Simple Explanation

Java is a programming language that lets you write instructions for computers. Think of it as teaching a robot to perform tasks—you write code, and the computer executes it. Java is used to build websites, mobile apps (Android), enterprise systems, and cloud applications.

• Role: Backend developer, full-stack engineer, Android developer, enterprise architect.
• Goal: Learn Java fundamentals in 12-16 weeks and land your first job.
• Salary: Junior: $70K-$85K | Mid: $95K-$125K | Senior: $130K-$165K.

Why Learn Java?

• High demand: 3 million+ jobs globally; used by Google, Amazon, Netflix, banks.
• Beginner-friendly: Clear syntax, strong community, tons of tutorials.
• Cross-platform: "Write once, run anywhere"—works on Windows, Mac, Linux.
• Career growth: Path from junior dev → senior → architect → team lead.
• Ecosystem: Spring Boot, Hibernate, Microservices, Cloud (AWS/Azure).

Quick Q&A

Do I need a CS degree? No. Self-taught developers get hired if they build projects and pass interviews.

How long to become job-ready? 12-16 weeks with 15-20 hours/week of focused practice.

What tools do I need? JDK, IntelliJ IDEA (or Eclipse), Git, and a GitHub account.

Is Java still relevant in 2025? Yes. Java powers 90% of Fortune 500 backend systems.

Your 12-Week Roadmap

• Weeks 1-4: Java basics (syntax, loops, conditionals, methods).
• Weeks 5-7: OOP (classes, inheritance, interfaces, polymorphism).
• Weeks 8-9: Collections, Exception Handling, File I/O.
• Weeks 10-11: Spring Boot basics, REST APIs, CRUD operations.
• Week 12: Build 2-3 portfolio projects and prep resume.

Setting Up Your Environment

Step 1: Download and install JDK (Java Development Kit) from Oracle or use OpenJDK.

Step 2: Install IntelliJ IDEA Community Edition (free) or Eclipse.

Step 3: Verify installation by typing java -version in terminal/command prompt.

Step 4: Create your first project: File → New Project → Java → Hello World.

Your First Java Program

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

Explanation:

• public class HelloWorld — Defines a class named HelloWorld.
• public static void main — Entry point; program starts here.
• System.out.println — Prints text to console.

Variables and Data Types

Variables store data. Java has 8 primitive types:

Operators and Expressions

int a = 10, b = 20;
int sum = a + b;        // Addition: 30
int diff = b - a;       // Subtraction: 10
int product = a * b;    // Multiplication: 200
int quotient = b / a;   // Division: 2
int remainder = b % a;  // Modulus: 0

boolean isEqual = (a == b);   // false
boolean isGreater = (b > a);  // true

Control Flow: If-Else

int score = 85;

if (score >= 90) {
    System.out.println("Grade: A");
} else if (score >= 80) {
    System.out.println("Grade: B");
} else if (score >= 70) {
    System.out.println("Grade: C");
} else {
    System.out.println("Grade: F");
}

Loops: For, While, Do-While

// For loop - when you know iteration count
for (int i = 0; i < 5; i++) {
    System.out.println("Count: " + i);
}

// While loop - condition-based
int count = 0;
while (count < 5) {
    System.out.println("Count: " + count);
    count++;
}

// Do-While - executes at least once
int num = 0;
do {
    System.out.println("Number: " + num);
    num++;
} while (num < 5);

Methods (Functions)

public class Calculator {
    // Method to add two numbers
    public static int add(int a, int b) {
        return a + b;
    }
    
    // Method with no return value
    public static void greet(String name) {
        System.out.println("Hello, " + name + "!");
    }
    
    public static void main(String[] args) {
        int result = add(10, 20);
        System.out.println("Sum: " + result);
        
        greet("John");
    }
}

Key Points:

• public — Accessible from anywhere.
• static — Can be called without creating an object.
• int — Return type (use void for no return).
• Parameters go inside parentheses: (int a, int b).

What is OOP?

OOP is a programming style that organizes code into objects—real-world entities with properties (data) and behaviors (methods). Java is 100% object-oriented.

Four Pillars of OOP:

• Encapsulation: Bundle data and methods; hide internal details.
• Inheritance: Reuse code by extending existing classes.
• Polymorphism: One interface, multiple implementations.
• Abstraction: Hide complexity; show only essential features.

Classes and Objects

// Define a class
public class Car {
    // Properties (fields)
    String brand;
    String model;
    int year;
    
    // Constructor
    public Car(String brand, String model, int year) {
        this.brand = brand;
        this.model = model;
        this.year = year;
    }
    
    // Method (behavior)
    public void displayInfo() {
        System.out.println(year + " " + brand + " " + model);
    }
}

// Create objects
public class Main {
    public static void main(String[] args) {
        Car car1 = new Car("Toyota", "Camry", 2023);
        Car car2 = new Car("Honda", "Accord", 2024);
        
        car1.displayInfo();  // Output: 2023 Toyota Camry
        car2.displayInfo();  // Output: 2024 Honda Accord
    }
}

Encapsulation (Getters and Setters)

public class BankAccount {
    private double balance;  // Private = hidden from outside
    
    // Constructor
    public BankAccount(double initialBalance) {
        this.balance = initialBalance;
    }
    
    // Getter
    public double getBalance() {
        return balance;
    }
    
    // Setter with validation
    public void deposit(double amount) {
        if (amount > 0) {
            balance += amount;
        }
    }
    
    public void withdraw(double amount) {
        if (amount > 0 && amount <= balance) {
            balance -= amount;
        }
    }
}

Why Encapsulation? Protects data from invalid changes. You control access via methods.

Inheritance

// Parent class
public class Animal {
    public void eat() {
        System.out.println("This animal eats food.");
    }
}

// Child class inherits from Animal
public class Dog extends Animal {
    public void bark() {
        System.out.println("The dog barks.");
    }
}

public class Main {
    public static void main(String[] args) {
        Dog myDog = new Dog();
        myDog.eat();   // Inherited from Animal
        myDog.bark();  // Defined in Dog
    }
}

Use Case: Avoid code duplication. Common features go in parent class.

Polymorphism

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

class Cat extends Animal {
    @Override
    public void makeSound() {
        System.out.println("Meow");
    }
}

class Dog extends Animal {
    @Override
    public void makeSound() {
        System.out.println("Bark");
    }
}

public class Main {
    public static void main(String[] args) {
        Animal myAnimal = new Dog();
        myAnimal.makeSound();  // Output: Bark (runtime polymorphism)
    }
}

Interfaces and Abstraction

// Interface defines a contract
interface Payable {
    void processPayment(double amount);
}

// Implement the interface
class CreditCard implements Payable {
    @Override
    public void processPayment(double amount) {
        System.out.println("Processing $" + amount + " via Credit Card");
    }
}

class PayPal implements Payable {
    @Override
    public void processPayment(double amount) {
        System.out.println("Processing $" + amount + " via PayPal");
    }
}

When to use: When multiple classes share the same behavior but implement it differently.

What are Collections?

Collections are data structures that store groups of objects. Think of them as smart containers that automatically resize and provide useful methods.

ArrayList - Dynamic Arrays

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList fruits = new ArrayList<>();
        
        // Add items
        fruits.add("Apple");
        fruits.add("Banana");
        fruits.add("Orange");
        
        // Access items
        System.out.println(fruits.get(0));  // Apple
        
        // Size
        System.out.println("Size: " + fruits.size());
        
        // Loop through
        for (String fruit : fruits) {
            System.out.println(fruit);
        }
        
        // Remove item
        fruits.remove("Banana");
    }
}

Use Case: When you need a list that grows/shrinks dynamically.

HashMap - Key-Value Pairs

import java.util.HashMap;

public class Main {
    public static void main(String[] args) {
        HashMap ages = new HashMap<>();
        
        // Add key-value pairs
        ages.put("John", 25);
        ages.put("Sarah", 30);
        ages.put("Mike", 28);
        
        // Get value by key
        System.out.println(ages.get("Sarah"));  // 30
        
        // Check if key exists
        if (ages.containsKey("John")) {
            System.out.println("John's age: " + ages.get("John"));
        }
        
        // Loop through
        for (String name : ages.keySet()) {
            System.out.println(name + " is " + ages.get(name) + " years old");
        }
    }
}

Use Case: Store data as pairs (like a dictionary). Fast lookups.

HashSet - Unique Items Only

import java.util.HashSet;

public class Main {
    public static void main(String[] args) {
        HashSet uniqueNames = new HashSet<>();
        
        uniqueNames.add("Alice");
        uniqueNames.add("Bob");
        uniqueNames.add("Alice");  // Duplicate - won't be added
        
        System.out.println(uniqueNames);  // [Alice, Bob]
        System.out.println("Size: " + uniqueNames.size());  // 2
    }
}

Use Case: Remove duplicates automatically.

Collections Comparison

What is Spring Boot?

Spring Boot is a framework that makes it easy to build production-ready web applications and REST APIs. It handles configuration automatically so you focus on business logic.

Why Spring Boot?

• 95% of Java backend jobs require Spring Boot experience.
• Build REST APIs in minutes without complex XML configuration.
• Includes embedded server (Tomcat) - no separate setup needed.
• Integrates with databases, security, testing tools out-of-the-box.

Creating Your First Spring Boot Project

Step 1: Go to start.spring.io

Step 2: Select: Maven, Java 17, Spring Boot 3.x

Step 3: Add dependencies: Spring Web, Spring Data JPA, H2 Database

Step 4: Generate and download the project

Step 5: Open in IntelliJ, run DemoApplication.java

Simple REST API Example

import org.springframework.web.bind.annotation.*;
import java.util.ArrayList;
import java.util.List;

@RestController
@RequestMapping("/api/users")
public class UserController {
    
    private List users = new ArrayList<>();
    
    // GET all users
    @GetMapping
    public List getAllUsers() {
        return users;
    }
    
    // GET user by ID
    @GetMapping("/{id}")
    public User getUserById(@PathVariable Long id) {
        return users.stream()
            .filter(u -> u.getId().equals(id))
            .findFirst()
            .orElse(null);
    }
    
    // POST create new user
    @PostMapping
    public User createUser(@RequestBody User user) {
        users.add(user);
        return user;
    }
    
    // PUT update user
    @PutMapping("/{id}")
    public User updateUser(@PathVariable Long id, @RequestBody User updatedUser) {
        User user = getUserById(id);
        if (user != null) {
            user.setName(updatedUser.getName());
            user.setEmail(updatedUser.getEmail());
        }
        return user;
    }
    
    // DELETE user
    @DeleteMapping("/{id}")
    public String deleteUser(@PathVariable Long id) {
        users.removeIf(u -> u.getId().equals(id));
        return "User deleted";
    }
}

Spring Boot Annotations Explained

Testing Your API

Using Postman:

• GET: http://localhost:8080/api/users - Fetch all users
• POST: http://localhost:8080/api/users - Send JSON body
• PUT: http://localhost:8080/api/users/1 - Update user with ID 1
• DELETE: http://localhost:8080/api/users/1 - Delete user

Why Databases Matter

Every real application needs to store data permanently. Java connects to databases using JDBC, JPA, and Hibernate.

Common Databases:

• MySQL: Most popular open-source database.
• PostgreSQL: Advanced features, JSON support.
• H2: In-memory database for testing.
• MongoDB: NoSQL database for JSON documents.

SQL Basics You Must Know

-- Create table
CREATE TABLE users (
    id INT PRIMARY KEY AUTO_INCREMENT,
    name VARCHAR(100),
    email VARCHAR(100),
    age INT
);

-- Insert data
INSERT INTO users (name, email, age) VALUES ('John Doe', '[email protected]', 25);

-- Read data
SELECT * FROM users;
SELECT name, email FROM users WHERE age > 21;

-- Update data
UPDATE users SET age = 26 WHERE id = 1;

-- Delete data
DELETE FROM users WHERE id = 1;

-- Joins
SELECT orders.id, users.name 
FROM orders 
INNER JOIN users ON orders.user_id = users.id;

JDBC Example

import java.sql.*;

public class DatabaseDemo {
    public static void main(String[] args) {
        String url = "jdbc:mysql://localhost:3306/mydb";
        String user = "root";
        String password = "password";
        
        try {
            // Connect to database
            Connection conn = DriverManager.getConnection(url, user, password);
            
            // Execute query
            Statement stmt = conn.createStatement();
            ResultSet rs = stmt.executeQuery("SELECT * FROM users");
            
            // Process results
            while (rs.next()) {
                System.out.println(rs.getString("name"));
            }
            
            // Close connection
            conn.close();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}

JPA and Hibernate

Instead of writing SQL manually, JPA (Java Persistence API) lets you work with objects.

import javax.persistence.*;

@Entity
@Table(name = "users")
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    
    private String name;
    private String email;
    private int age;
    
    // Getters and setters
}

// Repository interface
import org.springframework.data.jpa.repository.JpaRepository;

public interface UserRepository extends JpaRepository {
    List findByName(String name);
    List findByAgeGreaterThan(int age);
}

// Usage in Service
@Service
public class UserService {
    @Autowired
    private UserRepository userRepository;
    
    public List getAllUsers() {
        return userRepository.findAll();
    }
    
    public User saveUser(User user) {
        return userRepository.save(user);
    }
}

Why Build Projects?

Projects prove you can code. Recruiters want to see GitHub repos with working code, not just certificates.

Goal: Build 3-5 projects that demonstrate different skills.

Project 1: Todo List REST API

Skills: Spring Boot, REST API, CRUD, MySQL

Features:

• Create, read, update, delete tasks
• Mark tasks as complete/incomplete
• Filter tasks by status (pending, completed)
• Search tasks by title

Tech Stack: Spring Boot + JPA + MySQL + Postman

Timeline: 3-5 days

Project 2: Employee Management System

Skills: Full CRUD, Relationships, Validation

Features:

• Add, view, edit, delete employees
• Department management
• Employee-Department relationship (one-to-many)
• Search and filter by department

Tech Stack: Spring Boot + JPA + H2/MySQL

Timeline: 5-7 days

Project 3: E-Commerce Product API

Skills: Authentication, Pagination, File Upload

Features:

• Product catalog with categories
• User authentication (JWT)
• Add to cart and checkout
• Product image upload
• Pagination and sorting

Tech Stack: Spring Boot + Spring Security + JWT + PostgreSQL

Timeline: 10-14 days

Project 4: Blog Application

Skills: Full-stack, Relationships, Comments

Features:

• User registration and login
• Create, edit, delete blog posts
• Comment on posts
• Like/Unlike posts
• User profiles

Tech Stack: Spring Boot + React/Thymeleaf + MySQL

Timeline: 14-21 days

GitHub and Portfolio Tips

• Create a GitHub account and push all projects
• Write clear README files with setup instructions
• Include screenshots and API documentation
• Add a demo video or deployed link (Heroku, AWS)
• Keep code clean: comments, naming conventions, formatting

Java Interview Roadmap

Java interviews have 3 rounds:

• Round 1: Phone screen (30 mins) - Basics, projects, availability.
• Round 2: Technical (60-90 mins) - Coding problems, OOP, collections.
• Round 3: System design (45-60 mins) - Architecture, scalability (for mid/senior).

Top 20 Java Interview Questions

1. What is Java? Why is it platform-independent?

Java compiles to bytecode that runs on JVM (Java Virtual Machine). JVM is platform-specific but bytecode is universal.

2. Difference between JDK, JRE, and JVM?

• JDK: Development kit (compiler + libraries)
• JRE: Runtime environment (JVM + libraries)
• JVM: Executes bytecode

3. What is OOP? Explain the 4 pillars.

Encapsulation, Inheritance, Polymorphism, Abstraction (see OOP section above).

4. Difference between == and .equals()?

• == compares memory references
• .equals() compares content/values

5. What is a constructor?

Special method called when creating an object. Same name as class, no return type.

6. Difference between ArrayList and LinkedList?

• ArrayList: Fast random access, slow add/remove in middle
• LinkedList: Slow access, fast add/remove anywhere

7. What is a HashMap? How does it work?

Stores key-value pairs. Uses hashing to find values in O(1) time.

8. Difference between interface and abstract class?

• Interface: 100% abstract (before Java 8), can implement multiple
• Abstract class: Can have concrete methods, single inheritance

9. What is exception handling? Try-catch example.

try {
    int result = 10 / 0;
} catch (ArithmeticException e) {
    System.out.println("Cannot divide by zero");
} finally {
    System.out.println("Always executes");
}

10. What is multithreading?

Running multiple threads (tasks) concurrently. Use Thread class or Runnable interface.

Coding Challenges Practice

Easy Problems (Start Here):

• Reverse a string
• Find largest number in array
• Check if number is prime
• Count vowels in a string
• Factorial using recursion

Medium Problems:

• Find duplicates in array
• Two sum problem (LeetCode #1)
• Valid parentheses (LeetCode #20)
• Merge two sorted arrays
• Find missing number in 1 to N

Practice Platforms:

• LeetCode (75-100 problems minimum)
• HackerRank Java track
• CodeSignal
• InterviewBit

Spring Boot Interview Questions

1. What is Spring Boot? Why use it?

Framework to build production-ready apps fast. Auto-configuration, embedded server, less boilerplate.

2. Explain @RestController vs @Controller

• @RestController: Returns JSON/XML (REST APIs)
• @Controller: Returns HTML views

3. What is Dependency Injection?

Spring creates and manages objects (beans). Use @Autowired to inject dependencies.

4. Difference between @Component, @Service, @Repository?

• @Component: Generic bean
• @Service: Business logic layer
• @Repository: Data access layer

5. How to handle exceptions globally?

Use @ControllerAdvice and @ExceptionHandler.

Behavioral Questions

• Tell me about yourself (30-sec elevator pitch)
• Why Java? Why this company?
• Describe a challenging project
• How do you handle tight deadlines?
• Teamwork example
• Where do you see yourself in 3 years?

Interview Day Checklist

• Test your mic, camera, internet 1 hour before
• Have resume, projects, and portfolio links ready
• Keep notepad for notes and whiteboard coding
• Dress professionally (even for remote)
• Ask 2-3 questions at the end (team, tech stack, growth)

Free Courses & Tutorials

• Java Basics: Codecademy Java course, Java Programming MOOC
• Spring Boot: Spring.io guides, Amigoscode YouTube
• Data Structures: freeCodeCamp, GeeksforGeeks
• Projects: Java Brains, Programming with Mosh

Books Worth Reading

• Head First Java - Best for absolute beginners
• Effective Java - Advanced patterns and best practices
• Spring in Action - Deep dive into Spring framework
• Clean Code - Write professional, maintainable code

Community & Support

• Stack Overflow - Ask coding questions
• Reddit r/learnjava - Beginner-friendly community
• Discord servers - Java, Spring Boot communities
• LinkedIn - Connect with Java developers

Salary Benchmarks (US Market 2024-2025)

Note: Salaries vary by location. SF/NYC +25%, remote -10%, healthcare/finance +$10K-$15K premium.

Hot Skills in 2025

• Spring Boot: Required in 95% of Java jobs
• Microservices: Breaking monoliths into services
• Cloud: AWS (Lambda, ECS), Azure, GCP
• Docker & Kubernetes: Containerization and orchestration
• Kafka: Event streaming for real-time data
• GraphQL: Modern API alternative to REST

Job Titles to Search

• Java Developer
• Backend Developer (Java)
• Full Stack Java Developer
• Spring Boot Developer
• Java Software Engineer
• Microservices Developer

General Questions

Can I learn Java with no programming experience?

Yes. Java is beginner-friendly. Start with basics, practice daily, build projects. Most self-taught devs become job-ready in 12-16 weeks.

Do I need a computer science degree?

No. Many companies hire based on skills and projects. Build a strong GitHub portfolio and pass coding interviews.

What's the fastest way to learn Java?

Follow this path: 1) Java basics (4 weeks) → 2) OOP & Collections (3 weeks) → 3) Spring Boot (3 weeks) → 4) Build 3 projects (4 weeks) → 5) Interview prep (2 weeks).

Java vs Python - which to learn first?

Java for: Backend, enterprise, Android, higher salary. Python for: Data science, automation, AI/ML. Choose based on career goals.

Is Java still relevant in 2025?

Absolutely. Used by 90% of Fortune 500 companies. Banking, healthcare, e-commerce, government all run on Java.

How long until I'm job-ready?

12-16 weeks of focused learning (15-20 hours/week). Key milestones: basics → OOP → Spring Boot → 3 projects → interview prep.

Best IDE for Java?

IntelliJ IDEA (most popular), Eclipse (free), or VS Code with Java extensions.