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Interview Questions and Answers for 'ABS' - 51 question(s) found - Order By Newest

Very Very frequently asked. Among the first few questions in almost all interviews.Among Top 5 frequently asked questions. Frequently asked in Indian service companies (HCL,TCS,Infosys etc, Based on multiple feedback )
  Q1. Difference between == and .equals() ?Core Java
Anonymous
Ans. "equals" is the method of object class which is supposed to be overridden to check object equality, whereas "==" operator evaluate to see if the object handlers on the left and right are pointing to the same object in memory.

x.equals(y) means the references x and y are holding objects that are equal. x==y means that the references x and y have same object.

Sample code:

String x = new String("str");
String y = new String("str");

System.out.println(x == y); // prints false
System.out.println(x.equals(y)); // prints true

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Advanced level question. Frequently asked in High end product companies. Frequently asked in Cognizant ( Based on 2 feedbacks )
  Q2. Why is String immutable in Java ?Core Java
Anonymous
Ans. 1. String Pool - When a string is created and if it exists in the pool, the reference of the existing string will be returned instead of creating a new object. If string is not immutable, changing the string with one reference will lead to the wrong value for the other references.

Example -

String str1 = "String1";
String str2 = "String1"; // It doesn't create a new String and rather reuses the string literal from pool

// Now both str1 and str2 pointing to same string object in pool, changing str1 will change it for str2 too

2. To Cache its Hashcode - If string is not immutable, One can change its hashcode and hence it's not fit to be cached.

3. Security - String is widely used as parameter for many java classes, e.g. network connection, opening files, etc. Making it mutable might possess threats due to interception by the other code segment.

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Very frequently asked in different variations. Frequently asked in Deloitte ( 2 feedback ) , HCL Tech ( 3 feedback ) and Coginizant (CTS)
  Q3. Explain the scenerios to choose between String , StringBuilder and StringBuffer ?

or

What is the difference between String , StringBuilder and StringBuffer ?
Core Java
Anonymous
Ans. If the Object value will not change, use String Class because a String object is immutable.

If the Object value can change and will only be modified from a single thread, use StringBuilder because StringBuilder is unsynchronized(means faster).

If the Object value may change, and can be modified by multiple threads, use a StringBuffer because StringBuffer is thread safe(synchronized).

  Sample Code for String

  Sample Code for StringBuffer

  Sample Code for StringBuilder

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Frequently asked in Infosys and HCL Technologies ( Based on 2 feedback )
 Q4. What are different ways of object creation in Java ?Core Java
Admin
info@buggybread.com
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( )

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Very frequently asked to Fresh graduates.
  Q5. What is the difference between Encapsulation and Abstraction?Core Java
Anonymous
Ans. 1.Abstraction solves the problem at design level while encapsulation solves the problem at implementation level

2.Abstraction is used for hiding the unwanted data and giving relevant data. while Encapsulation means hiding the code and data into a single unit to protect the data from outside world.

3. Abstraction lets you focus on what the object does instead of how it does it while Encapsulation means hiding the internal details or mechanics of how an object does something.

4.For example: Outer Look of a Television, like it has a display screen and channel buttons to change channel it explains Abstraction but Inner Implementation detail of a Television how CRT and Display Screen are connect with each other using different circuits , it explains Encapsulation.

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Very frequently asked. Favorite question in Walk in Drive of many Indian service companies. Frequently asked in HCL Technologies.
  Q6. What is the difference between final, finally and finalize() ?Core Java
Admin
info@buggybread.com
Ans. final - constant variable, objects cannot be de-referenced, restricting method overriding, restricting class sub classing.

finally - handles exception. The finally block is optional and provides a mechanism to clean up regardless of what happens within the try block. Use the finally block to close files or to release other system resources like database connections, statements etc.

finalize() - method helps in garbage collection. A method that is invoked before an object is discarded by the garbage collector, allowing it to clean up its state.

  Sample Code for final

  Sample Code for finally

  Sample Code for finalize

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Very frequently asked. Favorite question in Walk in Drive of many Indian service companies.
  Q7. What is the difference between ArrayList and LinkedList ?Core Java
admin
info@buggybread.com
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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 Q8. Can you give a real world example of Encapsulation and Abstraction ?Core Java2016-05-29 13:47:31

Ans. Car Engine is an example of encapsulation and abstraction. You ignite the car using an interface called starter and least bothered about how the tire actually moves (This is abstraction). The engine encapsulates the complete process to itself only and doesn't allow you to start the other components like the radiator etc ( this is excapsulation )

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Very Frequently asked to fresh graduates and less experienced. Favorite question in Walk in drives.
  Q9. Difference between Overloading and Overriding ?Core Java
Anonymous
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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Very Frequently asked. Have been asked in HCL Technologies very frequently ( based on 2 feedback ). Among first few questions in many interviews.
  Q10. Differences between abstract class and interface ?Core Java
Admin
info@buggybread.com
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 ).

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Almost sure to be asked in every company using any Dependency Injection framework ( Spring, Guice etc )
  Q11. What is Dependency Injection or IOC ( Inversion of Control ) ?Design

Ans. It is a Design Pattern that facilitates loose coupling by sending the dependency information ( object references of dependent object ) while building the state of the object. Objects are designed in a manner where they receive instances of the objects from other pieces of code, instead of constructing them internally and hence provide better flexibility.

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 Q12. If an Abstract class has only abstract methods, What's the difference between such a class and an interface ?Core Java2017-05-07 07:26:29

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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 Q13. Difference between Abstract and Concrete Class ?Core Java
Anonymous
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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 Q14. Difference between Factory and Abstract Factory Design Pattern ?Design
Anonymous
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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 Q15. What are the design considerations while making a choice between using interface and abstract class ?Core Java
Anonymous
Ans. Keep it as a Abstract Class if its a "Is a" Relationsship and should do subset/all of the functionality. Keep it as Interface if its a "Should Do" relationship.

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 Q16. How is Abstraction implemented in Java ?Core Java
Admin
info@buggybread.com
Ans. Abstraction is provided in Java by following ways -

Coding to the ( Interfaces / Abstract Classes ) or contracts

By Encapsulating details within classes and exposing the minimal Door ( few public methods )

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Very frequently asked. Usually asked in this format or as difference with interfaces / concrete classes.
  Q17. What is an abstract class ?Core Java2017-01-10 13:12:20

Ans. Abstract class is the class that is not supposed to be instantiated. The purpose of the class to only have extension to the derived class.

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 Q18. Can we declare an abstract method private ?
Anonymous
Ans. No Abstract methods can only be declared protected or public.

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 Q19. Difference between Abstraction and Implementation hiding ?
Anonymous
Ans. Implementation Hiding is a broader concept. Abstraction is a way of implementation hiding in OOP's.

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 Q20. What are the examples of Abstraction in Java ?
Admin
info@buggybread.com
Ans. function calling - hides implementation details
wrapper classes
new operator - Creates object in memory, calls constructor

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  Q21. Write a Program to check if 2 strings are Anagrams ?Core Java2016-11-30 20:52:37

Ans. public void checkIfAnagram(String str1,String str2){
boolean anagram = true;
for(char c:str1.toCharArray()){
if(!str2.contains(String.valueOf(c))){
System.out.println("Strings are Anagrams");
anagram = false;
}

if(anagram == true){
System.out.println("Strings are not Anagrams");
}
}
}

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 Q22. what will be the output ?

class Animal {
public void eat() throws Exception {
}
}

class Dog2 extends Animal {
public void eat(){}
public static void main(){
Animal an = new Dog2();
an.eat();
}
}
Core Java
Admin
info@buggybread.com
Ans. Compile Time Error: Unhandled exception type Exception

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 Q23. Will this code Work ? If not , Why ?

java.util.Calendar c = new java.util.Calendar();
Core Java
Admin
info@buggybread.com
Ans. No. It gives the error "Cannot Instantiate the type Calendar". Calendar is an abstract class and hence Calendar object should be instantiated using Calendar.getInstance().

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 Q24. Is java.util.Date an abstract Class ? Is java.util.Calendar an abstract Class ?Core Java
Admin
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Ans. Date is not a abstract class whereas Calendar is.

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 Q25. Will this code compile ?

public class BuggyBread1{
abstract public void test();
}
Core Java
Admin
info@buggybread.com
Ans. No. It will give the compilation error saying "The abstract method test in type BuggyBread1 can only be defined by an abstract class".

We need to declare the class abstract for it to have any abstract method.

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 Q26. Will this Code compile ?

abstract public class BuggyBread1{
abstract public void test(){};
}
Core Java
Admin
info@buggybread.com
Ans. No. This will give a compilation error saying "Abstract methods do not specify a body".

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Frequently asked in face to face interviews.
  Q27. 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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 Q28. What is ADT or Abtstract Data Type ?Core Java

Ans. ADT is a container which holds different types of objects with specifications.

For example - Stack, Array, Liked list, Tree

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 Q29. 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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 Q30. In the following code , how many methods needs to be implemented in Class B ?

public interface A{
   public void method1();
   public void method2();
   public void method3();
}

abstract class B implements A{
}
Core Java

Ans. As Class B has been declared abstract , we can either implement any of these methods and just declare rest of them abstract.

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 Q31. What should a class do if its extending an abstract class ?Core Java

Ans. It should either implement the abstract methods or re-declare them abstract.

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Frequently asked to Fresh graduates.
 Q32. Write a program to implement Binary search ?Core Java2016-10-18 10:49:58

Ans. http://algs4.cs.princeton.edu/11model/BinarySearch.java.html

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 Q33. Can we declare a main method as abstract ?2016-06-21 13:11:31

Ans. No. Static methods cannot be overridden and hence make no sense to be declared abstract.

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 Q34. Implement the following method ?

int findMax(int[] items)
Core Java2016-12-01 16:04:18

Ans. int findMax(int[] items){
   int maxNumber = 0;
   for(int x:items){
      if(x > maxNumber){
         maxNumber = x;
      }
   }
      
   return maxNumber;
}

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 Q35. When should we use Abstract classes ?Core Java2016-12-23 20:12:52

Ans. Abstract classes provide a mechanism of interfacing ( using abstract method ) as well as inheritance ( extending abstract class ). So they should be used in place of interfaces in case there is some code ( methods ) or object body ( member elements ) that can be reused with inheritance.

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 Q36. Write program to create a linked list and perform different operations on it.Algorithm2017-02-24 14:18:01

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 Q37. write program to do matrix multiplicationCore Java2017-02-24 14:18:15

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 Q38. Can we overload abstract methods ?Core Java2017-03-01 09:43:18

Ans. Yes

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 Q39. Can we override abstract methods ?Core Java2017-03-01 09:45:08

Ans. Abstract methods are the methods that need to be overridden in the derived class ( either as implementing method or again abstract method ) so it's not only allowed but its required to override abstract method in the derived class.

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 Q40. Can we override an abstract method with abstract method in the derived class ?Core Java2017-03-01 09:46:35

Ans. Yes, but in that case the derived class itself should be abstract. We cannot have an object of a class without definition for the method which has been declared abstract in the parent.

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 Q41. Difference between log4j and slf4j ?Logging2017-03-03 14:18:37

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 Q42. What was the need of creating an abstract class in Java when we cannot instantiate an object of it ?Core Java2017-04-03 09:18:49

Ans. The objective of an abstract class is to provide an interface as well as code reuse. Though we cannot instantiate an instance of abstract class but we can reuse the code by extending such a class. Moreover abstract class provides interfacing to outside components just like interfaces.

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 Q43. Why can't we create an instance of abstract class ? Why is it restricted by compiler ?Core Java2017-04-03 13:27:57

Ans. Because Abstract class is incomplete i.e there are abstract methods that needs to be defined by extending classes.

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 Q44. What is synchronization ?Core Java2017-04-07 10:15:24

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 Q45. What is JVM ?Core Java2017-04-07 10:16:18

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 Q46. What is the importance of Abstract Class ?Core Java2017-04-30 09:37:41

Ans. Abstract classes provide a mechanism of interfacing ( using abstract method ) as well as code reuse through inheritance ( extending abstract class )

Comparing to concrete class they have an advantage of providing interface which a concrete class doesn't provide.

Comparing to interfaces they have an advantage of providing code reuse through inheritance which interfaces dont provide.

  Sample Code for abstract class

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 Q47. Write a Program for insertion sort.Core Java2017-06-12 07:50:44

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 Q48. What are the uses of Abstraction ?Core Java2017-08-03 07:41:32

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 Q49. 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 Java2017-08-17 19:28:33

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


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 Q50. Which of the following is not true for abstract classes ?Core Java
a. Abstract Class is only meant to be sub classed and not supposed to be instantiated.
b. Abstract class handlers can be used to handle derived class objects.
c. We can't have an abstract class without abstract methods.
d. Abstract class has member elements.

Ans.c. We can't have an abstract class without abstract methods.

 Q51. Which of the following can be declared abstract ?Core Java
a. static methods
b. instance methods
c. static variable
d. instance variables

Ans.b. instance methods



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