Classes & Objects in Java: Full Course

Introduction

You just started learning Java. Excited. Ready to write your first program.

You want to print "Hello World." Simple, right?

In Python or JavaScript, you write:

# In Python:
print("Hello World")  # Done! One line.

# In JavaScript:
console.log("Hello World"); # Done! One line.

But in Java, you write:

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

Wait... what?

Why do you need public? Why class? Why static? Why void? Why main? Why are there curly braces inside curly braces?

You're just trying to print text. Why is Java making you write 4 lines of ugly code?

You ask: "Can't I just write the code directly?"

And the answer is: No. Not in Java.

This is where most beginners get frustrated. They think: "Java is too complicated. Why not just use Python?"

But here's the secret: Java isn't making you write this code because it's evil. It's making you write this code because you're about to learn something powerful.

That structure you see? It's not a complication. It's the foundation of Object Oriented Programming. And once you understand why it's there, you'll understand why large systems use Java.

This section reveals the mystery. By the end, you won't just know HOW to write Java code. You'll know WHY Java works this way, and you'll appreciate the design.

Ready to go deeper?

What You'll Learn

1. The Problem: Why We Need Structure

Why can't Java be simple like Python? What is Java solving that Python isn't? We'll understand the real problem Java solves by forcing you to organize your code.

2. What Is a Class?

The fundamental concept. A class is a blueprint (template or model). We'll explain what that means, why it matters, and why Java makes you define templates before writing code.

3. What Is an Object?

Templates are useless unless you use them. An object is the real thing created from the template. The difference between a recipe and the actual cake.

4. Understanding Keywords: The Heart of Java Structure

This is where the mystery gets solved. We deep dive into every keyword you see:

• public -> Why Java needs it, what it prevents, what happens if you don't use it,...
• class -> Why this keyword exists, what it means, what error you get without it
• static -> One of the most confusing keywords. We'll explain when you use it, when you DON'T,...
• void -> What it means and when methods return things instead
• main -> Why Java looks for it, what happens if you name it differently, and what String[] args means
• String[] and args -> Command line arguments explained simply

For each keyword, we'll answer: What does it do? What error happens without it? Is there an alternative?

5. Properties: How Objects Store Data

What data does an object hold? Why each object has its own copy? How do properties differ? We'll deep dive into storing data in objects.

6. Methods: What Objects Can Do

How do objects perform actions? Methods vs properties. Methods that return values vs methods that just do something. We'll explore when to use each.

7. Constructors: Initializing Objects

A special method that runs when you create an object. Why constructors matter. What happens without one. How they save you from repetition.

8. A Complete Simple Example

Everything together. A real, working example showing classes, objects, properties, methods, and constructors working as a team.

9. Common Mistakes to Avoid

Beginners make predictable mistakes. We'll show what they are and how to spot them before they happen.

10. Cheat Sheet

A quick reference you can come back to. All the keywords, concepts, and syntax in one place.

11. Summary

What you've learned. Why it matters. And why the next part (Encapsulation) builds on this foundation.