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Interview Questions and Answers for 'Object' - 102 question(s) found - Order By Rating

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 Q1. Can a class extend itself in Java ?Core Java2018-07-20 18:16:21

Ans. No

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 Q2. Have you ever used java.util.Objects class ?Core Java2018-07-16 11:33:40

Ans. Yes, We are using requireNonNull method for validating if the object is null.

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 Q3. How many VTables are there for each class ? Core Java2018-07-16 07:57:04

Ans. There is one VTable for each class.

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 Q4. How is the virtual method table implemented in Java?Core Java2018-07-16 07:55:56

 This question is still unanswered. Can you please provide an answer.


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 Q5. What is virtual table with respect to method overriding in Java ?Core Java2018-07-16 06:54:03

 This question is still unanswered. Can you please provide an answer.


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 Q6. What could be the reasons for restricting inheritance using final class ?Core Java2018-07-16 06:52:00

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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 Q7. Does use of Final class enforces composition over Inheritance in Java ? Core Java2018-07-16 06:49:53

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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 Q8. Can we have multiple inheritance in java using abstract class ?Core Java2018-07-04 16:17:49

Ans. No

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 Q9. What is immutability and What are it's disadvantages ?Design2018-06-20 12:20:19

 This question was recently asked at 'Bloomberg'.This question is still unanswered. Can you please provide an answer.


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 Q10. Does immutability facilitates better performance ?Core Java2018-02-10 12:51:54

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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 Q11. What are the drawbacks of creating immutable objects ? Why don't we create all object immutable if mutability possesses so much threat ? Core Java2018-02-10 12:47:43

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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 Q12. What is composition ?Core Java2018-01-20 12:20:35

 This question was recently asked at 'MST Solutions'.This question is still unanswered. Can you please provide an answer.


  Sample Code for composition

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 Q13. What is the use of Object class ?Core Java2017-09-02 15:30:43

Ans. 1. Default definition for some of the methods , like equals, hashcode etc that gets carried to all objects even if you don't define anything. But that default definition is kind of like assigning 0 to integer, it just provide a safe state , nothing much for comparison.

2. Places where you have no idea about what object you may receive and just want to perform out of 8 basic methods of object class, something like printing their string representation ( defined by their toString method ), or equality ( using their equals method )

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 Q14. Is Runtime Polymorphism possible without Inheritance ?Core Java2017-08-06 19:12:26

Ans. Yes, Runtime Polymprohism requires either Class inheritance or Interface implementation.

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 Q15. How can we track of all objects created in an application ?Design2017-08-04 14:02:03

Ans. We can store the references in a collection by adding to those objects in the collection. We can create a class "ObjectRegistry" with a collection or multiple collections with a search algorithm to look for the already collected objects.

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 Q16. What will be the output of following code

class TestMain {
public static void main(String[] args) {
String s1 = "ravi";
String s2 = new String("ravi");
Integer i = 10;
Integer a1 = new Integer("10");
System.out.println(s1 == s1);
System.out.println(i == a1);
}
}
Core Java2017-07-17 08:07:19

Ans. false
false

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 Q17. Explain Inheritance and Polymorphism.Core Java2017-07-10 14:27:40

Ans. Getting parent class object properties into child class is called Inheritance.

The process of representing one form into multiple forms is called polymorphism.

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  Q18. Explain OOPs

or

Explain OOPs Principles

or

Explain OOPs Concepts

or

Explain OOPs features

or

Tell me something about OOPs
Core Java2017-06-13 08:25:45

Ans. OOPs or Object Oriented Programming is a Programming model which is organized around Objects instead of processes. Instead of a process calling series of processes, this model stresses on communication between objects. Objects that all self sustained, provide security by encapsulating it's members and providing abstracted interfaces over the functions it performs. OOP's facilitate the following features

1. Inheritance for Code Reuse
2. Abstraction for modularity, maintenance and agility
3. Encapsulation for security and protection
4. Polymorphism for flexibility and interfacing

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Very frequently asked. Usually among very first few questions.
 Q19. Define encapsulation in Java ?Core Java2017-06-12 07:55:47

Ans. Encapsulation is a feature of OOP's that binds the data and it's associated methods together as a single unit and facilitate protection and data hiding by providing minimal interface to outside. For example - member variables are declared private and are accessed through public methods. Moreover we have private methods that can only be used internally and hence providing minimal interface to outside class through use of public methods.

  Sample Code for encapsulation

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 Q20. How would you explain the statement "Java Programs are collection of objects" ?Core Java2017-06-08 07:24:57

Ans. Java Programs are collection of objects that communicates with each other to get a task accomplished. To add to those objects, there are common spaces ( static i.e common for objects belonging to a class ) that are used too.

We can visualize objects as departments of an organization in real world. Just like Task gets initiated in one department and then files are moved across different departments to get work done. In a similar fashion, a task is initiated in one object ( having main method ) and then information ( through POJOs / DTOs ) is moved across objects to accomplish a task.

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 Q21. How can we hide a class in Java ?Core Java2017-06-07 16:42:57

Ans. By encapsulating it within another class and declaring it private. In such a case, it will only be accessible through parent class or parent class object.

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 Q22. What happens to private variables during inheritance ?Core Java2017-06-07 16:35:35

Ans. Private variables are not inherited. They are simply ignored and not made part of the derived object body.

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 Q23. What is dynamic dispatch in Java ?Core Java2017-05-29 09:57:31

 This question is still unanswered. Can you please provide an answer.


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 Q24. What are the examples of immutable objects in Java ?Core Java2017-05-29 08:35:00

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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 Q25. What happens when you assign a pre initialized reference some other object ?Core Java2017-05-22 08:31:02

Ans. Now the reference points to a new object in memory. If that was the only reference for the previous object , it will be marked for garbage collection.

Foe example -

Object obj = new Object();
obj = new Object();

object created in first line will be eligible for garbage collection after line 2 as it looses all it's handlers.

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 Q26. What is the difference between reference and object ?Core Java2017-05-13 20:20:48

Ans. Object is an entity in Java , i.e which has a state ( instance variables ) and methods attached to it ( static or non static , through class definition ). References are the identifiers that are used to point to objects.

For example -

Employee emp = new Employee();
emp = new Employee();

In this code, emp is the reference that gets assigned to the new object created by the new operator. In the second line , we have assigned the same reference to another object. So with these 2 lines of code, we have 2 objects in memory with reference emp now pointing to second object.

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 Q27. When does an object cleaned or destroyed by garbage collection ?Core Java2017-05-07 20:41:46

Ans. When it is no longer referenced or it looses all references which were earlier pointing to it.

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 Q28. How can we destroy an object ? Core Java2017-05-07 20:39:20

Ans. We cannot explicitly initiate destruction of an object as no destroy method is available in java. Only Garbage collection can destroy an object.

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 Q29. What will be the output of following code

Integer x = 1;
Integer y = 2;
System.out.println(x == y);

What if you change 1 to "1" and Integer to String?
Core Java2017-05-07 19:30:13

Ans. It will print "true" with integers as well as strings. The reason is "Integer constant pool" and "String pool"

String pool maintains pool of string literals. When a string literal is used for the first time, a new string object is created and is added to the pool. Upon it's subsequent usage , the reference for the same object is returned. Similarly java uses integer constant pool.

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 Q30. What is "dynamic method dispatch" in java ?Core Java2017-05-07 16:27:26

Ans. dynamic method dispatch is a process in which a call to an
overridden method is resolved at runtime rather than at compile-time. This is used to achieve runtime polymorphism in java.

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 Q31. How does java identifies which method to be called in method overriding or runtime polymorphism, when both methods share the same name and signature ? Core Java2017-05-02 08:33:52

Ans. Java identifies the method to be called at runtime by the object that is being referenced.

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 Q32. Which of the two - compile time and run time polymorphism - requires signature of the method to be different ?Core Java2017-05-02 08:31:29

Ans. runtime polymorphism or method overriding doesn't require method name and signature to be different whereas compile time polymorphism or method overloading requires method name to be same but the signature to be different.

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 Q33. What could be the possible cause for following exception ?

org.hibernate.StaleObjectStateException: Row was updated or deleted by another transaction (or unsaved-value mapping was incorrect):

What could be the way to fix it ?
Hibernate2017-03-29 12:44:00

Ans. This looks like the case for optimistic locking wherein hibernate suspects that the information in table was updated by some other transaction after the entity was loaded by current transaction.

One way is to have synchronized entity state and don't detach the entity. Other could be to merge the entity with the table record rather than just directly persisting the entity.

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 Q34. What is ACL ?Operating System2017-03-01 09:45:12

Ans. Access Control List or ACL is the list of permissions attached to an object in the File System.

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 Q35. How are the concept of Association related to Composition and Inheritance ?Core Java2017-02-06 16:18:04

Ans. Composition and Inheritance are the different types of Associations for Classes.

Composition is a has-a association between classes.
Inheritance is a is-a association between classes.

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 Q36. What are the benefits of creating immutable objects ?Core Java2017-01-22 16:01:54

Ans. 1. Security and Safety - They can be shared across multiple threads as they are thread safe. Moreover, it protects then from bad state due to interception by the other code segment. One such problem due to mutability and access by alternate code segment could be the change of hash code and then the impact on its search with hash collections.

2. Reuse - In some cases they can be reused as only one copy would exist and hence it can be relied upon. For example - String Pool

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 Q37. Can we override wait() or notify() methods?Core Java2017-01-19 13:59:12

Ans. wait and notify are declared final in object class and hence cannot be overridden.

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 Q38. What is a class and object ?

or

How would you explain a fresher the concept of class and object in simple terms ?
Core Java2017-01-19 12:39:36

Ans. Class is a template using which objects are created in memory. It's kind of a mold using which objects with body / state are made.


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 Q39. Difference between shallow copy and object cloning ?Core Java2017-01-12 14:21:02

Ans. Shallow copy is one of the way for object cloning, other being deep copy.

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 Q40. Is there a way to know size of an object in Java ?Core Java2017-01-12 11:14:59

Ans. No, Java doesn't have a sizeOf operator. In C / C++ , its required to determine how much memory allocation is required which is not the case with Java. Java handles memory allocation and deallocation intrinsically.

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