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 Q31. What things should be kept in mind while creating your own exceptions in Java?Core Java
Ans. All exceptions must be a child of Throwable.

If you want to write a checked exception that is automatically enforced by the Handle or Declare Rule, you need to extend the Exception class.

You want to write a runtime exception, you need to extend the RuntimeException class.

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   Like         Discuss         Correct / Improve     java   exceptions   exception handling   user defined exceptions   throwable   architecture   library development   technical architect   technical lead

 Q32. What is JDBC? Describe the steps needed to execute a SQL query using JDBC.Database
Ans. The JDBC is a pure Java API used to execute SQL statements. It provides a set of classes and interfaces that can be used by developers to write database applications.

The steps needed to execute a SQL query using JDBC:

1. Open a connection to the database.
2. Execute a SQL statement.
3. Process th results.
4. Close the connection to the database.

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   Like         Discuss         Correct / Improve     java   jdbc   db connectivity     Asked in 2 Companies      basic        frequent

 Q33. How to implement an immutable class ?Core Java
Ans. We can make a class immutable by

1. Making all methods and variables as private.

2. Setting variables within constructor.

Public Class ImmutableClass{

private int member;
ImmutableClass(int var){
member=var;
}
}

and then we can initialize the object of the class as

ImmutableClass immutableObject = new ImmutableClass(5);

Now all members being private , you cant change the state of the object.

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 Q34. What things you would care about to improve the performance of Application if its identified that its DB communication that needs to be improved ?Solution
Ans. 1. Query Optimization ( Query Rewriting , Prepared Statements )

2. Restructuring Indexes.

3. DB Caching Tuning ( if using ORM )

4. Identifying the problems ( if any ) with the ORM Strategy ( If using ORM )

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   Like         Discuss         Correct / Improve     java   db   database   hibernate   orm   at&t   overstock.com   performance improvement   architecture   technical lead   architect      intermediate

 Q35. Why do we need Thread class even in case we execute thread using runnable interface ?Core Java
Ans. Thread class holds the definition of start method ( This is the method that starts execution of new thread and then calls run method within the scope of new thread ). Interfaces don't hold any definition and so does runnable. So it makes it necessary the usage of Thread class , whatever implementation you choose.

When your class extends the thread class, it carries the definition of start method from parent Thread class onto itself and hence new yourClass.start() helps starting a new thread and then executing run method in that new thread scope.

When you implement runnable interface , you are just making it sure to the JVM that you have implemented the required method ( run() ) which the Thread start method will look for upon executing start method.

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 Q36. 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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 Q37. Difference between Region , Availability zone and Edge Locations ?Amazon Web Services (AWS)
Ans. A Region is a Geographical entity like US-East , US-West etc. Each Region may have multiple availability zones where each zone comprise of 1 or more Data Center located with each other.

Edge Locations are the sites that hosts cached content for faster delivery and for saving network traffic as they feed content from sites that are local or near to consumption.

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   Like         Discuss         Correct / Improve     aws region  aws availability zone  aws edge location  region vs availabillity zone

Very frequently asked. Favorite question in Walk in Drive of many Indian service companies.
  Q38. 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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 Q39. When are static variables loaded in memory ?Core Java
Ans. They are loaded at runtime when the respective Class is loaded.

  Sample Code for static variable

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 Q40. Can we serialize static variables ?Core Java
Ans. No. Only Object and its members are serialized. Static variables are shared variables and doesn't correspond to a specific object.

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 Q41. Can we use null as a key for a map collection ?Core Java
Ans. Yes , for HashMap. HashMap implements Map interface. HashMap allows one null key and any number of null values.

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 Q42. Difference between Abstract and Concrete Class ?Core Java
Ans. Abstract classes are only meant to be sub classed and not meant to be instantiated whereas concrete classes are meant to be instantiated.

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  Q43. Difference between Vector and ArrayList ?Core Java
Ans. Vectors are synchronized whereas Array lists are not.

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  Q44. 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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Very Frequently asked. Usually asked along with String Class related questions.
  Q45. What is an immutable class ?Core Java
Ans. Class using which only immutable (objects that cannot be changed after initialization) objects can be created.

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Very frequently asked in phone and walk in interviews.
  Q46. What are Marker Interfaces ? Name few Java marker interfaces ?Core Java
Ans. An interface without any method declaration is called as marker interface. there are 3 in-built interfaces in JVM i.e. serializable, clonable, remote

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Frequently asked to fresh graduates.
  Q47. 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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 Q48. Difference between Factory and Abstract Factory Design Pattern ?Design
Ans. Factory Pattern deals with creation of objects delegated to a separate factory class whereas Abstract Factory patterns works around a super-factory which creates other factories.

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 Q49. Difference between Factory and Strategy Design Pattern ?Design
Ans. Factory is a creational design pattern whereas Strategy is behavioral design pattern. Factory revolves around the creation of object at runtime whereas Strategy or Policy revolves around the decision at runtime.

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 Q50. How can we create objects if we make the constructor private ?Core Java
Ans. We can do so through a static public member method or static block.

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 Q51. What are the disadvantages of using arrays ?Core Java
Ans. Arrays are of fixed size and have to reserve memory prior to use. Hence if we don't know size in advance arrays are not recommended to use.

Arrays can store only homogeneous elements.

Arrays store its values in contentious memory location. Not suitable if the content is too large and needs to be distributed in memory.

There is no underlying data structure for arrays and no ready made method support for arrays, for every requriment we need to code explicitly

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 Q52. Will the static block be executed in the following code ? Why ?

class Test
{
static
{
System.out.println("Why I am not executing ");
}
public static final int param=20;
}

public class Demo
{
public static void main(String[] args)
{
System.out.println(Test.param);
}
}
Ans. No the static block won't get executed as the referenced variable in the Test class is final. Compiler replaces the content of the final variable within Demo.main method and hence actually no reference to Test class is made.

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 Q53. If you are given a choice to implement the code to either Insert a Record or Update if already exist, Which approach will you follow ?

1. Insert into the DB Table. If exception occurs, update the existing record.
2. Check if the record exists and update it if it exists, If not insert a new record.
Solution
Ans. In first case, there would be 2 DB calls in worst case and 1 in best case. In 2nd approach there will be always 2 DB calls.

Decision on the approach should depend on the following considerations -

1. How costly is the call to DB ? Are we using indices , hibernate etc

If calls to DB are costly , 1st approach should be the choice.

2. Exception Book keeping load upon exception.

The benefit of saving 1st call in approach 1 should be bigger than the Book keeping for the exception.

3. Probability of the exception in first apparoach.

If the DB Table is almost empty, it makes sense to follow Approach 1 as majority of the 1st calls will pass through without exception.

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   Like         Discuss         Correct / Improve     database   insert-update   db exceution plan   db strategy   design   architecture   technical lead

 Q54. What is the use of @GeneratedValue annotation in Hibernate?Hibernate
Ans. This annotation is added to the auto increment column with the strategy to increment the column value. Usually this is added to the surrogate primary key column and specified with the Database Sequence.

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 Q55. What is the use of runnable interface if we can always create a new thread using Thread class ?Core Java
Ans. It's a choice to be made whether to use first approach ( Thread class ) or second approach ( runnable interface ) by the programmer. The second facility has been given for cases where your class is already extending some parent class and hence cannot extend another class ( for Thread ) as Java doesn't allow multiple inheritance.

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 Q56. Which of the following Java feature promotes Code Re usability ?

a. Abstraction
b. Encapsulation
c. Inheritance
d. Polymorphism
Core Java
Ans. Inheritance

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 Q57. What are different types of variables a class can have ?Core Java
Ans. 1. Method local variables - These are declared and defined within a method ( instance or static methods ) and their scope is limited to the method itself. They are destructed once the execution of method completes. They are stored in stack memory.

2. Instance variables - These are declared as non static variables as part of the class.They are initialized as part of object creation ( constructor ) and are destructed by java's garbage collection mechanism and hence stored in heap.

3. Static variables - These are declared with the static keyword and are part of the class. They are initialized at the time of class loading and are destructed by java's garbage collection mechanism and hence stored in heap.

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 Q58. If an Abstract class has only abstract methods, What's the difference between such a class and an interface ?Core Java
Ans. Such a class still can have member elements which can be inherited and hence facilitate code reuse. Moreover Abstract class can have non final static elements whereas interfaces are only allowed to have static final elements.

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 Q59. When we say final x = 10, is the reference final or the value ?Core Java
Ans. final keyword have meaning only to referenced and not the value. It means that the specified reference cannot be dereferenced. It doesn't control the value assigned to the memory that's being referenced. This is the reason that final object references doesn't mean that the object is immutable but means that the reference cannot be changed to point to new object.

In case of primitive types too, when we assign a reference to another, values are passed and not the object reference, and hence a new placeholder is created in memory with the same value. That is why final to that context means that you cannot change the assigned memory and there is no way we can have that memory place have another value.

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   Like         Discuss         Correct / Improve     final keyword  final variables  references

Very frequently asked. Usually asked with questions related to String. Frequently asked at CTS / Cognizant
  Q60. What is an Immutable Object ?Core Java
Ans. Object that can't be changed after instantiation.

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