Java Backend Interview: Key Concepts and Practical Coding Explained

Introduction

This summary covers core topics commonly asked in Java backend interviews, illustrated through a detailed Q&A format. It addresses thread management, exception handling, Spring Boot, and microservices, concluding with a practical Java 8 programming example. For a broader overview of Java fundamentals, see Java Programming: A Comprehensive Guide to Understanding Java and Its Concepts.

Java Threading Concepts

Ways to Create Threads

Extend Thread class and override run()
Implement Runnable interface
Use the Executor framework (recommended for multiple threads)

Deadlocks and Prevention

Deadlock occurs when threads wait indefinitely on each other's resources.
Prevent by consistent lock ordering, using tryLock(), and minimizing synchronized blocks.

Thread States

New (created but not started)
Runnable (ready to run or running)
Blocked/Waiting (waiting to acquire a lock or waiting for signal)
Timed Waiting (waiting for a specified time)
Terminated (completed execution)
Note: Threads cannot be restarted once terminated; new thread instances are needed.

Exception Handling in Java

throw is used to explicitly throw an exception.
throws declares exceptions a method might throw.
Exceptions: Checked (compile-time checked) and unchecked (runtime exceptions like NullPointerException).

Try-With-Resources

Allows automatic resource management.
Resources (e.g., BufferedReader) implementing AutoCloseable are closed automatically at block end.

Object Comparison

== compares object references.
equals() compares object content.

Example with strings:

String str1 = "ABC";
String str2 = new String("ABC");
str1 == str2 // false
str1.equals(str2) // true

Abstract Class vs Interface

Abstract class: can have abstract and concrete methods; allows variables.
Interface: primarily abstract methods; since Java 8, may include default and static methods.
A class can extend only one abstract class but implement multiple interfaces.

Spring Boot Fundamentals

Dependency Injection (DI)

DI injects dependencies into a class instead of creating them internally.
Supported via annotations like @Autowired and @Bean.
For detailed common Spring Boot interview topics, refer to Top 50 Spring Boot Interview Questions and Answers Explained.

Circular Dependencies

Handled using proxies or @Lazy annotation to defer bean initialization.

Repository Interfaces

CrudRepository: basic CRUD operations.
JpaRepository: extends CrudRepository, adds pagination and more features.
Preference depends on use case; e.g., JpaRepository for pagination support.

Performance Tuning Strategies

Use caching to reduce database calls.
Apply indexing on frequently queried columns.
Monitor with Spring Actuator.
Implement asynchronous processing with @Async.

Microservices Architecture

Monolith to Microservices Migration Steps

Identify domain boundaries and functional modules.
Develop independent services for each module.
Define service communication:
- Synchronous (REST APIs)
- Asynchronous (message brokers like Kafka)
Use API Gateway as a unified entry point.
Implement service discovery (e.g., Eureka).
Setup CI/CD pipelines for automated deployment.
Write unit and integration tests; automate regression testing.

Communication Patterns

Synchronous: client waits for response before continuing.
Asynchronous: client sends requests without waiting; consumers process messages independently.

Java 8 Coding Example: Unique Sorted Words

Input: A sentence string.
Process:
1. Split sentence into words.
2. Convert to lowercase.
3. Extract distinct words.
4. Sort alphabetically.
5. Collect into a list.

List<String> uniqueWords = Arrays.stream(sentence.split(" "))
    .map(String::toLowerCase)
    .distinct()
    .sorted()
    .collect(Collectors.toList());

uniqueWords.forEach(System.out::println);

Output: List of unique words sorted alphabetically, e.g., [and, fun, is, java, powerful].

For foundational coding exercises related to Java basics, consider Java Basics: Outputs, Variables, and User Input Explained.

Conclusion

This overview equips Java developers with concise explanations of core backend concepts, practical coding techniques, and strategic insights for interviews and real-world projects. To deepen interview preparation, explore Top 10 Spring Boot Interview Questions with Answers & Examples.

uh hello good evening how are you hi good evening I'm good thank you how are you I'm also doing well thank

you so this this call is regarding your Java interview and this is going to be your uh Java backend interview so do you

like to introduce yourself about technology stack and what kind of uh domains you have worked

on uh yes so I am currently uh having around 4.5 years of experience working as a Java developer so I know

Java writing Rest API is using spring boot uh basics of micros bed architecture and uh from database side I

know SQL and Oracle as a database and apart from this I have a little idea about UI Technologies as

well like angular HTML CSS but that is my secondary skill my primary skill is Ja so that's it from my side

okay great all right and uh which version of java are you currently working

on uh yes we are currently using jdk8 but trying to upgrade it to jdk 17 all right nice okay so if I ask you

to rate yourself in C Java out of five how much would you rate yourself okay so out of five I'll will

try to answer at least uh four questions so if you ask me to WR I can WR myself around

four okay great all right so let's get started what are the different ways to create threads do you have any idea

around this uh yes so we can create the thread in two or three ways uh first way is

like we can extend the thread class and then override the run method so that will give me a thread object then I can

use runable interface so my class should Implement that reenable interface and then I'll be able to implement or create

a thread and there is a executor framework okay so that can also use to create the thread so these are the three

ways which can be used to create threads in Java okay all right and what is the recommended way of creating

threats so you can uh if there is a need to create multiple threads then you can go with executor framework otherwise if

you want to create a single thread and your class is not extending any other class then you can go with thread class

otherwise uh it is simply go and Implement theal inter renable interface all right great okay uh so let's uh just

go one step further uh do you know what is deadlock and how can uh we avoid it uh yes so uh Deadlock occurs when two

or more threads are waiting on each other's resources okay so let us consider I have uh one thre thread one

is holding a resource one and uh thread two is waiting for resource to but that is hold by another thread so this

situation is called as deadlock now we can avoid this by different ways like uh we can use locks in a consistent order

or uh we can use uh trylock mechanisms like lock. TR function which is available in

java.util concurrent package or even we can minimize the synchronized code blocks so that will help me to reduce

the deadlock conditions great great uh what are the common thread States or what are the

different states in which the thread can be present uh yes so there are different

stats uh through which a thread can go uh so when we create a thread but that thread is not at started so we call it

as uh in the thread is in New State now when the thread starts running or it is ready to run then we call that state as

runable state then a blocked state or we can call it as a wetting state so thread is actually uh in this set the thread is

actually waiting to acquire the lock then we have another like waiting or time waiting uh state so where the

thread is waiting for a specific time and when we uh uh finish or the thread finish its task so basically it

completes uh the task execution then that thread goes into terminated State okay can I uh can I uh once thread is

terminated can I restart the thread uh once a thread is terminated uh I think you should be able to restart

that thread but that will be a new thread okay all right all right okay now uh let's let's try to go towards

exception uh what is the difference between and throws do you know about it uh yes so throw and throws keyword

are related to exception handling in Java so basically throw is used to explicitly throw an exception in the

code for example I want to throw an exception when I don't see the information which is uh let us consider

I'm trying to find an employee and that employee is not available so I can throw employee not found exception so I can

use throw keyword to throw this exception and throws is used with your method signature this indicates that the

method uh May throw an exception so this is the use of uh throw ski word okay okay all right and uh what

are the different types of exceptions in Java we have a different types of exceptions like checked exception and

unchecked exceptions so checked exceptions are the exceptions which are checked by your compiler at compile time

and unchecked exceptions are basically runtime exceptions which occurs when your code is running or this is uh not

checked by your compiler for example null pointer exception or array index out of bound exception so these are the

runtime exceptions which occurs at a run time okay great um there is something called as TR with resource what what do

you mean by TR with resource uh try with a resource means we are writing a resource along with your tribe

block for example uh uh let us consider there is a file okay and I want to create an object of buffer reader so

instead of creating separate object of buffer reader inside triblock or outside Tri block what I can do I can use the

buffer dater object along with a TR statement so what will happen I don't need to close that resource explicitly

so when you use try with resource the resource will be automatically closed once uh the uh execution or uh use of

that resource is completed so you don't need to uh explicitly close that resource so that is a try with resource

and the one more thing related to it is like the uh as I said I need a buffer rer res so that Bufford reader

should be implementing that autoc closable interface that is the requirement to use or try with

resource excellent in Java excellent excellent nice so uh let's try to um what is the difference between uh equal

to equal to operator and equals method these are the two ways to compare the objects so what what is the difference

between them uh so equal equal operator is used to compare the references for the

objects okay and equals method is used to check the content of the object for example uh now let me uh create two

strings uh string St str1 equal to uh ABC and string St str2 is equal to new string of ABC in this case if I use St

R1 equal equal to St str2 in this case I'll get uh the response as false because this St str1 and S str2 are

referencing to two different uh instances and if I use St str1 do equals St str2 then I'll get the response as

true because Str str1 is holding the string content as ABC and Str str2 is also hold in string as AB so both ABC

and ABC are same so I'll get uh true as a uh return value or the response okay okay and uh what about

what is the difference between abstract class and interface and when to go for which one do do you have any idea on

this uh so basically abstract class uh can have methods as well as variables but in interface we have only abstract

methods so in abstract class you can have abstract as well as non-abstract methods but in interface till Java we

have only abstract methods in Java we have static and default methods as well where we can provide the implementation

of uh the methods and the other thing is like uh a class can extend only one abstract class but it can Implement

multiple interfaces so interface can extend the interface but it cannot implement the

interface okay all right nice so these are the two differences okay right uh now let's try

to move towards uh spring boot questions okay how much would you rate yourself in Spring boot out of

five uh so even in Spring boot I will try to answer at least four questions out of five so I can rate around four

great great uh there's a basic fundamental what is the dependency injection and how does

spring wot support it okay so dependency injection is basically a design pattern where

dependencies are injected into a class instead of being created inside it okay so what I'm going to do is I'm going to

create uh the dependency instead of creating the dependencies it will be injected by this dependency injection

pattern now uh this spring boot uh supports this dependency injection using annotations like add direct aut or addir

bin where uh the bins will be injected by using these annotations for example let us consider I need a bean of type

employee so I can use object of employee along with atwi I'll get the instance or being of uh type

employee okay all right all right and uh uh do you know circular dependen dependencies and how does uh spring

handle it spring boot handle it uh yeah so circular dependencies can be handled using proxy

objects or you can use the annotations like add direct a lazy annotation to load the dependencies lazily so

developers actually uh can avoid these circular dependencies by refactoring the code or we can use constru injection

with a direct lazy annotation that will load the beans lazily so you can avoid the circular

dependency great great okay so now uh in Spring boot there are different types of repositories oh for example cred

repository and jpa repository what is the difference between cred repository and GP

repository uh so C repository as it is name suggest uh it is useful for performing CED operations like create

read update and a delete operations whereas JPI repository is built on top of BL repository where it provides the

operations which are available in C repository with additional attributes or uh functions like pation so you can have

uh I can say that there are uh more features which are available in jpa repository okay and in your project uh

what kind of repository are you using uh we are using JP repository because we need to the results as well

as we need pation as well and that is supported by JP repository so we use the JP

repository okay okay nice nice so um what are some of the best practices for tuning the performance of your

springboard application I have an application in I want to tune the performance of that application how

would you go about this okay uh so to tune the performance or to improve the application

performance I can use uh caching that is one way so what will happen uh caching means basically we are trying to avoid

the database calls frequently let us consider I need uh some data the database and that data is not changing

frequently so what I will do I'll cure that data at servers memory and then whenever there is a need it will uh go

to the server get the data from the cach and to the client every time I don't need to go to the database so that will

reduce the uh time uh of round trip from client to server apart from that on database side uh I can use indexing on

which are frequently used for search operations so I can uh optimize queries using uh this proper indexing then

uh I can even configure the actuator end points to get the insights of application or to monitor the

application health and performance that will help me to monitor the performance and then I can use those met to improve

the performance or I can use use the asynchronous processing uh using addir async

annotation okay all right so these are some of the nice ways to uh tune in the performance nice nice all right and um

how do you uh now we'll move to microservices consider you have one monolithic application and I want to

divide that application into microservices how would you go about dividing your monolithic to M

Services okay so dividing a monolithic application into microservice based applications first thing is I'll try to

identify the dos and what I can say as boundaries based on the functionalities so what I will do I'll try to analyze

the existing code and identify the modules which I can create or I can use to create the services then uh the next

thing is I'll try to create this separate services to handle a specific business functionality then we need to

decide how these services will interact with each other or communicate with each other so if there is a need to

consistently pass data from one service to another service then I can use synchronous

communication uh using uh messaging tools like uh capka or ACU mq or rabit mq and if there is a need to have a

synchronous communication like sending a request to a service and getting dat that service so I can use um rest apis

for this communication then I'll introduce API Gateway which will act as a front door for uh the client request

so whenever there is a request from client side it will go through the API and then uh the API get will collect

data from the different services and send back to the client and then I will use Discovery service so that service

will help me toover the instances of these services so whenever there is a need uh to communicate uh or to manage

the communication between Services I can use Discovery service to discover services for communication purpose even

I can use some load balancing to uh manage the load of these Services then I need to decide the technical

stack which will be required to implement these Services as these Services can be developed independently

and deployed independently then uh I uh need to decide the things related deployment of

these services for example I need to set up a cicd which will help me for continuous integration and

continuous deployment of this services so whenever developer commits the code the checkin should happen and it should

build the code and deploy the code to a VM or server by using Docker images or you can use kubernetes for managing

these containers then uh testing point of view I'll try to write the J units which will cover the coding

and uh from Qs point of view we uh have testing as well as Automation in place to uh for regression testing so these

are the few things which I will consider while migrating monolith application into uh microservice based application

yeah that that was a comp comprehensive answer all right thanks all right now uh let's try to uh do you know some of the

differences between synchronous and asynchronous communication in case of microservices uh yes so synchronous

communication is basically uh like send the request and wait for the response before proceeding with the next one so

basically client sends a request and for response from the uh server s uh before proceeding with the next request and I

uh we are using rest apis for uh this communication okay and uh asynchronous communication is like client continually

sends the request and uh it doesn't wait for the server to get the response for example message cues like app or rabbit

where the publisher will publish the messages continuously to that capka broker or q and the consumer will

consume it as and when are required is synchronous and asynchronous communication okay nice uh all right now

uh I am going to give you one program you can try to answer that program using Java 8 so do you want to share your

screen yes uh let me know once it is visible I can I can see your

screen okay now now uh the program statement is like this uh you want to write you need to

write a program to find the list of unique words from a sentence okay okay and sorted alphabetically in

alphabetical order using Java we need to find the unique words and sort it alphabetically using Java

it okay so what I will do uh here you can see I have written a class client and inside I write a man

that will be the starting point for my program execution now I'll take the input as a string

so Str Str equal to uh so what I will do I'll take uh input as something like Java is

fun and Java is powerful so this will be my input okay

okay now the next thing is uh I have to find the list of inals from this sentence and then I need to sort it

right so what I will do first I will try to find the and store it into list so I'll declare a variable of type list

which will hold all these unique words so I'll mention it as unique words okay now find the unique

words I can use split function which is available in string so what I can do uh and then I need to uh use stream

apis and then display the result so first thing is I'll need to uh split this so St str.

split will be used to split the uh sentence by spaces then I need to use a stream what I will do I'll simply create

a stream using array

dot stream and here I'll pass the uh Words which are splitted by using space function then what I will do I'll

convert this into lower case so I'll use map and then I'll use string colon colon to lower case so basically this lower

case is a function which is available in string and and I'm referring it using method reference object then what I will

do I'll try to find the distinct words using uh distinct function and then I'll sort all this using sorted function okay

now I got uh what I did here so I tried to split this sentence using equals and then I converted all these uh words into

low workus and then I tried to find the distinct or unique words using distinct function and then I sorted it and

finally I'll collect it into a list so I'll use collect method which takes collector and this

collectors dot to list will give me all the words into a list okay now I will get all the unique words and now I'll

simply print these words okay so I'll use system. out. print to print these words

now let me run this and see what is the output so here you can see and one is Java powerful so Java was occurred twice

but we have to find unique words so basically we need to avoid the duplicates so Java has been printed only

once then is is also printed only once and if you see all these are sorted in ascending order or alphabetical

order yes so that is uh from this Cod yeah nice all right wonderful so I think uh I'm done from my side do you

have any specific questions for me uh no I don't have any question all right so it was wonderful talking to you

and thanks a lot for your time and have a very uh good day ahead thank you yeah thank you Bye by

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