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Q31. Explain Inheritance and Polymorphism.

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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Q32. Difference between ApplicationContext and BeanFactory ?

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

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Q33. How do we define constant variables in Java ?

Ans. By using static and final modifiers.

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Q34. Give an example of how Object Oriented Programming Concepts can be implemented.

Ans. You can implement encapsulation in Java by keeping the fields (class variables) private and providing public getter and setter methods to each of them. Java Beans are examples of fully encapsulated classes. Encapsulation in Java: Restricts direct access to data members (fields) of a class.

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Q35. Explain OOPs

or

Explain OOPs Principles

or

Explain OOPs Concepts

or

Explain OOPs features

or

Tell me something about OOPs

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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Usually asked in relation to casting and ClassCastException.

Q36. What is instanceOf operator ? Explain it's use ?

Ans. The operator instanceOf is used to verify if the specified object is the instance of specified class or interface.

Syntax if(x instanceOf ABC)

where x is an object reference and ABC could be a class name or interface name. The above statement will be true if x holds an object that is an instance of ABC or any of the child class of ABC or if x holds an object that implements ABC.

instanceOf operator is used to verify in case of downcasting. For ex -

DerivedClass extends BaseClass

x is the reference of BaseClass but holds DerivedClass object ( Polymorphism )

There is an operation that is defined in Derived Class, let's say derivedClassMethod()

We cannot call derivedClassMethod() directly using x as x is reference of BaseClass and not DerivedClass and hence can only access methods that are defined in BaseClass and overridden in derived class.

Though we can cast it to DerivedClass as following
((DerivedClass)x).derivedClassMethod();

But it may throw ClassCastException in case x doesn't hold an instance of DerivedClass at that point.

So before casting it to DerivedClass we may like to make sure that it is an instance of DerivedClass and hence won't throw ClassCastException.

So we make a check for it

if(x instanceOf DerivedClass) {
((DerivedClass)x).derivedClassMethod();
}

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Very frequently asked. Usually among very first few questions.

Q37. Define encapsulation in Java ?

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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Q38. What is the use of final keyword ?

Ans. final keyword is a modifier that means differently when applied to variables, methods and classes.

1. Final variable cannot be changed once initialized
2. Final method cannot be overridden
3. Final class cannot be sub classed

Sample Code for final variable

Sample Code for final method

Sample Code for final class

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Q39. 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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Q40. What is the difference between reference and object ?

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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Q41. What is the difference between Enumeration and Iterator ?

Ans. Enumeration can iterate only legacy collections like Vector , HashTable and Stack whereas Iterator can iterate both legacy and non legacy collections.

Enumeration is less safer than Iterator

Enumeration is fail safe whereas Iterator is fail fast

Iterator allows for removal of element while traversal whereas Enumeration doesn't have remove method.

Enumerations were introduced in Java 1 whereas Iterators were introduced with Java 2

Enumerations have methods like hasMoreElements and nextElement whereas Iterators have methods like hasNext, next and remove

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Q42. What is the importance of Abstract Class ?

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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Q43. What are the benefits of Rest services over using SOAP ?

Ans. Rest services are light weight and hence facilitate faster development and involve less overheads compared to SOAP.

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Q44. What will happen if we don't have termination statement in recursion ?

Ans. It would result in endless function calls and hence eventually would result in stackoverflow exception.

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Q45. What is JVM ?

Ans. JVM (Java Virtual Machine) is an abstract machine. It is a specification that provides runtime environment in which java bytecode can be executed.JVMs are available for many hardware and software platforms (i.e. JVM is platform dependent).Runtime Instance Whenever you write java command on the command prompt to run the java class, an instance of JVM is createdThe JVM performs following operation:Loads codeVerifies codeExecutes codeProvides runtime environment

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Q46. What do you know about our company ?

Ans. [Open Ended Answer]

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Q47. What is your major weakness ?

Ans. [Open Ended Answer]

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Q48. Which are the most commonly used Design Patterns ?

Ans. Builder ( While Writing Unit Tests )
Prototype ( Cloning )
Adapter ( asList , toString )
Chain Of Responsibility ( Logging )
Singleton
Factory ( Action Mapping )
Proxy
Observer ( Event Listener )
MVC ( Web frameworks )
Filter ( Criteria )

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Q49. What are the core interfaces in Hibernate?

Ans. Session,SessionFactory,Configuration,Transaction,query and Criteria

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Q50. What are the core OOPs concepts ?

Ans. Abstraction, Encapsulation, Polymorphism , Composition and Inheritance

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Q51. Write any sorting algorithm.

Ans. https://javasearch.buggybread.com/InterviewQuestions/questionSearch.php?searchOption=label'

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Q52. What is the difference between authentication and authorization ?

Ans. Authentication is the process of verifying the identity and credentials of the user to authenticate him into the system.

whereas

Authorization is the process by which access to a segment , method or resource is determined.

Authorization is usually a step next to authentication.

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Q53. Can you name some of the Java frameworks in different domains ?

Ans. Web Framework - Spring , Struts, Play
Dependency Injection frameworks - Google Guice , PicoContainer and Dagger
ORM Framework - Hibernate
Big Data / ETL Frameworks - Apche Hadoop , Apache Crunch, Apache Spark.
View Frameworks - JSF , Apache Wicket,jtwig.
Java Gui Frameworks - SWT , AWT, JavaFX
Testing - Junit, Mockito, PowerMock, EasyMock, JMock, JMockit
Rest Web services - Jersey , Restlet , RestX, RestEasy ,Restfulie

Here is the big list for reference - http://javasearch.buggybread.com/home2.php?keyword=

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Q54. What are the benefits of creating immutable objects ?

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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Q55. How does Lazy Initialization helps improving performance of an application ?

Ans. Lazy Initialization means , Load Dependencies when required. Which means less load on application resources as only required data is loaded upfront. It's not only good for better performance but for better resource utilization too.

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Usually asked to entry level software developers.

Q56. Write a program to swap two variables without using third

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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Q57. Write an algorithm / java program for Heap Sort ?

Ans. https://www.geeksforgeeks.org/heap-sort/

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Q58. What is memory leak ? How Java helps countering memory leaks compared to C++ ?

Ans. Memory Leak is a situation wherein we have a reserved memory location but the program has lost its reference and hence has no way to access it.

Java doesn't have concept of Pointers, Moreover Java has concept of Garbage collection that frees up space in heap that has lost its references.

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Q59. What is a class and object ?

or

How would you explain a fresher the concept of class and object in simple terms ?

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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Q60. What is a Servlet Filter ?

Ans. It's an object that can intercept http request and response and hence we can take appropriate action on those requests.

There are different types of filters based on Specifications like

Authentication
Logging
Encryption
Tokenizing

etc

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