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Interview Questions and Answers - Order By Newest

Q41. How to implement a hashtable ?

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Q42. Make the following class immutable

public class Employee {

   private long employeeId;

   private String employeeName;

   public long getEmployeeId() {
      return employeeId;
   }

public void setEmployeeId(long employeeId) {
      this.employeeId = employeeId;
   }

   public String getEmployeeName() {
      return employeeName;
   }

public void setEmployeeName(String employeeName) {
      this.employeeName = employeeName;
   }
}

Ans. public class Employee {

   private long employeeId;

   private String employeeName;

   public Employee(long employeeId,String employeeName){
      this.employeeId = employeeId;
      this.employeeName = employeeName;
   }

   public long getEmployeeId() {
      return employeeId;
   }

   public long getEmployeeName() {
      return employeeName;
   }
}

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Q43. What are the examples of immutable objects in Java ?

Ans. Following Classes in Java SE creates immutable objects

String class
Wrapper Classes like Integer, Float etc.
StackTraceElement
Most Enum classes
File Class
Locale Class

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Q44. What is the difference between being final and being immutable ? Are string final or immutable in Java ?

Ans. Being final in the sense of objects or object reference means that the reference cannot be reassigned to a different object whereas being immutable means that the object contents cannot be changed.

Strings in Java are immutable.

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Q45. Does Wrapper classes produces immutable objects ?

Ans. Yes

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Q46. Why shouldn't we use string concatenation extensively or in a loop ?

Ans. Because String being an immutable object creates a new object upon each concatenation cycle. If there is any such need , we should use String Builder whose objects are mutable.

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Q47. Is immutability an advantage with Wrapper classes over primitive types ?

Ans. There is no concept of de referencing with primitive types and hence they are implicitly immutable. Having wrapper classes as mutable offers disadvantages compared to primitive types. Wrapper classes being immutable offer similar advantage as primitive types.It actually overshadows the disadvantage wrapper class could have if they are immutable.

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Q48. Which immutable classes have you worked with ? Can you name some immutable Collections ? How to make an immutable collection ?

Ans. string and wrapper class objects

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Q49. What are the benefits of immutability or Immutable classes ?

Ans. Immutable objects are thread-safe so you will not have any synchronization issues.Immutable objects are good for Map keys and Set elements, since these typically do not change once created.Immutability makes it easier to write, use and reason about the code (class invariant is established once and then unchanged)Immutability makes it easier to parallelize your program as there are no conflicts among objects.The internal state of your program will be consistent even if you have exceptions.References to immutable objects can be cached as they are not going to change.

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Q50. How ConcurrentHashMap provides better performance compared to HashTable ?

Ans. ConcurrentHashMap uses multiple buckets to store Data and hence avoid read locks which facilitates better performance.

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Q51. What are the drawbacks of creating immutable objects ? Why don't we create all object immutable if mutability possesses so much threat ?

Ans. Immutable objects relieves us from the problems of inconsistencies and security and helps with better read performance but at the same time are write performance and storage overheads. In case any modification is required , a new object is created and thus creating multiple copies of it.

This is the reason we use StringBuffer / StringBuilder when we have to append some text multiple times and then create a String out of it.

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Q52. Does immutability facilitates better performance ?

Ans. Yes , but only the read performance in case we don't need to modify it much. In case we need to modify it a lot , it creates write performance overheads as we would need to create many news objects instead of just changing one.

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Q53. What is the programming advantage of Immutability ?

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Q54. What are the disadvantages of immutability ?

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Q55. What is immutability and What are it's disadvantages ?

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Q56. What is virtual table with respect to method overriding in Java ?

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Q57. How is the virtual method table implemented in Java?

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Q58. How many VTables are there for each class ?

Ans. There is one VTable for each class.

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Q59. Have you heard of volatile Table in Teradata ?

Ans. If you create table with VOLATILE option, the life of the table will be only for the current session.
It will be automatically dropped by Teradata manager once session expires.

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Q60. Does Java SE has Immutable collections ?

Ans. Yes wef from Java 9

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Q61. Isnt the use of HashTable and ConcurrentHashMap the same, i.e providing synchronized map collection ?

Ans. Both provide the thread safety but the actual difference come when talk about performance. CHM gives best performance when no of writer threads are less in number.

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Q62. Which of the following class creates immutable objects ? Reference

a. String

b. StringBuffer

c. StringBuilder

d. None of these create immutable objects.

Ans.a. String

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