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Interview Questions and Answers

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 ?

Core Java

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 ?

Core 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 ?

Core Java

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 ?

Core Java

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?

Core Java

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 ?

Core 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 ?

Core Java

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)

Core Java

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 ?

Core Java

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>();

Core Java

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 ?

Core Java

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];

Core Java

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 ?

Amazon Web Services (AWS)

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 ?

Amazon Web Services (AWS)

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 ?

Amazon Web Services (AWS)

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 ?

Amazon Web Services (AWS)

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 ?

Amazon Web Services (AWS)

Ans. EC2 - Time and Reservation , Instance Type ( Memory , Computation power )
S3 - Accessibility Frequency , Data Transfer , Storage Space
CloudFront - Geography, No of Locations
EBS - IOPS, Access Frequency

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Q141. What are the different Route53 routing policies ?

Amazon Web Services (AWS)

Ans. Simple
Weighted
Latency
Failover
GeoLocation
Multivalue

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Q142. What is the relationship between VPC and subnet ? Can you tell with an example ?

Networking

Ans. A VPC is devided into multiple subnets. Subnet in a particular VPC could be public or private.

For Example - A particular account may have multiple vpc's for each of the business type. For each Business or VPC , One can have different subnets to cater to each department so that access across departments could be restricted. But still there could be identities that have control over different subnets. Moreover we could have multiple subnets for each department so as to have distinguished public and private networks having different types of applications ( public facing or internal consumption )

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Q143. What are the different types of inner classes ?

Core Java

Ans. Simple Inner Class,
Local Inner Class,
Anonymous Inner Class,
Static Nested Inner Class.

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Frequently asked in Indian Service Companies. Tech Mahindra ( Based on 3 inputs ) and Infosys ( 3 inputs )

Q144. What is the difference between List, Set and Map ?

or

What are the different Java Collections Interfaces ?

Core Java

Ans. List - Members are stored in sequence in memory and can be accessed through index.
Set - There is no relevance of sequence and index. Sets doesn't contain duplicates whereas multiset can have duplicates.
Map - Contains Key , Value pairs.

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Q145. Does Java support Multiple Inheritance ?

Core Java

Ans. Java doesn't support multiple inheritance. Interfaces does't facilitate inheritance and hence implementation of multiple interfaces doesn't make multiple inheritance.

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Frequently asked at HCL Technologies ( Based on 3 feedback )

Q146. Difference between Checked and Unchecked exceptions ?

Core Java

Ans. Checked exceptions are the exceptions for which compiler throws an errors if they are not checked whereas unchecked exceptions are caught during run time only and hence can't be checked.

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Very frequently asked. Favorite question in Walk in Drive of many Indian service companies.

Q147. What is a Final Variable ?

Core Java

Ans. Final variable is a variable constant that cannot be changed after initialization.

Sample Code for final variable

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Q148. When are static variables loaded in memory ?

Core Java

Ans. They are loaded at runtime when the respective Class is loaded.

Sample Code for static variable

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Q149. Can we serialize static variables ?

Core Java

Ans. No. Only Object and its members are serialized. Static variables are shared variables and doesn't correspond to a specific object.

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Q150. Can we use null as a key for a map collection ?

Core Java

Ans. Yes , for HashMap. HashMap implements Map interface. HashMap allows one null key and any number of null values.

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