Java is a versatile, object-oriented language known for its "write once, run anywhere" philosophy through the JVM. It's widely used in enterprise applications, Android development, and web services.

Installation & Setup:

Download JDK (Java Development Kit) from oracle.com
Install JDK and set JAVA_HOME environment variable
Verify: Open terminal and type java -version
Use IDE like IntelliJ IDEA, Eclipse, or VS Code

Your First Java Program:

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

Variables & Data Types

// Primitive data types
int age = 25;
long population = 8000000000L;
float height = 5.9f;
double pi = 3.14159;
boolean isStudent = true;
char initial = 'A';

// Reference types
String name = "John Doe";
int[] numbers = {1, 2, 3, 4, 5};

// Type casting
int x = 10;
double y = (double) x;  // Widening
double z = 10.5;
int w = (int) z;        // Narrowing (10)

// Final (constant)
final int MAX_SIZE = 100;

// String operations
String greeting = "Hello" + " " + "World";
System.out.println("Name: " + name);
System.out.printf("Age: %d\n", age);

Control Flow

// If-Else
int score = 85;
if (score >= 90) {
    System.out.println("A");
} else if (score >= 80) {
    System.out.println("B");
} else {
    System.out.println("C");
}

// Switch
int day = 2;
switch (day) {
    case 1:
        System.out.println("Monday");
        break;
    case 2:
        System.out.println("Tuesday");
        break;
    default:
        System.out.println("Other");
}

// For loop
for (int i = 0; i < 5; i++) {
    System.out.println(i);
}

// Enhanced for loop
int[] arr = {1, 2, 3, 4, 5};
for (int num : arr) {
    System.out.println(num);
}

// While loop
int count = 0;
while (count < 3) {
    System.out.println(count);
    count++;
}

Methods

public class Calculator {
    // Method with parameters and return value
    public static int add(int a, int b) {
        return a + b;
    }
    
    // Method with no return
    public static void greet(String name) {
        System.out.println("Hello, " + name);
    }
    
    // Method with default parameter (Java doesn't support, use overloading)
    public static void printMessage(String message) {
        System.out.println(message);
    }
    
    public static void printMessage(String message, String name) {
        System.out.println(message + ", " + name);
    }
    
    // Varargs (variable arguments)
    public static int sum(int... numbers) {
        int total = 0;
        for (int num : numbers) {
            total += num;
        }
        return total;
    }
    
    public static void main(String[] args) {
        System.out.println(add(5, 3));          // 8
        greet("Alice");                         // Hello, Alice
        System.out.println(sum(1, 2, 3, 4));    // 10
    }
}

Object-Oriented Programming

public class Car {
    // Properties
    private String brand;
    private int year;
    private double price;
    
    // Constructor
    public Car(String brand, int year) {
        this.brand = brand;
        this.year = year;
    }
    
    // Getters
    public String getBrand() {
        return brand;
    }
    
    public int getYear() {
        return year;
    }
    
    // Setters
    public void setBrand(String brand) {
        this.brand = brand;
    }
    
    // Method
    public void display() {
        System.out.println(brand + " (" + year + ")");
    }
}

// Usage
Car car = new Car("Toyota", 2020);
car.display();
System.out.println(car.getBrand());

Inheritance

// Parent class
public class Animal {
    protected String name;
    
    public Animal(String name) {
        this.name = name;
    }
    
    public void sound() {
        System.out.println("Animal makes a sound");
    }
}

// Child class
public class Dog extends Animal {
    public Dog(String name) {
        super(name);  // Call parent constructor
    }
    
    @Override
    public void sound() {
        System.out.println(name + " barks");
    }
    
    public void fetch() {
        System.out.println(name + " fetches the ball");
    }
}

// Usage
Dog dog = new Dog("Buddy");
dog.sound();   // Buddy barks
dog.fetch();   // Buddy fetches the ball

Interfaces & Abstract Classes

// Interface
public interface Shape {
    void draw();
    double getArea();
}

// Implementation
public class Circle implements Shape {
    private double radius;
    
    public Circle(double radius) {
        this.radius = radius;
    }
    
    @Override
    public void draw() {
        System.out.println("Drawing circle");
    }
    
    @Override
    public double getArea() {
        return Math.PI * radius * radius;
    }
}

// Abstract class
public abstract class Vehicle {
    public abstract void start();
    
    public void stop() {
        System.out.println("Vehicle stopped");
    }
}

Collections

import java.util.*;

// ArrayList
ArrayList fruits = new ArrayList<>();
fruits.add("Apple");
fruits.add("Banana");
fruits.add("Orange");
System.out.println(fruits.get(0));  // Apple

// LinkedList
LinkedList nums = new LinkedList<>();
nums.add(1);
nums.addFirst(0);
nums.addLast(2);

// HashMap
HashMap ages = new HashMap<>();
ages.put("John", 25);
ages.put("Jane", 28);
System.out.println(ages.get("John"));  // 25

// HashSet
HashSet uniqueNames = new HashSet<>();
uniqueNames.add("Alice");
uniqueNames.add("Bob");
uniqueNames.add("Alice");  // Won't be added again

// Iteration
for (String fruit : fruits) {
    System.out.println(fruit);
}

Exception Handling

public class ExceptionExample {
    public static void main(String[] args) {
        try {
            int[] numbers = {1, 2, 3};
            System.out.println(numbers[5]);  // Index out of bounds
        } catch (ArrayIndexOutOfBoundsException e) {
            System.out.println("Error: " + e.getMessage());
        } catch (Exception e) {
            System.out.println("Generic error");
        } finally {
            System.out.println("Cleanup code");
        }
    }
    
    // Custom exception
    static class InvalidAgeException extends Exception {
        public InvalidAgeException(String message) {
            super(message);
        }
    }
    
    static void validateAge(int age) throws InvalidAgeException {
        if (age < 0 || age > 150) {
            throw new InvalidAgeException("Age must be between 0 and 150");
        }
    }
}

Projects & Practice

Beginner Projects:

Student Management System
Bank Account System
Inventory Management
Library Management System
Simple Chat Application

Intermediate Projects:

Spring Boot REST API
E-commerce Application
Ticket Management System
File Management System
Data Analysis Tool

Advanced Projects:

Microservices Architecture
Android Mobile App
Distributed System
High-traffic Web Application
Game Development