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  Q41. What are Default Methods ?Core Java
Ans. With Java 8, We can provide method definitions in the Interfaces that gets carried down the classes implementing that interface in case they are not overridden by the Class. Keyword "default" is used to mark the default method.

  Sample Code for interface default

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 Q42. What are design situations to use Singleton and Prototype Design Pattern ?Design
Ans. Both are creational design patterns.

Singleton is used when we would like to reuse an object if object is not supposed to hold request or thread specific information. Inversely Prototype is used in situations where we would like to reuse the object information but the request / thread may require it own data to be persisted.

In short, Singleton is used in situations where we can live with single object being shared across multiple requests or threads whereas Prototype is used when we need duplicate copies of objects.

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 Q43. How Marker interfaces works internally ?Core Java
 This question was recently asked at 'GGK Technologies'.This question is still unanswered. Can you please provide an answer.


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   Like         Discuss         Correct / Improve     marker interfaces  interfaces     Asked in 1 Companies      expert        rare


 Q44. Why do we need to specify import statement ? Don't you think Java could have been designed in such a way to automatically import everything that's in the class path ?Core Java
Ans. Java could design it in such a manner but would result in ambiguity if there are multiple files with the same name. The only way to get over it to show the message when you use any such class to provide explicit import with the package prefix. The other problem could be that Java might have to change the early import to Late import and check what's being used to decide what needs to be imported as otherwise you will see errors regarding the ambiguous imports with duplicate file name.

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 Q45. Does use of Final class enforces composition over Inheritance in Java ? Core Java
Ans. Yes, to a certain extent. But the objective for Final class could be beyond just enforcing composition as certain classes might have been created without inheritance or composition in mind.

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   Like         Discuss         Correct / Improve     final class   composition  object oriented programming (oops)  oops concepts  inheritance  object oriented programming (oops)  oops concepts  composition  object oriented programming (oops)  oops concepts vs inheritance  object oriented programming (oops)  oops concepts      expert


 Q46. What could be the reasons for restricting inheritance using final class ?Core Java
Ans. 1. Enforcing composition over inheritance
2. Restricting overriding of certain methods
3. Final methods are faster than regular instance methods
4. Enforcing Immutability

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   Like         Discuss         Correct / Improve     final class   reasons for final class   restricting inheritance  object oriented programming (oops)  oops concepts      expert


 Q47. What will happen if there are no interfaces in Java ?Core Java
Ans. Abstract classes can take care of that to a certain extent. Though they are little heavier than Interfaces but An abstract class with all abstract methods and no instance variables will be able to help with everything that currently an interface does.

The only problem is that a class can only extend one class whereas it can implements multiple interfaces and that is the reason Interfaces were introduced in Java, i.e to get over the problem of multiple inheritance.

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 Q48. Difference between long.Class and Long.TYPE ?
Ans. They both represent the long primitive type. They are exactly the same.

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 Q49. Which memory segment loads the java code ?Core Java
Ans. Code segment.

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 Q50. Can a lock be acquired on a class ?
Ans. Yes, a lock can be acquired on a class. This lock is acquired on the class's Class object.

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 Q51. Shall we use abstract classes or Interfaces in Policy / Strategy Design Pattern ?
Ans. Strategy deals only with decision making at runtime so Interfaces should be used.

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 Q52. Will Compiler creates a default no argument constructor if we specify only multi argument constructor ?
Ans. No, Compiler will create default constructor only if we don't specify any constructor.

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 Q53. What is ThreadFactory ?Core Java
Ans. ThreadFactory is an interface that is meant for creating threads instead of explicitly creating threads by calling new Thread(). Its an object that creates new threads on demand. Using thread factories removes hardwiring of calls to new Thread, enabling applications to use special thread subclasses, priorities, etc.

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Frequently asked these days as there are major changes in Java 8.
  Q54. What are new features introduced with Java 8 ?Core Java
Ans. Lambda Expressions , Interface Default and Static Methods , Method Reference , Parameters Name , Optional , Streams, Concurrency.

  Sample Code for Lambda

  Sample Code for interface default

  Sample Code for Optional

  Sample Code for Streams

  Sample Code for java.time

  Sample Code for Predicate

  Sample Code for Consumer

  Sample Code for MapMerge

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   Like         Discuss         Correct / Improve     java   java8   technical lead   technical architect     Asked in 14 Companies      expert        frequent


 Q55. How is HashSet maintained in memory by Java ?Core Java
Ans. HashSet is maintained as HashMap by Java with values of the HashSet as Keys of the HashMap and value of the HashMap as the constant PRESENT.

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   Like         Discuss         Correct / Improve     ebay  java   collections   hashset   hashmap      expert


 Q56. What are the advantages and disadvantages of CopyOnWriteArrayList ?Core Java
Ans. This collections class has been implemented in such a manner that it can never throw ConcurrentModificationException. As it performs update and write operations by creating a new copy of ArrayList, It's slower compared to ArrayList.

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 Q57. Which UML diagrams you usually use for design ?Design
Ans. Use Case Diagram, Component Diagram for High level Design and Class Diagram , Sequence Diagram for low level design.

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Frequently asked in Tata Consultancy (TCS) interview ( Based on 2 Feedback )
 Q58. What are the technologies used in Jira ?Tools
Ans. JIRA is written in Java and uses the Pico IOC container,Apache OFBiz entity engine, and WebWork technology stack. For remote procedure calls (RPC), JIRA supports REST, SOAP, and XML RPC.

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   Like         Discuss         Correct / Improve     java   jira   issue management   defect management     Asked in 5 Companies      expert


 Q59. Which sorting algorithm is used by Collections.sort() in Java ?Core Java
Ans. The sorting algorithm is a modified mergesort. This algorithm offers guaranteed n log(n) performance.

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 Q60. Which tree is used to implement TreeMap in Java?
Ans. Red Black Tree

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 Q61. How can we catch an exception thrown by another thread ?Core Java
Ans. http://stackoverflow.com/a/6548203/2789764

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 Q62. What is serialVersionUID ?Core Java
Ans. Everytime an object is serialized the java serialization mechanism automatically computes a hash value by passing the meta information for the class. This id is used during deserialization to verify that the sender and receiver of a serialized object have loaded classes for that object that are compatible with respect to serialization

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 Q63. How is hashcode calculated in java ?

or

What are the rules for hashcode calculation ?
Core Java
Ans. General contract of hashCode is:

1.Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer,

2.If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result.

3.It is not required that if two objects are unequal according to the equals(java.lang.Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results.

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 Q64. Can constructor be synchronized in Java ?Core Java
Ans. No, constructors can not be synchronized in Java. In fact using the keyword synchronized with a constructor results in compilation error.

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 Q65. Which memory segment holds String Pool in Java ?Core Java
Ans. String Pool resides in runtime constant Pool which is a part of Heap memory segment.

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 Q66. Why Java doesn't provide a default constructor if we define an argument constructor ?Core Java
Ans. Java provides a default constructor ( no argument ) if no constructor is provided by the programmer. Java makes a check during pre compilation if there is any constructor provided by the programmer. If any constructor is provided, Java understands that programmer has provided all the mechanism for initialization and construction of the object and may not even intend to create objects by calling a no argument constructor. For example - One may like to have objects created with self defined state otherwise restrict its creation.

We make the constructor private if we want to make a class singleton as we would like to restrict creation of new objects using new operator and default constructor. Similar could be the situation wherein we would like to construct objects in a particular manner only.

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 Q67. In Java, would you like to see an explicit destructor?Core Java
Ans. Yes , a gc method to explicitly call garbage collection immediately on the object. It will provide better flexibility over application that require optimal usage of memory.

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 Q68. What are the advantage of Abstract classes over interfaces with respect to Java 7 ? and What changed in Java 8 to help facilitate that in Java 8 ?Core Java
Ans. Abstract Classes provide default implementations of methods that are inherited by the classes that extend them, which was not the case for Interfaces. This changed in Java 8, where default implementations are provided for methods.

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 Q69. Why can we have a static inner class but not a static Top level class in Java ?Core Java
 This question is still unanswered. Can you please provide an answer.


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 Q70. How ConcurrentHashMap provides better performance compared to HashTable ?Core Java
Ans. ConcurrentHashMap uses multiple buckets to store Data and hence avoid read locks which facilitates better performance.

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