TRUE");
} else {
System.out.println ("FALSE");
}		Core Java
Ans. FALSE. == operator compares object references, a and b are references to two different objects, hence the FALSE. .equals method is used to compare string object content.

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   Like         Discuss         Correct / Improve     string   string class   java   ==   object references   coding     Asked in 2 Companies      basic        frequent
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 Q106. What are transient variables in java?Core Java
Ans. Transient variables are variable that cannot be serialized.

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   Like         Discuss         Correct / Improve     java   serialization   transient     Asked in 15 Companies      intermediate        rare

Frequently asked in Infosys India
  Q107. What is a String Pool ?Core Java
Ans. String pool (String intern pool) is a special storage area in Java heap. When a string is created and if the string already exists in the pool, the reference of the existing string will be returned, instead of creating a new object and returning its reference.

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   Like         Discuss         Correct / Improve     java   oops   string   string class   string pool   heap memory     Asked in 31 Companies      intermediate        frequent
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Advanced level question. Frequently asked in High end product companies. Frequently asked in Google , Cognizant and Deloitte ( Based on 2 feedback )
  Q108. Why is String immutable in Java ?Core Java
Ans. 1. String Pool - When a string is created and if it exists in the pool, the reference of the existing string will be returned instead of creating a new object. If string is not immutable, changing the string with one reference will lead to the wrong value for the other references.

Example -

String str1 = "String1";
String str2 = "String1"; // It doesn't create a new String and rather reuses the string literal from pool

// Now both str1 and str2 pointing to same string object in pool, changing str1 will change it for str2 too

2. To Cache its Hashcode - If string is not immutable, One can change its hashcode and hence it's not fit to be cached.

3. Security - String is widely used as parameter for many java classes, e.g. network connection, opening files, etc. Making it mutable might possess threats due to interception by the other code segment.

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   Like         Discuss         Correct / Improve     java   oops   string   string class   immutable  immutability   advanced     Asked in 39 Companies      expert        frequent
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  Q109. Which are the different segments of memory ?Core Java
Ans. 1. Stack Segment - Contains primitives, Class / Interface names and references.

2. Heap Segment - Contains all created objects in runtime, objects only plus their object attributes (instance variables), Static variables are also stored in heap.

3. Code Segment - The segment where the actual compiled Java bytecodes resides when loaded

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   Like         Discuss         Correct / Improve     java   memory   memory management   stack memory   heap memory   code segment memory   advanced     Asked in 9 Companies      expert        frequent
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 Q110. Does garbage collection guarantee that a program will not run out of memory?Core Java
Ans. Garbage collection does not guarantee that a program will not run out of memory. It is possible for programs to use up memory resources faster than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection

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  Q111. What is casting?Core Java
Ans. There are two types of casting, casting between primitive numeric types and casting between object references. Casting between numeric types is used to convert larger values, such as double values, to smaller values, such as byte values. Casting between object references is used to refer to an object by a compatible class, interface, or array type reference

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   Like         Discuss         Correct / Improve     java   data types   casting  type casting   basic interview question     Asked in 8 Companies      basic        frequent

Very frequently asked. Usually asked in different variants like Diff between StringBuffer , String Builder ; Difference between StringBuilder and String class; Choice between these classes etc.
  Q112. What is the difference between StringBuffer and String class ?Core Java
Ans. A string buffer implements a mutable sequence of characters. A string buffer is like a String, but can be modified. At any point in time it contains some particular sequence of characters, but the length and content of the sequence can be changed through certain method calls.

The String class represents character strings. All string literals in Java programs, such as "abc" are constant and implemented as instances of this class; their values cannot be changed after they are created.

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   Like         Discuss         Correct / Improve     java   string   stringbuffer   frequently asked     Asked in 18 Companies      basic        frequent
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  Q113. What is JSON ?Json
Ans. JSON is "JavaScript Object Notation", primarily used for client-server or server-server communication. Its a much lighter and readable alternative to XML. JSON is language independent and is easily parse-able in all programming languages.

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   Like         Discuss         Correct / Improve     json   markup language   client server communication     Asked in 26 Companies      basic        frequent
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Very frequently asked to Fresh graduates.
  Q114. What is the difference between Encapsulation and Abstraction?Core Java
Ans. 1.Abstraction solves the problem at design level while encapsulation solves the problem at implementation level

2.Abstraction is used for hiding the unwanted data and giving relevant data. while Encapsulation means hiding the code and data into a single unit to protect the data from outside world.

3. Abstraction lets you focus on what the object does instead of how it does it while Encapsulation means hiding the internal details or mechanics of how an object does something.

4.For example: Outer Look of a Television, like it has a display screen and channel buttons to change channel it explains Abstraction but Inner Implementation detail of a Television how CRT and Display Screen are connect with each other using different circuits , it explains Encapsulation.

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Very Frequently asked to fresh graduates and less experienced. Favorite question in Walk in drives. Frequently asked in Indian Services companies.
  Q115. Difference between Overloading and Overriding ?Core Java
Ans. Overloading - Similar Signature but different definition , like function overloading.

Overriding - Overriding the Definition of base class in the derived class.

  Sample Code for overloading

  Sample Code for overriding

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  Q116. Difference between Vector and ArrayList ?Core Java
Ans. Vectors are synchronized whereas Array lists are not.

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   Like         Discuss         Correct / Improve     java   basic interview question   vector   arraylist   collections   synchronization   vector vs arraylist     Asked in 35 Companies      basic        frequent

Frequently asked question for intermediate developers. Frequently asked in HCL Technologies and EPAM.
  Q117. What is Volatile keyword used for ?Core Java
Ans. Volatile is a declaration that a variable can be accessed by multiple threads and hence shouldnt be cached.

  Sample Code for volatile

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Frequently asked in Cognizant (CTS)
  Q118. What is Serialization ? Why do we need it ?Core Java
Ans. Storing the state of an object in a file or other medium is called serialization.

Classes can communicate only if they are built together ( as they need Byte code for communication ). What if we need to enable communication between different applications ( i.e they have been built independently or even they reside at different locations ), We need a mechanism that will transfer the Bean state to a Medium than can be transferred to the receiving application.

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  Q119. What is the use of Transient Keyword ?Core Java
Ans. It in Java is used to indicate that a field should not be serialized.

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   Like         Discuss         Correct / Improve     java   oops   serialization   transient   java keywords     Asked in 39 Companies      intermediate        frequent
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Very Frequently asked.Favorite question in Walk in drive for many Indian service companies.
 Q120. What is a Final Class ?
Ans. A Class that cannot be sub classed.

  Sample Code for final class

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