Java Backend Development is a highly in-demand and well-paying job in the tech industry.

If you're looking to become an excellent Java Developer, here's a roadmap that includes various online resources:

1. Core Java

First comes the fundamentals of Java Programming, here you will understand OOP concepts, conditional statements, collection frameworks,and many more.

https://youtube.com/playlist?list=PLd3UqWTnYXOnjGmyjD3zbIkyLXP15-6w0&si=UYvQhVNlZQYCH1Il

2. JDBC

A  Java API that allows you to access tabular data, especially data stored in a relational database.

https://youtube.com/playlist?list=PLsyeobzWxl7rU7Jz3zDRpqB-EODzBbHOI&si=Cc45vpeRXGIUKbfn

3. SQL

Get a good understanding of SQL, which is necessary to work with JDBC to write queries.

https://youtu.be/7S_tz1z_5bA

4. JSP + Servlet

Learn JSP and Servlet, which are used for writing web pages for web applications and handling client requests on the server-side.

https://youtube.com/playlist?list=PLE0F6C1917A427E96&si=TQeOIgluG3R29622

5. Spring Framework

5.1. Core Spring Framework

https://youtu.be/GB8k2-Egfv0?si=dHYp6rFkGVl9RMt8

5.2. Spring REST & Spring DATA

https://youtu.be/GwYUjzPrQTM?si=hJfmpukii2WnofxQ

5.3. Spring Security
Spring Security is the primary choice for implementing application-level security

https://youtu.be/sm-8qfMWEV8?si=uMkpBxOUXIptZif9

6. Hibernate Framework

Hibernate is a Java framework that simplifies the development of Java application to interact with the database. It is an open source, lightweight, ORM (Object Relational Mapping) tool.

https://youtu.be/Yv2xctJxE-w?si=q4FIOBxxX-Y0Qj5q

7. Spring Boot

Spring Boot is the ultimate framework and Makes Java Web Development less boilerplate, it can help you make production-ready applications in no time.

https://youtu.be/msXL2oDexqw?si=XB19MCCECAggcB18

8. Basic DevOps

8.1. Learn to Use AWS & Deploy Java Apps

https://youtube.com/playlist?list=PLVz2XdJiJQxxurKT1Dqz6rmiMuZNdClqv&si=bJyg0Np11PkGDIKO

8.2. Learn Basic Docker:

https://youtu.be/p28piYY_wv8?si=Xoqje7MJad1NeE9g

8.3. Learn Basic Kubernetes:

https://youtu.be/rTNR7vDQDD8?si=dgzwxQof7yGHUP4n

8.4. Deploy Spring Boot App on Kubernetes:

https://youtu.be/EZolJ4lNiYc?si=Kw09qKwMp_LxHTvw

9. Basic of git and GitHub
https://youtu.be/apGV9Kg7ics?si=MHtCgD8o7XqpU4W3

10. Java Microservices

https://youtu.be/tuJqH3AV0e8?si=1jjrWl4l4uYGrE5i

14. Java Quick revision for Interviews

https://youtu.be/oYXivKMSEqM?si=tngbZrvgbAwsShEb

15. Spring JMS(java messaging services)

JMS is a standard Java API that allows a Java application to send messages to another application.

https://youtube.com/playlist?list=PLmCsXDGbJHdibOx6axScPiuHA8ftsrmut&si=J0qjBSbB4GDCeTOv

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Preparing for a Java developer interview can be a bit overwhelming,
but breaking it down by difficulty and experience level can make it more manageable.

Whether you're a fresher or an experienced developer, here's a guide to help you focus your preparation and walk into your interview with confidence.

𝗙𝗼𝗿 𝗔𝗹𝗹 𝗟𝗲𝘃𝗲𝗹𝘀 (𝗜𝗻𝗰𝗹𝘂𝗱𝗶𝗻𝗴 𝗙𝗿𝗲𝘀𝗵𝗲𝗿𝘀)

➤ Topic 1: Project Flow and Architecture (Medium)
- These questions are designed to gauge your understanding of project development, teamwork, and problem-solving. Be ready to discuss a project you've worked on, including the tech stack used, the challenges you faced, and how you overcame them.

𝗙𝗼𝗿 𝗗𝗲𝘃𝗲𝗹𝗼𝗽𝗲𝗿𝘀 𝘄𝗶𝘁𝗵 𝗖𝗼𝗿𝗲 𝗝𝗮𝘃𝗮 𝗦𝗸𝗶𝗹𝗹𝘀 (𝟭-𝟯 𝗬𝗲𝗮𝗿𝘀 𝗘𝘅𝗽𝗲𝗿𝗶𝗲𝗻𝗰𝗲)

➤ Topic 2: Core Java (Medium to Hard)
- Fundamental Java concepts. You'll likely face questions on strings, object-oriented programming (OOP), collections, exception handling, and multithreading.

𝗙𝗼𝗿 𝗘𝘅𝗽𝗲𝗿𝗶𝗲𝗻𝗰𝗲𝗱 𝗗𝗲𝘃𝗲𝗹𝗼𝗽𝗲𝗿𝘀 (𝟯+ 𝗬𝗲𝗮𝗿𝘀 𝗘𝘅𝗽𝗲𝗿𝗶𝗲𝗻𝗰𝗲)

➤ Topic 3: Java 8/11/17 Features (Hard)
- This is where the interview gets more challenging. You'll asked advanced features introduced in recent Java versions, such as lambda expressions, functional interfaces, the Stream API, and modules.

➤ Topic 4: Spring Framework, Spring Boot, Microservices, and REST API (Hard)
- Expect questions on popular frameworks and backend development architectures. Be prepared to explain concepts like dependency injection, Spring MVC, and microservices.

𝗙𝗼𝗿 𝗗𝗲𝘃𝗲𝗹𝗼𝗽𝗲𝗿𝘀 𝘄𝗶𝘁𝗵 𝗗𝗮𝘁𝗮𝗯𝗮𝘀𝗲 𝗘𝘅𝗽𝗲𝗿𝗶𝗲𝗻𝗰𝗲

➤ Topic 5: Hibernate/Spring Data JPA/Database (Hard)
- This section focuses on data persistence with JPA and working with relational (SQL) or NoSQL databases. Be ready to discuss JPA repositories, entity relationships, and complex querying techniques.

𝗙𝗼𝗿 𝗗𝗲𝘃𝗲𝗹𝗼𝗽𝗲𝗿𝘀 𝘄𝗶𝘁𝗵 𝗔𝗱𝗱𝗶𝘁𝗶𝗼𝗻𝗮𝗹 𝗦𝗸𝗶𝗹𝗹𝘀

➤ Topic 6: Coding (Medium to Hard)
- You'll likely encounter coding challenges related to data structures and algorithms (DSA), as well as using the Java Stream API.

➤ Topic 7: DevOps Questions on Deployment Tools (Advanced)
- These questions are often posed by managers or leads, especially if you're applying for a role that involves DevOps. Be prepared to discuss deployment tools like Jenkins, Kubernetes, and cloud platforms.

➤ Topic 8: Best Practices (Medium)
- Interviewers may ask about design patterns like Singletons, Factories, or Observers to see how well you write clean, reusable code.

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2 Month Roadmap to learn Java and Spring from basics to advanced

Week 1-2: Core Java

- Basic Syntax: Data types, operators, loops (for, while, do-while)
- OOP Concepts: Classes, objects, inheritance, polymorphism, encapsulation, abstraction
- Collections Framework: List, Set, Map, Queue, Iterator, etc.
- Exception Handling: Try-catch, custom exceptions, multi-catch
- File I/O: Reading/writing files using java.io and java.nio
- Java 8+ Features: Lambdas, Streams, Optional, Functional Interfaces
- Multithreading: Threads, Runnable, ExecutorService, Future, and basic synchronization

Week 3-4: Advanced Java & JDBC

- JVM Internals: ClassLoader, JIT, memory management, garbage collection
- Generics: Usage and wildcards
- Design Patterns: Singleton, Factory, Strategy, Observer, Dependency Injection (Intro)
- JDBC: Connecting with databases (CRUD operations), PreparedStatement, Connection pooling

Week 5-6: Spring Framework (Basics)

- Spring Core
- Dependency Injection (DI) and Inversion of Control (IoC)
- Beans, Scopes, and Bean Life Cycle
- Autowiring and Spring Annotations
- Spring AOP: Aspect-Oriented Programming fundamentals
- Spring Data JPA: Basic CRUD operations with JPA
- Entities, Repositories, and Custom Queries

Week 7: Spring Boot (Basics)

- Spring Boot Fundamentals: Understanding the Spring Boot architecture
- REST API creation
- Spring Boot Annotations (@RestController, @RequestMapping, etc.)
- Running a Spring Boot application
- Spring Boot Auto Configuration: Application properties and profiles
- Spring Boot with Database: Integrate Spring Boot with MySQL using Spring Data JPA

Week 8: Spring Boot (Advanced)

- Security: Introduction to Spring Security (Basic Authentication)
- Advanced Spring Boot Features: Exception handling (@ControllerAdvice)
- Logging with Spring Boot
- Pagination and Sorting
- Testing: Write unit tests using JUnit and Mockito in Spring Boot

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Java coding interview questions

1. Reverse a String:
Write a Java program to reverse a given string.
2. Find the Largest Element in an Array:
Find and print the largest element in an array.
3. Check for Palindrome:
Determine if a given string is a palindrome (reads the same backward as forward).
4. Factorial Calculation:
Write a function to calculate the factorial of a number.
5. Fibonacci Series:
Generate the first n numbers in the Fibonacci sequence.
6. Check for Prime Number:
Write a program to check if a given number is prime.
7. String Anagrams:
Determine if two strings are anagrams of each other.

8. Array Sorting:
Implement sorting algorithms like bubble sort, merge sort, or quicksort.

9. Binary Search:
Implement a binary search algorithm to find an element in a sorted array.

10. Duplicate Elements in an Array:
Find and print duplicate elements in an array.

11. Linked List Reversal:
Reverse a singly-linked list.

12. Matrix Operations:
Perform matrix operations like addition, multiplication, or transpose.

13. Implement a Stack:
Create a stack data structure and implement basic operations (push, pop).

14. Implement a Queue:
Create a queue data structure and implement basic operations (enqueue, dequeue).

15. Inheritance and Polymorphism:
Implement a class hierarchy with inheritance and demonstrate polymorphism.

16. Exception Handling:
Write code that demonstrates the use of try-catch blocks to handle exceptions.
17. File I/O:
Read from and write to a file using Java's file I/O capabilities.
18. Multithreading:
Create a simple multithreaded program and demonstrate thread synchronization.
19. Lambda Expressions:
Use lambda expressions to implement functional interfaces.
20. Recursive Algorithms:
Solve a problem using recursion, such as computing the factorial or Fibonacci sequence.

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🚀 Roadmap to Become a Java Developer 🔰

📂 Programming Basics
 ∟📂 Master Java Syntax, Variables & Data Types
  ∟📂 Learn Control Flow, Loops & Exception Handling
   ∟📂 Practice with Basic Programs

📂 Object-Oriented Programming (OOP)
 ∟📂 Understand Classes, Objects & Inheritance
  ∟📂 Dive into Encapsulation, Polymorphism & Abstraction
   ∟📂 Explore Design Patterns in Java

📂 Tools & Build Systems
 ∟📂 Get Comfortable with IDEs (e.g., IntelliJ, Eclipse)
  ∟📂 Master Maven & Gradle for Dependency Management
   ∟📂 Learn Git & Version Control Systems

📂 Data Structures & Algorithms
 ∟📂 Learn Java Collections (Lists, Sets, Maps, etc.)
  ∟📂 Understand Algorithms & Problem Solving
   ∟📂 Tackle Coding Challenges

📂 Web Development & APIs
 ∟📂 Learn Servlets & JSP Basics
  ∟📂 Dive into Spring Boot & RESTful Services
   ∟📂 Understand Microservices Architecture

📂 Database & Persistence
 ∟📂 Master JDBC & SQL Fundamentals
  ∟📂 Explore ORM Tools like Hibernate & JPA
   ∟📂 Practice with Real-World Data

📂 Testing & Debugging
 ∟📂 Get Started with JUnit & Test-Driven Development
  ∟📂 Learn Debugging Techniques & Best Practices
   ∟📂 Automate Testing with Tools

📂 Advanced Topics
 ∟📂 Learn Multithreading & Concurrency
  ∟📂 Understand JVM Internals & Garbage Collection
   ∟📂 Explore Performance Optimization

📂 Projects & Real-World Applications
 ∟📂 Build End-to-End Java Applications
  ∟📂 Contribute to Open-Source Projects
   ∟📂 Showcase on GitHub & Portfolio

📂 Interview Preparation & Job Hunting
 ∟📂 Solve Java Coding Challenges
  ∟📂 Master System Design & Architecture Concepts
   ∟📂 Network & Apply for Java Roles

✅️ Get Hired

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Java developer - Realistic Approach 💪🩵

1. Learn Java as a whole:
📍Beginner :
- Java Core: Java syntax , Collections framework , Exception Handling , Multithreading ,
File Handling
- Java Intermediate - JDBC , Design Pattern , Generics etc.

💪Pro :
- Advanced Java - Lambdas , streams , time , concurrency utilities , JVM internals
- Design Patterns - Creational , Structural , Behavioral

2. Build Tools:
- Learn and use popular build tools like :
📍Beginner : Maven (Web development) Gradle (App development)
💪Pro : Ant

3. Version Control:
- Master a version control system like Git. Master the skills for
📍Beginner : Github
💪Pro : GitLab , BitBucket

4. Command Line (This can be done parallel to the above 4)
Believe me when it comes to Java development Command line skills will be a boon for you guys.
Start with the basics for eg : install and setup java with Command Line only.

Start using Linux distributions ( it's very necessary ) go to a virtual box or dual boot your systems with any of Ubuntu , Kali Linux , Manjaro etc

5. Learn Servlets and JSP and then go for a framework ( Spring boot

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Polymorphism in Java

📍 Polymorphism allows a single interface to be used for different types of actions.

📍 It is of two types:

Method Overloading (Compile-time Polymorphism)

Method Overriding (Runtime Polymorphism)


*Method Overloading* (Same method name, different parameters)

class MathOperations {
int add(int a, int b) {
return a + b;
}

double add(double a, double b) {
return a + b;
}
}

*Method Overriding* (Same method in parent and child class)

class Animal {
void makeSound() {
System.out.println("Animal makes a sound");
}
}

class Dog extends Animal {
void makeSound() {
System.out.println("Dog barks");
}
}

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1. What is Java?
2. What are the features of Java?
3. What are the OOP concepts?
4. What is data encapsulation, and why is it useful?
5. What is polymorphism?
6. What are the types of polymorphism, and how do they differ?
7. What does an interface in Java refer to?
8. What is a class in Java?
9. What is a Java object?
10. What is a singleton class, and how can it be used?
11. What is a constructor in Java?
12. What does the term constructor overloading mean?
13. How are non-primitive variables used in Java?
14. In Java, what is a static variable?
15. What are Java data types, and how are they grouped?
16. How do you define primitive data types and describe each by size and description?
17. What do the terms autoboxing and unboxing mean in Java?
18. What are wrapper classes in Java?
19. What is inheritance in Java?
20. What types of inheritance does Java support?
21. What is Java exception handling?
22. What are the differences between unchecked exceptions, checked exceptions, and errors?
23. What are loops in Java?
24. What are the types of loops in Java, and how are they used?
25. What is an infinite loop?
26. What is the difference between the continue and break statement?
27. What is the entry point in Java, and how is it written?
28. In Java, what’s the purpose of static methods and static variables?
29. How do you use, call, and access a static method in Java?
30. How do you use, call, and access a non-static method in Java?

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Leetcode patterns you should definitely checkout to Learn DSA(Java) from scratch

1️⃣ Arrays: Data structures, such as arrays, store elements in contiguous memory locations. They are versatile and useful for a wide variety of purposes.
LeetCode Problems:
• Search in Rotated Sorted Array (Problem #33)
• Product of Array Except Self (Problem #238)
• Find the Missing Number (Problem #268)

2️⃣Two Pointers: In Two Pointers, two pointers are maintained in the collection and can be manipulated to solve a problem efficiently.
LeetCode problems:
• Trapping Rain Water (Problem #42)
• Longest Substring Without Repeating Characters (Problem #3)
• Squares of a Sorted Array (Problem #977)

3️⃣In-place Linked List Traversal: As an explanation, in-place traversal is a technique for modifying linked list nodes without using extra space.
LeetCode Problems:
• Remove Nth Node From End of List (Problem #19)
• Reorder List (Problem #143)

4️⃣Fast & Slow Pointers: This pattern uses two pointers to traverse a sequence at different speeds (fast and slow), often used to detect cycles or find a specific position in the sequence.
LeetCode Problems:
• Happy Number (Problem #202)
• Subarray Sum Equals K (Problem #560)
• Intersection of Two Linked Lists (Problem #160)

5️⃣Merge Intervals: This pattern involves merging overlapping intervals in a collection, often used in problems dealing with intervals or ranges.
LeetCode problems:
• Non-overlapping Intervals (Problem #435)
• Minimum Number of Arrows to Burst Balloons (Problem #452)

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Typical java interview questions sorted by experience


Junior
* Name some of the characteristics of OO programming languages
* What are the access modifiers you know? What does each one do?
* What is the difference between overriding and overloading a method in Java?
* What’s the difference between an Interface and an abstract class?
* Can an Interface extend another Interface?
* What does the static word mean in Java?
* Can a static method be overridden in Java?
* What is Polymorphism? What about Inheritance?
* Can a constructor be inherited?
* Do objects get passed by reference or value in Java? Elaborate on that.
* What’s the difference between using == and .equals on a string?
* What is the hashCode() and equals() used for?
* What does the interface Serializable do? What about Parcelable in Android?
* Why are Array and ArrayList different? When would you use each?
* What’s the difference between an Integer and int?
* What is a ThreadPool? Is it better than using several “simple” threads?
* What the difference between local, instance and class variables?

Mid
* What is reflection?
* What is dependency injection? Can you name a few libraries? (Have you used any?)
* What are strong, soft and weak references in Java?
* What does the keyword synchronized mean?
* Can you have “memory leaks” on Java?
* Do you need to set references to null on Java/Android?
* What does it means to say that a String is immutable?
* What are transient and volatile modifiers?
* What is the finalize() method?
* How does the try{} finally{} works?
* What is the difference between instantiation and initialisation of an object?
* When is a static block run?
* Why are Generics are used in Java?
* Can you mention the design patterns you know? Which of those do you normally use?
* Can you mention some types of testing you know?

Senior
* How does Integer.parseInt() works?
* Do you know what is the “double check locking” problem?
* Do you know the difference between StringBuffer and StringBuilder?
* How is a StringBuilder implemented to avoid the immutable string allocation problem?
* What does Class.forName method do?
* What is Autoboxing and Unboxing?
* What’s the difference between an Enumeration and an Iterator?
* What is the difference between fail-fast and fail safe in Java?
* What is PermGen in Java?
* What is a Java priority queue?
* *s performance influenced by using the same number in different types: Int, Double and Float?
* What is the Java Heap?
* What is daemon thread?
* Can a dead thread be restarted?

Source: medium.