Comments

Part of the Fundamentals section of Coddy's Java journey — lesson 3 of 73.

Comments are notes you write inside your code. The compiler completely ignores them - they exist only to help humans understand the code.

To write a single-line comment, use //. Everything after // until the end of the line is ignored:

// This is a comment
System.out.println("Hello, Coddy!");

A comment can also be written at the end of a line, after the code:

System.out.println("Hello, Coddy!"); // This prints Hello, Coddy!

For comments that span several lines, use /* to start and */ to end:

/* This is a multi-line comment.
   The compiler ignores all of it. */
System.out.println("Welcome!");

Comments can also temporarily disable a line of code without deleting it:

// System.out.println("This line will NOT run");
System.out.println("This line will run");

Challenge

Beginner

Fix the code so that only Hello, Java! is printed.

Replace the ? with the symbols that turn a line into a comment
The line printing Goodbye! should become a comment so it does NOT run
Only change the line that starts with ?

Cheat sheet

Comments in Java:

Notes for humans - the compiler ignores them
Can disable code temporarily without deleting it

Single-line comment:

// This is a comment
System.out.println("Hello!"); // Comment after code

Multi-line comment:

/* This is a
   multi-line comment */

Disabling code:

// System.out.println("This will NOT run");
System.out.println("This will run");

Try it yourself

public class Main {
    public static void main(String[] args) {
        // Type your code below
        ? System.out.println("Goodbye!");
        System.out.println("Hello, Java!");
    }
}

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

All lessons in Fundamentals

1Introduction

Java Hello World!CommentsBasic Program Structure

4Operators Part 1

Arithmetic Operators Modulo Operator Increment/Decrement Post Increment/Decrement Arithmetic Shortcuts Comparison Operators String Comparison

7Basic IO

System.out Methods Formatting Output Scanner Class Recap - Till 120 Recap - True or False

10Methods (Functions)

Declaring Methods Method Parameters Return Types Method Overloading Recap - Sigma Function Recap - Validation Function Void Methods

13String Operations

String Methods Part 1 String Methods Part 2 String Formatting String Comparison

2Variables Part 1

Numbers String Boolean char Type Declaration

5Operators Part 2

Logical Operators Part 1 Logical Operators Part 2 Recap - Simple Logic Logical Operators Part 3 Logical Operators Part 4

8Calculator App

Welcome Message Getting Numbers

Basic Operations

Formatted Output

11Arrays Basics

Declaring Arrays Accessing Elements Modifying Arrays Array Methods Recap - Product Array Recap - Reversed Array

14Final Challenges

Number Pattern Pattern Finder Student Grade Calculator

3Variables Part 2

Constants Naming Conventions Recap - Initialize Variables Type Casting Part 1 Type Casting Part 2

6Decision Making

If Statement If - Else Switch Statement Ternary Operator Recap - If Else Nested If - Else

9Loops

For Loop Part 1 While Loop Do While Loop Break Continue For Loop Part 2 Nested Loop Infinite Loop Recap - Dynamic Input

12Iterating Over Arrays

Using For Loop Enhanced For Loop Common Array Operations