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Comments

Learn single-line (//) and multi-line (/* */) comments.

Why write comments?

Comments are text that Java ignores but explains the code to humans. Comments help you remember what code does later and help others understand it.

A single-line comment starts with //. Everything from the sign to the end of the line is a comment:

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

A multi-line comment is wrapped in /* */. A documentation comment /** */ is used for the javadoc tool.

Practice Add a comment and println

Print Welcome to the screen with println. Add a // Greeting message comment above it.

Sample solution

Main.java
// Greeting message
System.out.println("Welcome");

Expected output

Welcome

When should you write comments?

Comments are best used to explain why code is written a certain way. But instead of commenting every line, it is better to pick descriptive variable and method names.

A good comment explains the step of a complex calculation, the reason for an external dependency, or why a hack is needed. A bad comment just repeats what the code does line by line.

Documentation comments (/** */) are written for public methods and classes; javadoc generates HTML docs from them.

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Complete the comment line

Question 
// This line is a ____
Answer 
// This line is a comment

Explanation: A line starting with // is a comment.

2 Which of these is a valid comment?

  • // Greeting message //
  • // Greeting message Correct answer
  • ** Greeting message **
  • /* Greeting message

Explanation: A single-line comment starts with //.

3 Match the comment types

Term Meaning
// Single line
/* */ Multi-line
/** */ Documentation
javadoc Doc tool

All lessons in Beginner

1 Java basics

  1. What is Java?
  2. The main method
  3. Comments
  4. Class skeleton
  5. javac and java
  6. Summary - Java basics

2 Variables

  1. int and double
  2. boolean
  3. String
  4. Naming variables
  5. final variables
  6. Summary - Variables

3 Data types

  1. Primitive types
  2. Type casting
  3. String concatenation
  4. Math class
  5. BigInteger and BigDecimal
  6. Summary - Data types

4 Project: Calculator

  1. Project design
  2. Add/subtract
  3. Multiply/divide
  4. Error handling
  5. Putting it together

5 Operators

  1. Arithmetic
  2. Comparison
  3. Logical
  4. Bitwise
  5. Assignment and increment
  6. Summary - Operators

6 String basics

  1. length and charAt
  2. equals and equalsIgnoreCase
  3. indexOf and substring
  4. toUpperCase and trim
  5. String.format
  6. Summary - String basics

7 Conditionals

  1. The if statement
  2. else and else if
  3. Nested if
  4. switch
  5. Ternary
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. The while loop
  3. do-while
  4. break and continue
  5. for-each
  6. Summary - Loops

9 Project: Shopping cart

  1. Cart design
  2. Adding items
  3. Calculating totals
  4. Updating quantity
  5. Putting it together

10 Arrays

  1. Declaring arrays
  2. Array access
  3. Filling arrays
  4. Multi-dimensional arrays
  5. Copying arrays
  6. Summary - Arrays

11 Array methods

  1. Arrays.sort
  2. Arrays.toString
  3. Arrays.binarySearch
  4. Arrays.copyOf
  5. Arrays.fill
  6. Summary - Array methods

12 Collections

  1. ArrayList
  2. add and get
  3. HashMap
  4. put and get
  5. HashSet
  6. Summary - Collections

13 Methods

  1. Defining methods
  2. return
  3. void methods
  4. Method overloading
  5. Scope
  6. Summary - Methods

14 Method parameters

  1. Primitive parameters
  2. Object parameters
  3. varargs
  4. Multiple values via array
  5. final parameters
  6. Summary - Method parameters

15 Project: To-do CLI

  1. Task model
  2. Adding tasks
  3. Listing tasks
  4. Completing tasks
  5. Putting it together

16 String builder

  1. StringBuilder
  2. append
  3. insert and delete
  4. reverse
  5. toString
  6. Summary - String builder

17 Input and output

  1. Scanner
  2. nextInt and nextLine
  3. System.out.printf
  4. BufferedReader
  5. try-with-resources
  6. Summary - Input and output

18 Enum

  1. Declaring enums
  2. values and valueOf
  3. Enum fields
  4. Enum methods
  5. switch with enum
  6. Summary - Enum

19 Exception basics

  1. try/catch
  2. finally
  3. throw
  4. Exception hierarchy
  5. Multiple catch
  6. Summary - Exception basics

20 Beginner final

  1. Project design
  2. Data model
  3. Business logic
  4. Error handling
  5. Putting it together