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Frequently asked. Among first few questions in the J2EE segment of interview.
  Q121. 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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 Q122. 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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 Q123. 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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  Q124. 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

  Q125. 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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Very frequently asked. Favorite question in Walk in Drive of many Indian service companies.
  Q126. 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 )
  Q127. 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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Very Frequently asked across all type of companies and across all levels.
  Q128. 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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 Q129. 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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  Q130. 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
  Q131. 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 )
  Q132. 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.
  Q133. 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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Very frequently asked. Favorite question in Walk in Drive of many Indian service companies.
  Q134. 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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Frequently asked.
  Q135. What is an Iterator?Core Java
Ans. Iterator is an interface that provides methods to iterate over any Collection.

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 Q136. Which interface provides the capability to store objects using a key-value pair?Core Java
Ans. java.util.map

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Very frequently asked across all types of companies.
  Q137. 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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 Q138. 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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Frequently asked to fresh graduates.
 Q139. What are the difference between Threads and Processes ?Operating System
Ans. 1. when an OS wants to start running program it creates new process means a process is a program that is currently executing and every process has at least one thread running within it.
2). A thread is a path of code execution in the program, which has its own local variables, program counter(pointer to current execution being executed) and lifetime.
3. When the JavaVirtual Machine (JavaVM, or just VM) is started by the operating system, a new process is created. Within that process, many threads can be created.
4. Consider an example : when you open Microsoft word in your OS and you check your task manger then you can see this running program as a process. now when you write something in opened word document, then it performs more than one work at same time like it checks for the correct spelling, it formats the word you enter , so within that process ( word) , due to different path execution(thread) all different works are done at same time.
5. Within a process , every thread has independent path of execution but there may be situation where two threads can interfere with each other then concurrency and deadlock come is picture.
6. like two process can communicate ( ex:u open an word document and file explorer and on word document you drag and drop another another file from file explorer), same way two threads can also communicate with each other and communication with two threads is relatively low.
7. Every thread in java is created and controlled by unique object of java.lang.Thread class.
8. prior to jdk 1.5, there were lack in support of asynchronous programming in java, so in that case it was considered that thread makes the runtime environment asynchronous and allow different task to perform concurrently.

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 Q140. How can we run a java program without making any object?Core Java
Ans. By putting code within static method. With Java 6 and earlier versions, even static block can be used.

  Sample Code for static block

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Frequently asked to fresh graduates and less experienced developers.
 Q141. Explain multithreading in Java ?Core Java
Ans. 1. Multithreading provides better interaction with the user by distribution of task

2. Threads in Java appear to run concurrently, so it provides simulation for simultaneous activities.The processor runs each thread for a short time and switches among the threads to simulate sim-ultaneous execution (context-switching) and it make appears that each thread has its own processor.By using this feature, users can make it appear as if multiple tasks are occurring simultaneously when, in fact, each is running for only a brief time before the context is switched to the next thread.

3. We can do other things while waiting for slow I/O operations.In the java.iopackage, the class InputStreamhas a method, read(), that blocks until a byte is read from the stream or until an IOExceptionis thrown. The thread that executes this method cannot do anything elsewhile awaiting the arrival of another byte on the stream.

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Frequently asked in all types of companies especially Indian Services companies. Frequently asked in CTS (Based on 2 feedback)
  Q142. What is the use of hashcode in Java ?Core Java
Ans. Hashcode is used for bucketing in Hash implementations like HashMap, HashTable, HashSet etc. The value received from hashcode() is used as bucket number for storing elements. This bucket number is the address of the element inside the set/map. when you do contains() then it will take the hashcode of the element, then look for the bucket where hashcode points to and if more than 1 element is found in the same bucket (multiple objects can have the same hashcode) then it uses the equals() method to evaluate if object are equal, and then decide if contain() is true or false, or decide if element could be added in the set or not.

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 Q143. Why java doesn't support multiple Inheritence ?Core Java
Ans. class A {
void test() {
System.out.println("test() method");
}
}

class B {
void test() {
System.out.println("test() method");
}
}

Suppose if Java allows multiple inheritance like this,

class C extends A, B {
}

A and B test() methods are inheriting to C class.

So which test() method C class will take? As A & B class test() methods are different , So here we would Facing Ambiguity.

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Frequently asked to fresh graduates.
 Q144. What is the difference between yield() and sleep()?Operating System
Ans. When a object invokes yield() it returns to ready state. But when an object invokes sleep() method enters to not ready state.

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 Q145. What are wrapper classes ?Core Java
Ans. They are wrappers to primitive data types. They allow us to access primitives as objects.

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Usually asked only to fresh graduates.
 Q146. What is the initial state of a thread when it is created and started?
Ans. Ready state.

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Frequently asked in Infosys India
  Q147. 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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Advanced level question. Frequently asked in High end product companies. Frequently asked in Google , Cognizant and Deloitte ( Based on 2 feedback )
  Q148. 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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  Q149. 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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  Q150. 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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