Comments in Java

In Plain English

Code is read far more often than it is written. Java offers three distinct styles of comments to explain design decisions, leave reminders, and generate automated API documentation without affecting runtime performance.

The Mental Model

"Comments are like sticky notes left on equipment. The machinery (the compiler) completely ignores the notes, but other engineers reading the manual will thank you."

Deep Dive: How It Works

Single-line comments start with `//` and extend until the end of that line.

Multi-line block comments start with `/*` and end with `*/`. They can span across multiple lines.

Javadoc comments start with `/**` and end with `*/`. Special tags like `@param` and `@return` allow the `javadoc` tool to automatically generate web documentation.

Syntax Blueprint

// Single line note
/* Multi-line
   comment block */
/** Javadoc documentation */

Comments are stripped out by javac during compilation and produce zero bytecode overhead.

Core Rules to Remember

Zero Runtime Cost: Comments do not make your compiled class files slower or bigger.

Explain the "Why": Use comments to explain why you wrote something, not just restating obvious code.

Quick Debugging: Temporarily comment out a line with // to test alternative code paths.

Common Beginner Traps & How to Fix Them

Mistake:Nesting multi-line comments /* /* */ */

Why it happens: The first */ encountered immediately closes the entire block comment.

How to fix: Do not nest /* inside another /*. Use single-line // for inner notes instead.

Pro Tip: Use Ctrl+/ (or Cmd+/) in any code editor to quickly toggle single-line comments on selected lines.

Live Interactive Example

Hit Run Code to see it live

Comment Types in Action

Java 13

Output Console

Click "Run Code" to view the rendered output.

How it works: All comments are ignored by the compiler, leaving only the executable print statement.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Uncomment the greeting

The output line in the starter code is commented out.
Remove the // comment markers to enable printing "Comments Mastered!".

Sandbox Output

Click "Run & Check" to test your solution.

Finished reading and practicing?

Mark this lesson as completed to update your course progress.

Deep Dive: How It Works

Single-line comments start with `//` and extend until the end of that line.

Multi-line block comments start with `/*` and end with `*/`. They can span across multiple lines.

Javadoc comments start with `/**` and end with `*/`. Special tags like `@param` and `@return` allow the `javadoc` tool to automatically generate web documentation.