Java Comments

Single-line, multi-line, and JavaDoc comments — when and how to use each.

What are Comments?

Comments are notes for humans reading your code — the compiler ignores them. Java has three flavours.

1. Single-line — `//`

java
// This is a single-line comment
int x = 5;          // can also follow code

2. Multi-line — `/* ... */`

java
/*
   This block can span
   as many lines as you need.
*/

3. JavaDoc — `/** ... */`

Special multi-line comments parsed by the javadoc tool to generate API docs.

java
/**
 * Returns the area of a circle.
 * @param r radius (must be non-negative)
 * @return  the computed area
 */
public double area(double r) { return Math.PI * r * r; }

Example

java
public class Main {
    // entry point
    public static void main(String[] args) {
        /* multi-line:
           prints a friendly greeting */
        System.out.println("Hello!");   // greeting
    }
}

Hello!

When to comment

✅ Why — context the code can't express.
✅ TODO / FIXME markers for follow-ups.
✅ JavaDoc on every public class & method.
❌ Restating what the code already says clearly.

Common Mistakes

Commenting out big blocks of code instead of deleting (use Git).
Stale comments — they lie when the code changes and the comment doesn't.

Practice Exercises

Add a JavaDoc comment to your add(int, int) method describing the params and return value.
Run javadoc on your file and open the generated HTML to see your comments rendered as API docs.

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# program

Program

Java

/**
 * A tiny JavaDoc demo.
 */
public class Main {
    // single-line
    public static void main(String[] args) {
        /* multi-line */
        System.out.println("Documented!");
    }
}

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