Java Fundamentals · Getting Started · Lesson 3 of 22

Comments and Class Structure

Concept

Comments and Class Structure

Two slashes start a single-line comment, and /* */ wraps a multi-line one. Every class name should match its filename exactly, which is a rule the compiler enforces.

By the end of this lesson

Explain the core idea behind Comments and Class Structure.
Predict output before running a change.
Test one realistic and one unusual input.
Use the result to make the next decision.

How to study this page

1. Read one concept.
2. Change the example.
3. Run, compare, and explain.
4. Complete the challenge below.

ExampleRunnable

// Greets the user
public class Main {
    public static void main(String[] args) {
        System.out.println("Welcome!"); // prints a greeting
    }
}

Try it Yourself »

Self-check before continuing

Without looking at the example, describe what changes when you modify one input in Comments and Class Structure. Then reopen the editor and prove your explanation with a small test.

You are ready to continue when you can predict, test, and explain the result.

Your turn

Add a comment above one line of code explaining what it does.

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Console

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