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 Q2011. Why doesn't Java even allow void return types for Constructors ?
Ans. Because having a void return type with the Constructor name will make it as a method name. As methods with the Class names are perfectly legit in Java, It needs some way to recognize the constructor and having none return type helps identify that.

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 Q2012. Does Constructor returns the reference of the newly created object ?
Ans. No, New operator actually returns the reference of newly created object. Constructor's purpose is to initialize the default state of the object by setting the initial values of the elements and hence returns nothing.

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   Like         Discuss         Correct / Improve     Constructor

 Q2013. Is New Keyword a method Name ?Core Java
Ans. No, Its an Operator.

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   Like         Discuss         Correct / Improve     new keyword  operator

All these acronyms JDK,JRE,JVM etc are very frequently asked.
 Q2014. Difference between JDK and JRE ?Core Java
Ans. JRE or Java Runtime Environment is the Java Virtual Machine on which class files are executed. It includes borwser plugins that facilitates execution of class files in browsers.

JDK or Java Development Kit includes JRE , compiler and development tools.

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   Like         Discuss         Correct / Improve     JDK  JRE  JDK vs JRE  Difference between     Asked in 4 Companies      basic        frequent

 Q2015. What will happen if we have our own constructor but only initialize some of the member elements ?
Ans. Its always advisable to initialize all the member elements in case we don't use the compiler's default constructor as leftover member elements will have garbage values and may reflect and unstable state for the object.

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   Like         Discuss         Correct / Improve     constructor

 Q2016. Is it advisable to set the member variables through constructors instead of setting them through setters ?
Ans. Yes , If the values to be set are known at the time of initialization and doesn't involve polymorphic behavior.If it's using Dependency Injection , then Constructor injection must be available. If it suffice the above conditions, then definitely its advisable to have them set through constructor as they eagerly load the values into the memory and save it fro multiple values assignment ( one through default constructor and then through assignment )

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   Like         Discuss         Correct / Improve     constructor  setter vs constructor

 Q2017. 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

 Q2018. What are fail-fast Iterators ?Core Java
Ans. Fail-fast iterators detect illegal concurrent modification during iteration and fail quickly and cleanly rather than risking arbitrary, non deterministic behavior at an undetermined time in future. Example could be of an Iterator failing if it smells ConcurrentModificationException.

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   Like         Discuss         Correct / Improve     fail-fast Iterators  Iterators     Asked in 5 Companies      intermediate

Frequently asked. Among first few questions in the J2EE segment of interview.
  Q2019. 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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 Q2020. What are constructors and Destructors in Java ?Core Java
Ans. Constructors are used for initializing the object state once it is initialized and memory has been reserved for it.

Destructor is used to de-allocate memory allocated by objects.

There are no destructors in Java. Alternatively, Java provides Automatic garbage collection i.e automatically releasing the un-referenced memory.


  Sample Code for constructor

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   Like         Discuss         Correct / Improve     constructors  destructors     Asked in 2 Companies
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 Q2021. What are instance methods ?
Ans. All methods defined in a class that are not marked static are actually instance methods.

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   Like         Discuss         Correct / Improve     methods  functions

 Q2022. Can you give a real world example of Encapsulation and Abstraction ?Core Java
Ans. Car Engine is an example of encapsulation and abstraction. You ignite the car using an interface called starter and least bothered about how the tire actually moves (This is abstraction). The engine encapsulates the complete process to itself only and doesn't allow you to start the other components like the radiator etc ( this is excapsulation )

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   Like         Discuss         Correct / Improve     encapsulation  object oriented programming (oops)  oops concepts  abstraction  oops  oops features  java features     Asked in 13 Companies

This question can be asked in different variants like performace improvement of an application, improving performance for DB communication etc.
 Q2023. What are the ways in which execution time of the code can be improved ?Solution
Ans. This depends on entirely upon the code type. For example

1.If its just logic, we can try it to short circuit or put the if / switch case with maximum probability in the beginning.

2. Can use faster data structures, for eg - random retrieval instead of sequence / iterator

3. Working with primitive types or even bytes instead of Objects, even though it may result in marginal improvement.

4. If its service call, then service call with bulk load can help

5. If DB Operation, then with use of Indices , Views or using ORM , cache etc.


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   Like         Discuss         Correct / Improve     performance improvement  code optimization     Asked in 1 Companies

 Q2024. Which of the following Java features facilitates Dependency Injection - Inheritance or Composition ?
Ans. Composition for sure. Inheritance in case the reference of a parent class is used. If its interface, no Inheritance involved.

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   Like         Discuss         Correct / Improve     dependency injection  IOC

 Q2025. What is the difference between declaration, instantiation and initialization ?Core Java
Ans. Declaration is intimation to the compiler about the nature of Data a reference is going to hold.

For example - List myList;

Instantiation is reservation of memory.

For example

myList = new ArrayList();

Initialization or construction is setting the default values for member elements.

For example

myList = new ArrayList(mySet);

** Example 2nd is both for instantiation as well as initialization. The only difference is that 2nd will initialized the member elements to their default values whereas 3rd will initialized it with the elements from set.


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   Like         Discuss         Correct / Improve     declaration   instantiation   initialization   construction   declaration vs instantiation   instantiation vs initialization   declaration vs initialization     Asked in 1 Companies      basic        frequent

 Q2026. What are different types of variables in Java ?
Ans. 1. Function Local Variables
2. Instance Variables
3. Class variables or static variables

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   Like         Discuss         Correct / Improve     variables     Asked in 1 Companies      basic        frequent

  Q2027. 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

 Q2028. Can we reduce the visibility of the overridden method ?Core Java
Ans. No

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   Like         Discuss         Correct / Improve     overriding   access specifier   oops   java   yes-no

  Q2029. 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.
  Q2030. 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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   Like         Discuss         Correct / Improve     java   collections   map   treemap   hashmap   treemap vs hashmap     Asked in 31 Companies      basic        frequent
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Frequently asked in Indian Service Companies. Tech Mahindra ( Based on 3 inputs ) and Infosys ( 3 inputs )
  Q2031. 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.
  Q2032. 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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 Q2033. What is servlet Chaining ?Java EE
Ans. Multiple servlets serving the request in chain.

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   Like         Discuss         Correct / Improve     java   web application   servlets     Asked in 1 Companies        rare

 Q2034. What are the Wrapper classes available for primitive types ?Core Java
Ans. boolean - java.lang.Boolean
byte - java.lang.Byte
char - java.lang.Character
double - java.lang.Double
float - java.lang.Float
int - java.lang.Integer
long - java.lang.Long
short - java.lang.Short
void - java.lang.Void

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   Like         Discuss         Correct / Improve     java   java5   data types   wrapper classes   adapter design pattern        rare

 Q2035. What are concepts introduced with Java 5 ?Core Java
Ans. Generics , Enums , Autoboxing , Annotations and Static Import.

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 Q2036. Does Constructor creates the object ?Core Java
Ans. New operator in Java creates objects. Constructor is the later step in object creation. Constructor's job is to initialize the members after the object has reserved memory for itself.

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   Like         Discuss         Correct / Improve     java   constructor   object creation      intermediate        rare

 Q2037. Can static method access instance variables ?Core Java
Ans. Though Static methods cannot access the instance variables directly, They can access them using instance handler.

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   Like         Discuss         Correct / Improve     static   static method   java   oop   variables     Asked in 1 Companies      basic        frequent

  Q2038. Does Java support Multiple Inheritance ?Core Java
Ans. Java doesn't support multiple inheritance. Interfaces does't facilitate inheritance and hence implementation of multiple interfaces doesn't make multiple inheritance.

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   Like         Discuss         Correct / Improve     java   oop   oop concepts   inheritence   multiple inheritence   basic interview question     Asked in 10 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
  Q2039. 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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Frequently asked at HCL Technologies ( Based on 3 feedback )
  Q2040. 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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