Java: The Complete Beginner-to-Advanced Guide (With Practical Code Examples)

Source: Dev.to

What Is Java and Why Is It Popular?

Java is a class‑based, object‑oriented, platform‑independent, strongly typed programming language.

Key Features

• Platform Independent (“Write Once, Run Anywhere” – WORA)
• Object‑Oriented (OOP: classes, objects, inheritance, polymorphism)
• Multi‑threaded
• Secure & Robust
• Huge Ecosystem (Spring Boot, Maven, Gradle, JavaFX)
• Massive Community Support

Setting Up Java

Install Java

Download the JDK from:

Check version

java -version
javac -version

Your First Java Program

Create a file named Main.java:

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

Compile and run:

javac Main.java
java Main

Variables and Data Types

Java is strongly typed, so every variable must have a type.

public class Main {
    public static void main(String[] args) {
        int age = 25;
        double price = 99.99;
        char grade = 'A';
        boolean isActive = true;
        String name = "Farhad";

        System.out.println(name + " - " + age);
    }
}

Control Flow Statements

If / Else

int score = 80;

if (score >= 90) {
    System.out.println("Excellent");
} else if (score >= 50) {
    System.out.println("Pass");
} else {
    System.out.println("Fail");
}

Switch

String day = "Monday";

switch (day) {
    case "Monday":
        System.out.println("Start of week");
        break;
    default:
        System.out.println("Unknown day");
}

Loops

for (int i = 1; i  names = new ArrayList<>();
        names.add("Farhad");
        names.add("Rahimi");

        System.out.println(names);
    }
}

Object‑Oriented Programming (OOP)

Class and Object

class Car {
    String model;
    int year;

    void start() {
        System.out.println(model + " is starting...");
    }
}

public class Main {
    public static void main(String[] args) {
        Car c = new Car();
        c.model = "Toyota";
        c.year = 2024;
        c.start();
    }
}

Constructors

class User {
    String name;
    int age;

    User(String name, int age) {
        this.name = name;
        this.age = age;
    }
}

public class Main {
    public static void main(String[] args) {
        User u = new User("Farhad", 23);
        System.out.println(u.name);
    }
}

Inheritance

class Animal {
    void sound() {
        System.out.println("Makes a sound");
    }
}

class Dog extends Animal {
    void sound() {
        System.out.println("Barks");
    }
}

public class Main {
    public static void main(String[] args) {
        Dog d = new Dog();
        d.sound();
    }
}

Polymorphism

class Shape {
    void draw() {
        System.out.println("Drawing shape");
    }
}

class Circle extends Shape {
    void draw() {
        System.out.println("Drawing circle");
    }
}

public class Main {
    public static void main(String[] args) {
        Shape s = new Circle();  
        s.draw();
    }
}

Encapsulation

class BankAccount {
    private double balance;

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

    public double getBalance() {
        return balance;
    }
}

public class Main {
    public static void main(String[] args) {
        BankAccount acc = new BankAccount();
        acc.deposit(100);
        System.out.println(acc.getBalance());
    }
}

Abstraction (Interfaces & Abstract Classes)

Interface

interface Animal {
    void eat();
}

class Cat implements Animal {
    public void eat() {
        System.out.println("Cat eating");
    }
}

Abstract Class

abstract class Vehicle {
    abstract void move();
}

class Bike extends Vehicle {
    void move() {
        System.out.println("Bike moves fast");
    }
}

Exception Handling

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

File Handling

import java.io.FileWriter;

public class Main {
    public static void main(String[] args) {
        try {
            FileWriter fw = new FileWriter("output.txt");
            fw.write("Java file writing...");
            fw.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

Java and OOP in Real Projects

Java is widely used in:

• Spring Boot (Enterprise Backends)
• Android Development
• Financial Services
• Large‑scale Microservices
• Distributed Systems
• Big Data with Hadoop
• Cloud Applications (AWS, GCP, Azure)

Most production environments prefer Java due to its reliability and scalability.

A Mini Java Project: Simple To‑Do Console App

import java.util.ArrayList;
import java.util.Scanner;

public class TodoApp {
    public static void main(String[] args) {
        ArrayList tasks = new ArrayList<>();
        Scanner sc = new Scanner(System.in);

        while (true) {
            System.out.println("1. Add Task  2. View Tasks  3. Exit");
            int choice = sc.nextInt();
            sc.nextLine();

            switch (choice) {
                case 1:
                    System.out.print("Enter task: ");
                    tasks.add(sc.nextLine());
                    break;
                case 2:
                    System.out.println("Your tasks:");
                    for (String t : tasks) System.out.println("- " + t);
                    break;
                case 3:
                    System.exit(0);
                default:
                    System.out.println("Invalid choice");
            }
        }
    }
}

Conclusion

Java remains one of the most powerful languages for backend development, enterprise systems, Android applications, and scalable architectures. Its mature ecosystem, strong community, and long‑term stability make it a top choice for developers worldwide.

Whether you’re building microservices with Spring Boot, high‑performance servers, or cross‑platform applications, Java gives you the tools to succeed.