Understand the idea

Comments are notes for readers; they do not produce output. Use them to explain decisions or constraints rather than repeat what a clear statement already says. A useful comment stays true when the code changes.

Explore the details

Java Comments

Comments can be used to explain Java code, and to make it more readable. It can also be used to prevent execution when testing alternative code.

Single-line Comments

Single-line comments start with two forward slashes (//).

Any text between // and the end of the line is ignored by Java (will not be executed).

This example uses a single-line comment before a line of code:

This example uses a single-line comment at the end of a line of code:

Java Multi-line Comments

Multi-line comments start with /* and ends with */.

Any text between /* and */ will be ignored by Java.

This example uses a multi-line comment (a comment block) to explain the code:

// Single-line comment: ignored by Java
int x = 10;  // x stores the value 10

/* Multi-line comment:
   Explain multiple lines here.
   Nothing inside runs. */
System.out.println(x);  // 10

// Tip: use comments to disable code temporarily
// System.out.println("This line is skipped");

A complete runnable example

This complete Main.java program focuses on the lesson idea. Reference fragments above may demonstrate individual statements; this example includes the imports, entry point, and supporting types it needs.

public class Main {
    public static void main(String[] args) throws Exception {
        // A welcome message for a new learner
        System.out.println("Welcome!");
        /* This comment can span
           more than one line. */
        System.out.println("Ready to practice.");
    }
}

Welcome!
Ready to practice.

Change it. Run it. Understand it.

Load this lesson example into the editor. Change one value or statement, predict the result, then press RUN to execute Java.

Make it your own

Your learning checkpoint

• Explain the example without reading the output first.
• Make the change requested in the practice task.
• Describe why the result changed—or why compilation failed.

Ready to move on? Follow the next lesson in course order, or return to the curriculum to review a prerequisite.