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Frequently asked in Cognizant (CTS)
  Q11. 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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Frequently asked. Favorite question in Walk in Drive of many Indian service companies.
  Q12. What are the methods of Object Class ?Core Java
Ans. clone() - Creates and returns a copy of this object.

equals() - Indicates whether some other object is "equal to" this one.

finalize() - Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.

getClass() - Returns the runtime class of an object.

hashCode() - Returns a hash code value for the object.

toString() - Returns a string representation of the object.

notify(), notifyAll(), and wait() - Play a part in synchronizing the activities of independently running threads in a program.

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Very frequently asked. Favorite question in Walk in Drive of many Indian service companies.
  Q13. 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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Almost sure to be asked in every company using any Dependency Injection framework ( Spring, Guice etc )
  Q14. What is Dependency Injection or IOC ( Inversion of Control ) ?Design
Ans. It is a Design Pattern that facilitates loose coupling by sending the dependency information ( object references of dependent object ) while building the state of the object. Objects are designed in a manner where they receive instances of the objects from other pieces of code, instead of constructing them internally and hence provide better flexibility.

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Advanced level question. Recently asked in few Indian service companies ( Based on 3 inputs )
 Q15. What are various types of Class loaders used by JVM ?Core Java
Ans. Bootstrap - Loads JDK internal classes, java.* packages.

Extensions - Loads jar files from JDK extensions directory - usually lib/ext directory of the JRE

System - Loads classes from system classpath.

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  Q16. Difference between SAX and DOM Parser ?Xml
Ans. A DOM (Document Object Model) parser creates a tree structure in memory from an input document whereas A SAX (Simple API for XML) parser does not create any internal structure.

A SAX parser serves the client application always only with pieces of the document at any given time whereas A DOM parser always serves the client application with the entire document no matter how much is actually needed by the client.

A SAX parser, however, is much more space efficient in case of a big input document whereas DOM parser is rich in functionality.

Use a DOM Parser if you need to refer to different document areas before giving back the information. Use SAX is you just need unrelated nuclear information from different areas.

Xerces, Crimson are SAX Parsers whereas XercesDOM, SunDOM, OracleDOM are DOM parsers.

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Very frequently asked.Usually among first few questions.
  Q17. What is MVC ? Design
Ans. MVC is a Design Pattern that facilititates loose coupling by segregating responsibilities in a Web application

1. Controller receives the requests and handles overall control of the request
2. Model holds majority of the Business logic, and
3. View comprise of the view objects and GUI component

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Frequently asked to fresh graduates and less experienced developers.
 Q18. 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 to fresh graduates.
  Q19. What is a Deadlock ?Operating System
Ans. When two threads are waiting each other and cant precede the program is said to be deadlock.

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Frequently asked to fresh graduates.
 Q20. Explain Thread States ?Operating System
Ans. Runnable - waiting for its turn to be picked for execution by the thread schedular based on thread priorities.

Running - The processor is actively executing the thread code. It runs until it becomes blocked, or voluntarily gives up its turn.

Waiting: A thread is in a blocked state while it waits for some external processing such as file I/O to finish.

Sleeping - Java threads are forcibly put to sleep (suspended) with Thread.sleep. they can resume using Thread.resume method.

Blocked on I/O - Will move to runnable after I/O condition like reading bytes of data etc changes.

Blocked on synchronization - Will move to Runnable when a lock is acquired.

Dead - The thread is finished working.

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 Q21. what is covariant return type? Core Java
Ans. co-variant return type states that return type of overriding method can be subtype of the return type declared in method of superclass. it has been introduced since jdk 1.5

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 Q22. What is a ConcurrentHashMap ?Core Java
Ans. ConcurrentHashMap is a hashMap that allows concurrent modifications from multiple threads as there can be multiple locks on the same hashmap.

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Frequently asked in Indian Service Companies. Tech Mahindra ( Based on 3 inputs ) and Infosys ( 3 inputs )
  Q23. 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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  Q24. 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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Frequently asked at HCL Technologies ( Based on 3 feedback )
  Q25. 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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Usually asked only to fresh graduates.
 Q26. What is the difference between time slicing and preemptive scheduling ?Operating System
Ans. In preemptive scheduling, highest priority task continues execution till it enters a not running state or a higher priority task comes into existence. In time slicing, the task continues its execution for a predefined period of time and reenters the pool of ready tasks.

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  Q27. Difference between Vector and ArrayList ?Core Java
Ans. Vectors are synchronized whereas Array lists are not.

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  Q28. 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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Frequently asked to fresh graduates.
  Q29. Difference between Process and Thread ?Operating System
Ans. Process is a program in execution whereas thread is a separate path of execution in a program.

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 Q30. What is race condition ?Operating System
Ans. A source of possible errors in parallel programming, where one thread can cause an error in another thread by changing some aspect of the state of the program that the second thread is depending on (such as the value of variable).

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 Q31. What is meant by an "asynchronous event." Give some examples ?Operating System
Ans. An asynchronous event is one that occurs at an unpredictable time outside the control of the program that the CPU is running. It is not "synchronized" with the program.

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Frequently asked in CTS ( Based on 2 feedback )
  Q32. What is the difference between comparable and comparator in java.util pkg?Core Java
Ans. Comparable interface is used for single sequence sorting i.e.sorting the objects based on single data member where as comparator interface is used to sort the object based on multiple data members.

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  Q33. Advantage of Collection classes over Arrays ?Core Java
Ans. Collections are re-sizable in nature. We can increase or decrease the size as per recruitment.
Collections can hold both homogeneous and heterogeneous data's.
Every collection follows some standard data structures.
Collection provides many useful built in methods for traversing,sorting and search.

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  Q34. Write a Program to check if 2 strings are Anagrams ?Core Java
Ans. public void checkIfAnagram(String str1,String str2){
boolean anagram = true;
for(char c:str1.toCharArray()){
if(!str2.contains(String.valueOf(c))){
System.out.println("Strings are Anagrams");
anagram = false;
}

if(anagram == true){
System.out.println("Strings are not Anagrams");
}
}
}

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Very frequently asked if being interviewed for hibernate. Frequently asked in Tata Consultancy (TCS) and Overstock.com
  Q35. Difference between load and get ?Hibernate
Ans. If id doesnt exist in the DB load throws an exception whereas get returns null in that case.get makes the call to DB immediately whereas load makes the call to proxy.

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 Q36. What is System.in in Java ?Core Java
Ans. It is an InputSream which is usually connected to the keyboard input of console program.

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 Q37. Explain System.out.println ?Core Java
Ans. System is a class within java.lang package that contains several useful class fields and methods. It cannot be instantiated and hence can use only statically.even in this case this has been used statically i.e with class name itself and without creating an instance.

out is the static reference of Printstream declared as following in the System Class -

public final static PrintStream out = null;

println is the method of PrintStream class.

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  Q38. What is database Normalization ?Database
Ans. https://en.wikipedia.org/wiki/Database_normalization

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 Q39. Write a Program to print factorial of a number using recursionCore Java
Ans. public class BuggyBread {
public static void main(String args[]) {
System.out.println(factorial(5));
}

private static int factorial(int number){
if(number == 1){
return 1;
} else {
return number * factorial(number - 1);
}
}
}

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 Q40. What is a Webdriver ?Testing
Ans. Selenium WebDriver is a tool for automating web application testing.It helps in replicating the manual tester behavior like keyboard entry, mouse events etc and then matching the output against the expected.

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