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 Q971. What was the driving force to introduce default methods in Interfaces wef from Java 8 ?Core Java
Ans. Earlier any class implementing an interface was supposed to implement all methods declared in an interface. There was no place for optionally implementing all or subset of methods.Though we have abstract classes wherein we could have provided such a mechanism by declaring some methods as abstract while providing definition for some. But as Abstract classes have a body and are comparatively heavier than interfaces and interfaces associate closely to the concept of providing interfacing than abstract classes, Java might have though of providing optional implementation for default methods. This way same interface can be reused in variety of ways rather than making copies of an interface to suit different needs.

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   Like         Discuss         Correct / Improve     java 8   java8  interface default methods  default methods     Asked in 6 Companies

 Q972. Why HashTable has been deprecated ?Core Java
Ans. HashTable has been deprecated. As an alternative, ConcurrentHashMap has been provided. It uses multiple buckets to store data and hence much better performance than HashTable. Moreover, there is already a raw type HashMap. The only difference between the HashTable and HashMap is that Hashtable is synchronized whereas HashMap is not. Most of the synchronized collections have been deprecated and their raw alternative have been presented as preferred.Synchronization has a cost. Using synchronized collection in places where there is no need of it leads to useless utilization of resources. As these collections are rarely used in a static context or shared among threads, Java might have thought it better to just provide the raw collection and let developers implement synchronization if he feels the need to do so. HashMap is now presented as the default and the preferred way of using Map with read optimized hashing, and ConcurrentHashMap has been provided for synchronized access which provides better performance than HashTable. Because of this, Java thought it right to deprecate the use of HashTable.'

Synchronization has a cost. Using synchronized collection at a place where there is hardly any need of it would means useless utilization of resources. As these collections are rarely used in static context or shared among threads, Java might have thought it better to just provide the raw collection and let developer implement synchronization if he feels the need to do so.

As HashMap has been presented as default and preferred way of using Map with read optimized hashing, and ConcurrentHashMap has been provided for synchronized access which provides better performance than HashTable, Java thought it right to deprecate the use of HashTable.

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   Like         Discuss         Correct / Improve     hashtable  synchronized collections  Why synchronized collections have been deprecated  Why HashTable has been deprecated  HashTable vs HashMap      expert
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 Q973. What is the difference between the following two code lines ?

1. new OuterClass().new InnerClass();

2. new OuterClass.InnerClass();		Core Java
Ans. In first case we are trying to initialize Inner class object using the instance of Outer Class whereas in second case we are trying to initialize the Inner class object directly using the Outer class name.

In second case , Inner class is "static inner class" as we cannot access "non static inner class" using Classname alone.

In first case, the inner class could be either "static inner class" or "non static inner class".

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   Like         Discuss         Correct / Improve     inner classes  inner class   static inner class   new operator

 Q974. What is multilevel inheritance ?Core Java
Ans. Multi Level Inheritance is multi level hierarchy of classes. Here is the example - http://www.buggybread.com/2015/09/java-se-class-diagram-classes-that_603.html

Class RoleList extends ArrayList which in turn extends AbstractList which in turn extends AbstractCollection.

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   Like         Discuss         Correct / Improve     multilevel inheritance  object oriented programming (oops)  oops concepts  inheritance  object oriented programming (oops)  oops concepts  oops concepts     Asked in 3 Companies      basic
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 Q975. What is POJO ?
Ans. POJO is Plain Java Object that holds only Member Elements , getters are setters and minimal processing on that. The primary purpose of such object is to either transfer information or keeping state for a while. They are not intended to provide any processing or transformation.

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   Like         Discuss         Correct / Improve     POJO   Plain Java Objects

 Q976. How does the String Class stores the string in memory ?Core Java
Ans. It stores the string as a character array with 2 bytes for each character.

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   Like         Discuss         Correct / Improve     String  String Class

 Q977. What are the disadvantages of running multiple apps in single JVM ?Core Java
Ans. 1. Issue with Jar and ClassPath Conflicts

2. Killing JVM will terminate all applications

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   Like         Discuss         Correct / Improve     JVM     Asked in 1 Companies

 Q978. 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.
 Q979. 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

 Q980. 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.
  Q981. 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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 Q982. 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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 Q983. 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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 Q984. 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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  Q985. 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

 Q986. 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

  Q987. 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.
  Q988. 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 )
  Q989. 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.
  Q990. 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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 Q991. 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

 Q992. 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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 Q993. What are concepts introduced with Java 5 ?Core Java
Ans. Generics , Enums , Autoboxing , Annotations and Static Import.

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 Q994. 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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 Q995. 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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  Q996. 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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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
  Q997. 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 )
  Q998. 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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Very frequently asked. Favorite question in Walk in Drive of many Indian service companies.
  Q999. 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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 Q1000. Which String class does not override the equals() and hashCode() methods, inheriting them directly from class Object?Core Java
Ans. java.lang.StringBuffer.

  Sample Code for StringBuffer

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