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Ans. co-variant return type states that return type of overriding method can be subtype of the return type declared in method of superclass. it has been introduced since jdk 1.5

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Q113. What is a ConcurrentHashMap ?

Ans. ConcurrentHashMap is a hashMap that allows concurrent modifications from multiple threads as there can be multiple locks on the same hashmap.

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Q114. What are the contents of Hibernate configuration file ( hibernate.cfg.xml ) ?

Hibernate

Ans. HBM Files ( Mapping )
DB Connection ( DB Connection String , User Name , Password , Pool Size )
SQL Dialect ( SQL variant to be generated )
Show SQL ( Show / No show SQL on Console )
Auto Commit ( True / False )

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Q115. Difference between Predicate, Supplier and Consumer ?

Ans. Predicate represents an anonymous function that accepts one argument and produces a result.

Supplier represents an anonymous function that accepts no argument and produces a result.

Consumer represents an anonymous function that accepts an argument and produces no result.

Sample Code for Predicate

Sample Code for Supplier

Sample Code for Consumer

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Q116. What things you would care about to improve the performance of Application if its identified that its DB communication that needs to be improved ?

Solution

Ans. 1. Query Optimization ( Query Rewriting , Prepared Statements )

2. Restructuring Indexes.

3. DB Caching Tuning ( if using ORM )

4. Identifying the problems ( if any ) with the ORM Strategy ( If using ORM )

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Q117. Why do we need Thread class even in case we execute thread using runnable interface ?

Ans. Thread class holds the definition of start method ( This is the method that starts execution of new thread and then calls run method within the scope of new thread ). Interfaces don't hold any definition and so does runnable. So it makes it necessary the usage of Thread class , whatever implementation you choose.

When your class extends the thread class, it carries the definition of start method from parent Thread class onto itself and hence new yourClass.start() helps starting a new thread and then executing run method in that new thread scope.

When you implement runnable interface , you are just making it sure to the JVM that you have implemented the required method ( run() ) which the Thread start method will look for upon executing start method.

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Q118. Collections.sort can only be performed on ..

a. Set
b. List
c. Map
d. Any Collection implementation

Ans. List

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Q119. Which of the following Java feature promotes access protection or Hiding ?

a. Inheritance
b. Encapsulation
c. Abstraction
d. Composition

Ans. Encapsulation

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Q120. Enums cannot be declared ..

a. private
b. public
c. static
d. final

Ans. final

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Q121. Difference between parameters and arguments ?

Ans. Parameters are the variables that the method is expected to receive along with the method call. Arguments are the values which are passed on while calling the methods.

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Q122. Why Java is not considered pure OOP's language ?

Ans. There are 2 reasons for it.

1. Usage of Primitive types - Though Java provides classes for the primitive data types but as the usage of primitives is permissible, its considered unpure OOP's language.

2. Usage of Static members - Static members belong to the class and not objects and hence not considered fit for pure OOP's programming.

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Q123. What are constructors and Destructors in Java ?

Ans. Constructors are used for initializing the object state once it is initialized and memory has been reserved for it.

Destructor is used to de-allocate memory allocated by objects.

There are no destructors in Java. Alternatively, Java provides Automatic garbage collection i.e automatically releasing the un-referenced memory.

Sample Code for constructor

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Frequently asked. Among first few questions in the J2EE segment of interview.

Q124. What is deployment descriptor ?

or

What is the use of deployment descriptor ?

Java EE

Ans. Deployment Descriptor which is usually web.xml is used to specify the classes, resources and configuration of the application and how the web server uses them to serve web requests.This file is usually added to WEB-INF folder and contains following

* Servlet entries and url mapping
* Plugins
* Some info regarding authentication / filters
* Landing Page
* Event Handlers

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Q125. Why HashTable has been deprecated ?

Ans. HashTable has been deprecated. As an alternative, ConcurrentHashMap has been provided. It uses multiple buckets to store data and hence much better performance than HashTable. Moreover, there is already a raw type HashMap. The only difference between the HashTable and HashMap is that Hashtable is synchronized whereas HashMap is not. Most of the synchronized collections have been deprecated and their raw alternative have been presented as preferred.Synchronization has a cost. Using synchronized collection in places where there is no need of it leads to useless utilization of resources. As these collections are rarely used in a static context or shared among threads, Java might have thought it better to just provide the raw collection and let developers implement synchronization if he feels the need to do so. HashMap is now presented as the default and the preferred way of using Map with read optimized hashing, and ConcurrentHashMap has been provided for synchronized access which provides better performance than HashTable. Because of this, Java thought it right to deprecate the use of HashTable.'

Synchronization has a cost. Using synchronized collection at a place where there is hardly any need of it would means useless utilization of resources. As these collections are rarely used in static context or shared among threads, Java might have thought it better to just provide the raw collection and let developer implement synchronization if he feels the need to do so.

As HashMap has been presented as default and preferred way of using Map with read optimized hashing, and ConcurrentHashMap has been provided for synchronized access which provides better performance than HashTable, Java thought it right to deprecate the use of HashTable.

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Q126. Difference beween HashSet and TreeSet ?

Ans. 1. HashSet doesnt maintain its elements in any specific order and is all random whereas TreeSet maintains elements in natural order 9 order defined by the equals method of TreeSet element type )

2. TreeSet doesnt allow null elements whereas HashMap does.

3. As TreeSet orders elements and is hence insertion is comparatively slower.

4. HashSet performs basic operations like add(), remove(), contains(), size() etc. in a constant size time. A TreeSet performs these operations at the order of log(n) time.

5. HashMap in Java internally backs a HashSet. A NavigableMap backs a TreeSet internally.

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Q127. Can you write a "Hello World" program without using any ";" within it?

Ans. Yes, That is possible

class A {
public static void main(String args[]){
if(System.out.printf("Hello World")==null){}
}
}

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Q128. Why two types of memory - stack and heap are required by Java ?

Ans. Because of the life cycle requirement for different type of values in java.

variables initialized and used in functions needs to be destructed with the execution of function and hence kept in stack. Same is applicable for the object references initialized within the method. If objects would have been created in stack, they wouldnt have been passed around across methods and hence they are created on heap.

So anything that is required beyond the scope of a method or function is kept on heap which is usually garbage collected by Java.

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Q129. Which elements of a class are ignored during serialization ?

Ans. 1. Objects are serialized and not classes and hence Static variables are ignored.

2. Transient is an explicit declaration to ignore the variable during serialization and hence transient instance variables are ignored too.

3. Base class instance variables if the base class hasn't been declared serializable.

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Q130. What is the difference between these two method declarations ?

private static void method(String[] arg)

and

private static void method(String... arg)

Ans. First expects the argument as a string array whereas second expects variable number of string arguments or a string array.

So we can call both by providing string array as an argument but second can be called with 0 to n string arguments which cannot be done for first.

for example - We can call second method with any of following

method();
method("Hello");
method("Hello","World");
method(new String[4]);

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Q131. What is the difference between inner class and sub class ?

Ans. Inner Class is a class that is nested within another class whereas sub class is a class that extends or inherit another class.

Inner class can only be accessed using reference of outer class ( Class name , if inner class is static or object reference, if inner class is non static ) whereas Sub class is accessed directly.

In terms of memory, inner class is stored just like another class having it's own body whereas sub class carries the body of parent class as well as it's own fields in memory.

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Q132. What is the difference between Graph's Breadth first and Depth First algorithm ?

Algorithm

Ans. In Breadth first algorithm, all the adjacent nodes of the starting node is visited first and then the same rule is followed while moving inwards whereas

In Depth first algorithm, all the nodes of a single traversal path are visited first till a cycle or an end is found.

For example , given the following entries of adjacent nodes

1,2
1,3
1,6
2,4
2,5
3,6

The Breadth first path would be

1,2,3,6,4,5

and Depth first path would be

1,2,4,5,3,6

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Q133. Is this a valid initialization ? Explain.

Collection<Collection> collection = new LinkedList<LinkedList>();

Ans. No. It will result in a type mismatch error.

Collection<Collection> collection = new LinkedList<Collection>();

is a valid initialization as collection being reference of "Collection" class can hold object of derived Class "LinkedList" due to runtime Polymorphism. Runtime polymorphism is not applicable to type arguments.

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Q134. What are the benefits of using Wrapper classes over primitive types ?

Ans. With Collection classes , we cannot use primitive types. Moreover for any class using generic types, we cannot use primitive types.

They add more functionality by means of additional methods.

As their reference can be null , they offer consistent check for uninitialized state.

They facilitate caching and reuse by means of constant Pools.

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Q135. If arrays cannot be resized , Why is this code valid

String[] strArray = new String[2];
strArray = new String[5];

Ans. We are not resizing the first array here but assigning the reference strArray to a new Array with size 5.

So after line 2, We have 2 arrays in memory, one with size 2 and other with size 5 with strArray referring to second array with size 5.

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Q136. Design an e commerce application using AWS ? What AWS services will you use ?

Ans. CloudFront for Static Media and cached content
EC2 and EKS ( Docker ) for main application computing
RDS and DynamoDB for Database
Lambda ( computing ) for running back end cron jobs
Kinesis for streaming and SQS for queuing
CloudWatch for Monitoring
CloudFormation for Infrastructure templating
S3 for Object Storage

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Q137. Difference between Region , Availability zone and Edge Locations ?

Ans. A Region is a Geographical entity like US-East , US-West etc. Each Region may have multiple availability zones where each zone comprise of 1 or more Data Center located with each other.

Edge Locations are the sites that hosts cached content for faster delivery and for saving network traffic as they feed content from sites that are local or near to consumption.

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Q138. What are the different S3 storage Tiers ?

Ans. S3 Standard
S3 IA - Infrequently Accessed
S3 One Zone IA - One Zone only , Infrequently Accessed
S3 Glacier - achieved

S3 Standard is most expensive , Glacier is least expensive

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Q139. What are the factors on which customers are charged for using S3 ?

Ans. Storage Tier ( IA , Standard , Glacier etc )
Storage ( Volume of Data )
No of Requests
Meta Data
Data Transfer Speed acceleration through cache and Edge Location

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Q140. What are the factors on which price classes are based on in case of EC2, S3, CloudFront and EBS ?