Java

Introduction

Overview

Java is a high-level, object-oriented programming language designed for portability and platform independence through the JVM (Java Virtual Machine).

Origin of Java

Java was developed by James Gosling at Sun Microsystems in 1995. Originally called "Oak", it was renamed to Java and designed with the principle "Write Once, Run Anywhere" (WORA).

Java Architecture

Java architecture consists of three main components: - Java Development Kit (JDK) - Java Runtime Environment (JRE) - Java Virtual Machine (JVM)

Key Components

• JVM: Executes Java bytecode
• JRE: Provides libraries and JVM for running Java applications
• JDK: Complete development kit including JRE, compiler, and tools

Key Features

• Platform Independent (Write Once, Run Anywhere)
• Object-Oriented
• Strongly Typed
• Automatic Memory Management (Garbage Collection)
• Multi-threaded
• Secure

Core Concepts

OOP Principles

Encapsulation

public class BankAccount {
    private double balance;

    public double getBalance() {
        return balance;
    }

    public void deposit(double amount) {
        if (amount > 0) balance += amount;
    }
}

Inheritance

public class Animal {
    public void eat() {
        System.out.println("Eating...");
    }
}

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

Polymorphism

public interface Shape {
    double area();
}

public class Circle implements Shape {
    private double radius;

    public Circle(double radius) {
        this.radius = radius;
    }

    @Override
    public double area() {
        return Math.PI * radius * radius;
    }
}

public class Rectangle implements Shape {
    private double width, height;

    public Rectangle(double width, double height) {
        this.width = width;
        this.height = height;
    }

    @Override
    public double area() {
        return width * height;
    }
}

Abstraction

public abstract class Vehicle {
    abstract void start();

    public void stop() {
        System.out.println("Vehicle stopped");
    }
}

Collections Framework

// List
List<String> list = new ArrayList<>();
list.add("Java");
list.add("Python");

// Set
Set<Integer> set = new HashSet<>();
set.add(1);
set.add(2);

// Map
Map<String, Integer> map = new HashMap<>();
map.put("one", 1);
map.put("two", 2);

Streams API (Java 8+)

List<String> names = Arrays.asList("John", "Jane", "Bob", "Alice");

// Filter and collect
List<String> filtered = names.stream()
    .filter(name -> name.startsWith("J"))
    .collect(Collectors.toList());

// Map and reduce
int totalLength = names.stream()
    .mapToInt(String::length)
    .sum();

Lambda Expressions

// Before Java 8
Runnable r1 = new Runnable() {
    @Override
    public void run() {
        System.out.println("Hello");
    }
};

// With Lambda
Runnable r2 = () -> System.out.println("Hello");

// With parameters
Comparator<String> comp = (s1, s2) -> s1.compareTo(s2);

Exception Handling

public void readFile(String path) {
    try {
        BufferedReader reader = new BufferedReader(new FileReader(path));
        String line = reader.readLine();
        reader.close();
    } catch (FileNotFoundException e) {
        System.out.println("File not found: " + e.getMessage());
    } catch (IOException e) {
        System.out.println("IO error: " + e.getMessage());
    } finally {
        System.out.println("Cleanup complete");
    }
}

Multithreading

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

// Using Runnable
class MyRunnable implements Runnable {
    public void run() {
        System.out.println("Runnable running");
    }
}

// Usage
new MyThread().start();
new Thread(new MyRunnable()).start();
new Thread(() -> System.out.println("Lambda thread")).start();

Java Versions Highlights

• Java 8: Lambdas, Streams, Optional
• Java 11: var keyword, HTTP Client
• Java 17: Sealed classes, Pattern matching
• Java 21: Virtual threads, Record patterns