List of topics you need to cover if you're preparing for Java Interviews based on current Job market:

1. Core Java Fundamentals (Refer to already posted topics)
2. Advanced Java
- Design Patterns
- Multithreading
- Java Memory Model
- Performance Optimization
- Reflection & Dynamic Proxies
3. Spring Framework
- Spring core concepts
- Spring boot
- Spring Data JPA
- Spring Security
- Spring cloud
- Spring webflux
4. Hibernate
5. Testing (JUnit, Mockito, Integration, Functional, Performance Testing)
6. Build Tools (Maven / Gradle)
7. Logging
8. RDBMS, NoSQL DBs
9. WebSecurity Concepts
10. REST API concepts
11. CI/CD (Jenkins, GitHub Actions)
12. Containerization (Docker, Kubernetes)
13. Version Control (GitHub)
14. Monitoring (Grafana, ELK Stack etc)
15. Cloud (AWS, Azure, GCP (Very rare) )
16. Spring boot microservices
16. Messaging systems
17. Caching Strategies
18. System Design
19. Data Structures
20. Algorithms
21. Agile Methodologies
22. Behavioral questions

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Here's a detailed A-Z explanation of essential Java programming concepts:

A - Abstraction

Abstraction is the process of hiding complex implementation details and showing only essential features. In Java, it's achieved using abstract classes and interfaces. It helps reduce complexity and increase code reusability.

B - Boolean Logic

Java uses Boolean logic to control program flow, typically through if, while, for, etc. It deals with true or false values, using logical operators like && (AND), || (OR), and ! (NOT).


C - Classes and Objects

Java is an object-oriented language. A class is a blueprint for creating objects, which are instances of classes. Classes encapsulate data (fields) and behaviors (methods).

class Car {
String color;
void drive() { System.out.println("Driving..."); }
}


D - Data Types

Java has two types:

Primitive (int, float, char, boolean, etc.)

Reference (arrays, classes, interfaces).
This allows the developer to handle different types of data efficiently.


E - Encapsulation

Encapsulation is bundling data and methods that operate on that data within a class and restricting direct access to some of the object's components (usually with private fields and public getters/setters).

F - Functions (Methods)

Functions in Java are called methods. They define the behavior of objects and are declared inside classes.

void greet(String name) {
System.out.println("Hello, " + name);
}

G - Generics

Generics allow classes and methods to operate on objects of various types while providing compile-time type safety.

List<String> names = new ArrayList<>();

H - HashMap

A part of the Collections Framework, HashMap stores data in key-value pairs and allows constant-time performance for basic operations like get() and put().


I - Inheritance

Inheritance allows one class to acquire the properties and behaviors of another. Java supports single inheritance through the extends keyword and interface inheritance through implements.

class Dog extends Animal {
void bark() { System.out.println("Woof!"); }
}


J - Java Virtual Machine (JVM)

JVM is the engine that runs Java bytecode on your machine. It provides platform independence, memory management, and runtime optimization.


K - Keywords

Java has reserved keywords like class, if, else, while, static, final, etc., that have predefined meanings and cannot be used as identifiers.

L - Loops

Used to execute a block of code repeatedly:

for (known iterations)

while (condition-based)

do-while (runs at least once)


M - Multithreading

Java supports multithreading, allowing concurrent execution of two or more threads for maximum CPU utilization and performance in applications.

class MyThread extends Thread {
public void run() { System.out.println("Thread running"); }
}


N - Null Pointer Exception

One of the most common runtime errors in Java. Occurs when you try to access a method or property on an object that is null.

O - Object-Oriented Programming (OOP)

Java follows OOP principles:

1. Encapsulation
2. Inheritance
3. Polymorphism
4. Abstraction

This makes the code more modular, reusable, and easier to manage.


P - Polymorphism

It allows methods to behave differently based on the object. Java supports compile-time (method overloading) and runtime (method overriding) polymorphism.


Q - Queue (Data Structure)

A First-In-First-Out (FIFO) data structure. Java provides several implementations like LinkedList, PriorityQueue, and concurrent queues.


R - Recursion

A method that calls itself to solve smaller instances of a problem. Often used for problems like factorials, tree traversals, etc.

int factorial(int n) {
return (n == 0) ? 1 : n * factorial(n - 1);
}


S - Streams (Java 8+)

A powerful feature to process sequences of elements (like collections) using functional programming.
Example:

list.stream().filter(x -> x > 10).forEach(System.out::println);


T - Time Complexity

An important concept in analyzing code performance. It measures how the time to run an algorithm grows with input size (e.g., O(n), O(log n), O(n²)).

U - Unit Testing

Used to verify individual parts of code (methods, classes) using tools like JUnit or TestNG. Ensures each part works correctly in isolation.


V - Variables

Containers for storing data values. Java supports:

Instance variables
Static variables
Local variables
Parameters


W - Wrapper Classes

Provide object versions of primitive data types. E.g., int → Integer, double → Double. Useful when working with collections or for autoboxing.


X - XML (eXtensible Markup Language)

Used to store and transport data. In Java, XML is commonly used for configuration (e.g., Spring) and data interchange.


Y - Yield (Threading)

The Thread.yield() method hints to the scheduler that the current thread is willing to yield its current use of a processor.


Z - Zero-Based Indexing

Java arrays and collections are zero-indexed, meaning the first element is accessed at index 0.

int[] arr = {1, 2, 3};
System.out.println(arr[0]); // Outputs 1


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

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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.

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8. Basic DevOps

8.1. Learn to Use AWS & Deploy Java Apps

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9. Basic of git and GitHub
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10. Java Microservices

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14. Java Quick revision for Interviews

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15. Spring JMS(java messaging services)

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Java Basics every beginner should learn to build a strong foundation:

1. Hello World & Setup

Install JDK and an IDE (like IntelliJ or Eclipse)

Write your first program: public class HelloWorld

2. Data Types & Variables

Primitive types: int, double, char, boolean

Non-primitive types: String, Arrays, Objects

Type casting (implicit & explicit)

3. Operators

Arithmetic: + - * / %

Comparison: == != > < >= <=

Logical: && || !

4. Control Flow

If, else if, else

Switch-case

Loops: for, while, do-while

break and continue

5. Functions (Methods)

Syntax: public static returnType methodName(params)

Method overloading

Return types & parameter passing

6. Object-Oriented Programming (OOP)

Classes & Objects

this keyword

Constructors (default & parameterized)

7. OOP Concepts

Encapsulation (private variables + getters/setters)

Inheritance (extends keyword)

Polymorphism (method overriding)

Abstraction (abstract classes & interfaces)

8. Arrays & ArrayList

Declaring and iterating arrays

ArrayList methods: add, remove, get, size

Multidimensional arrays

9. Exception Handling

Try-catch-finally blocks

throw and throws

Custom exceptions

10. Basic Input/Output

Scanner class for user input

System.out.println() for output

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