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Interview Questions and Answers - Order By Rating

   
 Q31. When do we generally get this database error - integrity constraint violated - parent key not found ?
Ans. This error most likely will be thrown during an insert statement, while inserting a value within a foreign key column which doesnt exist within the Parent column. For example - Trying to add a dept number reference within a Employee Table when the Dept doesnt exist in the Dept Table.

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   Like         Discuss         Correct / Improve     database  integrity constraint error  integrity constraint violated

Very Frequently asked across all type of companies and across all levels.
  Q32. 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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Frequently asked in Infosys India
  Q33. 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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  Q34. 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 and less experienced. Favorite question in Walk in drives. Frequently asked in Indian Services companies.
  Q35. 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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Frequently asked question for intermediate developers. Frequently asked in HCL Technologies and EPAM.
  Q36. 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 CTS ( Based on 2 feedback )
  Q37. 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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   Like         Discuss         Correct / Improve     java   java.util   comparable   comparator   collections     Asked in 23 Companies      basic        frequent

Very frequently asked.Usually among first few questions.
  Q38. 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 in Infosys and HCL Technologies ( Based on 2 feedback )
 Q39. What are different ways of object creation in Java ?Core Java
Ans. Using new operator - new xyzClass()

Using factory methods - xyzFactory.getInstance( )

Using newInstance( ) method - (Class.forName(xyzClass))emp.newInstance( )

By cloning an already available object - (xyzClass)obj1.clone( )

  Sample Code for object initialization using clone

  Sample Code for object initialization using getInstance

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   Like         Discuss         Correct / Improve     java   oops   object creation   new operator   class.forname   cloning   ebay     Asked in 9 Companies      expert

 Q40. What is Generalization and Specialization in terms of casting ?Core Java
Ans. Generalization or UpCasting is a phenomenon where a sub class is prompted to a super class, and hence becomes more general. Generalization needs widening or up-casting. Specialization or DownCasting is a phenomenon where a super class is narrowed down to a sub class. Specialization needs narrowing or down-casting.

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   Like         Discuss         Correct / Improve     java   data types   casting  type casting   up casting  type casting   downcasting  type casting   generalization   specialization     Asked in 2 Companies

Advanced level question usually asked to senior developers , leads and architects.
 Q41. How does volatile affect code optimization by compiler?Core Java
Ans. Volatile is an instruction that the variables can be accessed by multiple threads and hence shouldn't be cached. As volatile variables are never cached and hence their retrieval cannot be optimized.

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   Like         Discuss         Correct / Improve     java   java keywords   volatile   synchronization   compiler optimization   variable caching   architecture     Asked in 3 Companies      expert

 Q42. What is a Class ?Core Java
Ans. A class is a blue print or Mold using which individual objects are created. A class can contain fields and methods to describe the behavior of an object.

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 Q43. What is constructor chaining and how is it achieved in Java?Core Java
Ans. A child object constructor always first needs to construct its parent. In Java it is done via an implicit call to the no-args constructor as the first statement

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  Q44. What is the use of HashCode in objects ?Core Java
Ans. Hashcode is used for bucketing in Hash implementations like HashMap, HashTable, HashSet etc.

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   Like         Discuss         Correct / Improve     java   string   hashcode   hash code   string comparison  hashtable     Asked in 17 Companies      basic        frequent

Frequently asked to fresh graduates and less experienced.
  Q45. Difference between Composition and Inheritance ?Core Java
Ans. Inheritance means a object inheriting reusable properties of the base class. Compositions means that an abject holds other objects.

In Inheritance there is only one object in memory ( derived object ) whereas in Composition , parent object holds references of all composed objects.

From Design perspective - Inheritance is "is a" relationship among objects whereas Composition is "has a" relationship among objects.

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 Q46. What does String intern() method do?Core Java
Ans. intern() method keeps the string in an internal cache that is usually not garbage collected.

Moreover provide reference for scp object for corresponding string object present in heap memory.

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   Like         Discuss         Correct / Improve     java   string class   string   intern method   garbage collection   advanced     Asked in 2 Companies      expert

  Q47. Can we have null keys in TreeMap ?Core Java
Ans. No, results in exception.

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   Like         Discuss         Correct / Improve     java   collections   treemap     Asked in 6 Companies      Basic        frequent

Very frequently asked. Favorite question in Walk in Drive of many Indian service companies.
  Q48. What is the difference between ArrayList and LinkedList ?Core Java
Ans. Underlying data structure for ArrayList is Array whereas LinkedList is the linked list and hence have following differences -

1. ArrayList needs continuous memory locations and hence need to be moved to a bigger space if new elements are to be added to a filled array which is not required for LinkedList.

2. Removal and Insertion at specific place in ArrayList requires moving all elements and hence leads to O(n) insertions and removal whereas its constant O(1) for LinkedList.

3. Random access using index in ArrayList is faster than LinkedList which requires traversing the complete list through references.

4. Though Linear Search takes Similar Time for both, Binary Search using LinkedList requires creating new Model called Binary Search Tree which is slower but offers constant time insertion and deletion.

5. For a set of integers you want to sort using quicksort, it's probably faster to use an array; for a set of large structures you want to sort using selection sort, a linked list will be faster.

  Sample Code for ArrayList

  Sample Code for LinkedList

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  Q49. Which Software Development methodology is being used in your current Job ?General
Ans. We are using Agile methodology. I attend daily stand up where the development leads takes the status of assigned stories, achievements, any bottlenecks or challenges. We follow iteration of 2 weeks.

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Very Frequently asked. Have been asked in HCL Technologies very frequently ( based on 3 feedback ). Among first few questions in many interviews.
  Q50. Differences between abstract class and interface ?Core Java
Ans. Abstract classes can have both abstract methods ( method declarations ) as well as concrete methods ( inherited to the derived classes ) whereas Interfaces can only have abstract methods ( method declarations ).

A class can extend single abstract class whereas it can implement multiple interfaces.

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 Q51. Which of the following combination of keywords is illegal in Java ?

a. static and transient
b. transient and final
c. static and synchronized
d. abstract and final		Core Java
Ans. abstract and final

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Frequently asked in face to face interviews.
  Q52. Write a program to print fibonacci series.Core Java
Ans. int count = 15;
int[] fibonacci = new int[count];
fibonacci[0] = 0;
fibonacci[1] = 1;
for(int x=2; x < count; x++){
fibonacci[x] = fibonacci[x-1] + fibonacci[x-2];
}

for(int x=0; x< count; x++){
System.out.print(fibonacci[x] + " ");
}

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 Q53. Why can't we declare a class abstract as well as final ?Core Java
Ans. Abstract means that the class is only meant to be subclassed whereas final means that it cannot be subclassed so both concepts - abstract and final are actually mutually exclusive and hence not permitted together.

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 Q54. The use of volatile keyword facilitates ..Core Java
a. Making Use of Cache for better Performance
b. Avoiding use of Cache
c. Making use of Backward as well as Forward Cache
d. Keeping only one copy of variable in Cache

Ans.b. Avoiding use of Cache
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