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Very Frequently asked across all type of companies and across all levels.

Q11. Difference between Public, Private, Default and Protected ?

Ans. Private - Not accessible outside object scope.

Public - Accessible from anywhere.

Default - Accessible from anywhere within same package.

Protected - Accessible from object and the sub class objects.

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Frequently asked question for intermediate developers. Frequently asked in HCL Technologies and EPAM.

Q12. What is Volatile keyword used for ?

Core Java

Ans. Volatile is a declaration that a variable can be accessed by multiple threads and hence shouldnt be cached.

Sample Code for volatile

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Very frequently asked.Usually among first few questions.

Q13. What is MVC ?

Design

Ans. MVC is a Design Pattern that facilititates loose coupling by segregating responsibilities in a Web application

1. Controller receives the requests and handles overall control of the request
2. Model holds majority of the Business logic, and
3. View comprise of the view objects and GUI component

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Frequently asked to fresh graduates and less experienced.

Q14. Difference between Composition and Inheritance ?

Core Java

Ans. Inheritance means a object inheriting reusable properties of the base class. Compositions means that an abject holds other objects.

In Inheritance there is only one object in memory ( derived object ) whereas in Composition , parent object holds references of all composed objects.

From Design perspective - Inheritance is "is a" relationship among objects whereas Composition is "has a" relationship among objects.

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Frequently asked in high end product companies. Frequently asked in Deloitte.

Q15. How is Hashmap internally implemented in Java ?

Core Java

Ans. https://medium.com/javarevisited/internal-working-of-hashmap-in-java-97aeac3c7beb#:~:text=HashMap%20internally%20uses%20HashTable%20implementation,the%20entries%20into%20the%20map.

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Q16. What is Generalization and Specialization in terms of casting ?

Core Java

Ans. Generalization or UpCasting is a phenomenon where a sub class is prompted to a super class, and hence becomes more general. Generalization needs widening or up-casting. Specialization or DownCasting is a phenomenon where a super class is narrowed down to a sub class. Specialization needs narrowing or down-casting.

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Q17. What is constructor chaining and how is it achieved in Java?

Core Java

Ans. A child object constructor always first needs to construct its parent. In Java it is done via an implicit call to the no-args constructor as the first statement

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Usually asked to entry level software developers.

Q18. Write a program to swap two variables without using third

Core Java

Ans. public static void main(String[] args) {
   int num1 = 1;
   int num2 = 2;
   num1 = num1^num2;
   num2 = num1^num2;
   num1 = num1^num2;
   System.out.print("num1 = " + num1 +", num2 = "+num2);
}

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Q19. What is JSON ?

Json

Ans. JSON is "JavaScript Object Notation", primarily used for client-server or server-server communication. Its a much lighter and readable alternative to XML. JSON is language independent and is easily parse-able in all programming languages.

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Frequently asked in CTS ( Based on 2 feedback )

Q20. What is the difference between comparable and comparator in java.util pkg?

Core Java

Ans. Comparable interface is used for single sequence sorting i.e.sorting the objects based on single data member where as comparator interface is used to sort the object based on multiple data members.

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Q21. What does String intern() method do?

Core Java

Ans. intern() method keeps the string in an internal cache that is usually not garbage collected.

Moreover provide reference for scp object for corresponding string object present in heap memory.

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Q22. What is a Webdriver ?

Testing

Ans. Selenium WebDriver is a tool for automating web application testing.It helps in replicating the manual tester behavior like keyboard entry, mouse events etc and then matching the output against the expected.

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Q23. What is abstraction ?

Design

Ans. Abstraction is a process of hiding the implementation details and describing only the functionality to the user.

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Q24. What is a Class ?

Core Java

Ans. A class is a blue print or Mold using which individual objects are created. A class can contain fields and methods to describe the behavior of an object.

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Q25. What is the use of HashCode in objects ?

Core Java

Ans. Hashcode is used for bucketing in Hash implementations like HashMap, HashTable, HashSet etc.

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Q26. Can we have null keys in TreeMap ?

Core Java

Ans. No, results in exception.

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Q27. Which Software Development methodology is being used in your current Job ?

General

Ans. We are using Agile methodology. I attend daily stand up where the development leads takes the status of assigned stories, achievements, any bottlenecks or challenges. We follow iteration of 2 weeks.

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Frequently asked in face to face interviews.

Q28. 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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Q29. When do we generally get this database error - integrity constraint violated - parent key not found ?

Ans. This error most likely will be thrown during an insert statement, while inserting a value within a foreign key column which doesnt exist within the Parent column. For example - Trying to add a dept number reference within a Employee Table when the Dept doesnt exist in the Dept Table.

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Q30. 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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Q31. When do we generally get the database error - Unique Constraint Violated ?

Ans. This error can result either on an insert, update or delete when any change in data results in duplicate record or subset of it having unique constraint on record or its subset. For example - If we have a unique constraint on a combination of columns and then trying to insert duplicate column values.

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Q32. When should we use Abstract classes ?

Core Java

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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Q33. Write a program in Java that prints the numbers from 1 to 100. But for multiples of three print Fizz instead of the number and for the multiples of five print Buz. For numbers which are multiples of both three and five print FizzBuzz

Core Java

Ans. public class Class{
public static void main(String[] args){
for(int i=1;i<= 100;i++){
if(i%3 == 0){
if(i%5 == 0){
System.out.println("FizzBuzz");
}
System.out.println("Fizz");
} else if(i%5 == 0){
System.out.println("Buzz");
} else {
System.out.println(i);
}

}
}
}

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Q34. What do you make out of this error

org.hibernate.exception.ConstraintViolationException: could not insert: [<Entity>]
Caused by: java.sql.SQLIntegrityConstraintViolationException: ORA-00001: unique constraint (<Constraint Name>) violated

and how would you go about debugging it ?

Database

Ans. Application is unable to insert a record as it violates a unique constraint.

The exception states the constraint and Table can be located by the Entity mapping. So I will go to the DB and will first check to which all columns the unique constraint applies. And then I will go and check the code and logs to see how come the duplicate column values were attempted to be inserted when they were not supposed to be.

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Q35. What is ConstraintViolationException in Hibernate ?

Hibernate

Ans. The exception is thrown when a database constraint is violated.

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Q36. Write a Program for insertion sort.

Core Java

^{This question was recently asked at 'Ola Cabs'.This question is still unanswered. Can you please provide an answer.}

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Q37. Will the following code compile ?

final HashMap hm = new HashMap();
hm.put("Ravi", 221);

Core Java

Ans. Yes, it will compile without error. final in this context means that the reference hm cannot be assigned to a new hash map but didn't restrict from changing the state of collection already held by hm.

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Q38. How can i split a linked list in two parts in java 8?

Core Java

Ans. This can be done using a Spliterator.

LinkedList list = Arrays.asList("names","numbs","birds","animals");
Spliterator split1 = list.Spliterator();
Spliterator split2 = split1.Spliterator();

Now, the LinkedList is split into split1 and split2.
use split2 first then split1 to check the output.

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Q39. Difference between Arrays and LinkedList ?

Data Structure

^{This question was recently asked at 'Spillman Technologies,Motorola Solutions'.This question is still unanswered. Can you please provide an answer.}

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Q40. What is your role in current project ?

General

Ans. Possible Answer - Java Developer

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