✅ Core Java Fundamentals

Variables, Data Types, Operators
Control Flow (if, switch, loops)
Methods (overloading, parameters, return types)
Arrays & Strings
Exception Handling (try-catch-finally, throw, custom exceptions)

🏛️ Object-Oriented Programming (OOP)

Classes & Objects
Inheritance (extends, overriding)
Polymorphism (compile-time & runtime)
Encapsulation (private fields, getters/setters)
Abstraction (abstract class, interface)

📚 Java Collections Framework

🧩 Generics

Why use generics (List<String> vs List<Object>)
Create generic classes & methods

📂 Java I/O & Serialization

File handling (FileReader, BufferedReader)
Serialization (Serializable interface)
JSON parsing (Jackson basics)

🔀 Java Concurrency (Basics)

Thread class & Runnable interface
ExecutorService
synchronized keyword

🆕 Java 8+ Features

Lambda Expressions
Functional Interfaces (Consumer, Predicate, Supplier)
Streams API (map, filter, reduce)
Optional

🏷️ Annotations & Reflection

Built-in annotations (@Override, @Deprecated)
Create custom annotations
Reflection basics

⚒️ Build Tools

Maven basics
Gradle basics

📂 Mini-Projects Before Spring Boot

Student Management System (Console App) – CRUD with Collections
File-based Todo App – Read/write tasks from a file
Multithreaded Downloader – Use threads for multiple downloads
Library Management System – OOP + Collections + Exception handling

🌱 Ready for Spring Boot?

Install JDK + Maven/Gradle
Build Hello World Spring Boot project
Create your first REST API (@RestController)
Connect Spring Boot with Database (JPA + MySQL/Postgres)
Deploy a simple Spring Boot app (local/Heroku/Docker)

👉 Resource: Java Tutorial (W3Schools) JDK - Java Development Kit, set of development tools and libraries used to create Java programs. Works together with JVM and JRE to run and build Java applications. (JRE, javac, debugger etc.)

Converts .java to .class - Class Loader - Byte Code verifier - Interpreter (Executes Byte Code and make appropriate calls to the hardware)

JRE - Java Runtime Environment, provides an environment to run Java programs on the system. The environment includes standard libraries and JVM (rt.jar etc)

JVM - Java Virtual Machine, responsible for executing Java program. (ClassLoader, JIT, Compiler, Garbage Collector) (converts bytecode into native machine code). (Interpreter)

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Data Types

Primitive Data Types

boolean
char
byte
short
int
long
float
double

Non Primitive Data Types

These are reference types that store memory addresses of objects (SACIO)

String
Array
Class
Interface
Object

public class Geeks {
    public static void main(String[] args) {
        int p = 20000;
        int d = 15000;

        System.out.println(p);
        System.out.println(d)
    }
}

String cannot be modified after creation, use StringBuilder for heavy string manipulation

class Car {
    String model;
    int year;
    Car(String model, int year) {
        this.model = model;
        this.year = year;
    }
    void display() {
        System.out.println(year);
    }
}

public class Geeks {
    public static void main(String[] args) {
        Car myCar = new Car("Toyota", 2025);
        myCar.display();
    }
}

interface Animal {
    void sound();
}

class Dog implements Animal {
    public void sound() {
        System.out.println("Woof")
    }
}

public class InterfaceExample {
    publis static void main(String[] args) {
        Dog myDog = new Dog();
        myDog.sound();
    }
}

public class Geeks {
    public static void main(String[] args) {
        int[] num = {1,2,3,4,5};
        String[] arr = {"Geeks", "Geeks2", "Geeks3"};

        System.out.println("First number: " + num[0]);
        System.out.println("Second Number: " + num[1]);
    }
}

Number arrays in Java are initialized to 0 by default.

Scanner

Scanner sc = new Scanner(System.in)
sc.nextInt();
sc.nextLine();