Comments & Identifiers

 

🟩 1. COMMENTS in Java

Comments are used to make code more readable, explain what it does, or temporarily disable code. They are ignored by the Java compiler during execution.

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✅ Types of Comments:

1. Single-line Comment (//)

Used for short explanations or notes.

java

// This is a single-line comment
int age = 25; // declaring age

2. Multi-line Comment (/* */)

Used when the comment spans multiple lines.

java
/*
This is a multi-line comment
Used to describe a method or block of code
*/
int salary = 50000;

3. Documentation Comment (/** */)

Used to generate external documentation (API docs) using tools like javadoc. It’s written before class, method, or interface declarations.

java

/**
 * This class performs basic arithmetic operations.
 * @author Swami
 * @version 1.0
 */
public class Calculator {
    /**
     * Adds two integers.
     * @param a First number
     * @param b Second number
     * @return Sum of a and b
     */
    public int add(int a, int b) {
        return a + b;
    }
}

🟨 2. IDENTIFIERS in Java

Identifier Rules in Java:

1. Identifiers can contain only:

     - Letters (A-Z, a-z)

     - Digits (0-9)

     - Special characters: $ (dollar sign) and _ (underscore)

2. Identifiers **cannot start** with a digit.

     - Example: 1name → ❌ Invalid

     - Example: name1 → ✅ Valid

3. Identifiers cannot contain **any special characters** other than $ and _.

     - Example: na#me → ❌ Invalid

4. Identifiers **cannot contain spaces**.

     - If needed, use underscore `_` as a connector.

     - Example: first_name → ✅ Valid

5. Java is **case-sensitive**.

     - Example: `Name` and `name` are different identifiers.

6. Identifiers **cannot be Java keywords or reserved words**.

     - Example: `int`, `class` → ❌ Invalid

     - But you can use predefined class/variable/method names, if referred properly using packages.

7. From Java 9 onwards, a single underscore `_` **is not allowed** as an identifier as it became a keyword.

Note:

- There's **no limit** on identifier length in Java (unlike C).

- But using long identifiers is **not recommended** for readability and maintainability.

*/

// Java program demonstrating Identifier Rules

public class IdentifierRulesDemo {

    public static void main(String[] args) {

        // Valid identifiers

        int _age = 25;

        int $salary = 50000;

        int first_name = 10;

        int age1 = 30;

        int Age = 35; // Different from age due to case sensitivity

        // Invalid identifiers (Uncommenting will cause compile-time errors)

        // int 1number = 100;       // ❌ Cannot start with digit

        // int first name = 20;     // ❌ Cannot contain space

        // int na#me = 50;          // ❌ Cannot contain special character #

        // int int = 60;            // ❌ Cannot use reserved keyword

        // int _ = 5;               // ❌ Invalid from Java 9 onwards

        System.out.println("All valid identifiers compiled and executed successfully!");

    }

}

An identifier is the name used to identify:

• Variables

• Methods

• Classes

• Packages

• Interfaces

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✅ Rules for Naming Identifiers:

Rule	Description	Example
1. Allowed characters	Letters (A-Z, a-z), digits (0–9), _ and $ only	studentName, roll_no, $_count
2. Must not start with a digit	Cannot begin with 0–9	1name ❌
3. No special characters	Except _ and $, no @, #, ! etc.	user#id ❌
4. No spaces	Use _ to separate words	first name ❌ → first_name ✅
5. Case-sensitive	age and Age are different	score, Score → separate identifiers
6. Cannot use keywords	e.g. int, class, public, new etc.	int = 5; ❌
7. _ is reserved from Java 9+	Single underscore cannot be used alone	int _ = 5; ❌

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🔷 Examples of Valid Identifiers:

java
int age;
String userName;
double $amount;
int _counter;

🔴 Examples of Invalid Identifiers:

java
// int 2value;     // Starts with digit ❌
// int first name; // Contains space ❌
// int class;      // Java keyword ❌
// int user@id;    // Special character ❌
// int _;          // Single underscore (Java 9+) ❌

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📘 Best Practices for Naming Identifiers

Element	Convention	Example
Class	PascalCase	StudentInfo, BankAccount
Method	camelCase	getDetails(), calculateTotal()
Variable	camelCase	totalMarks, isVerified
Constant	UPPER_SNAKE_CASE	MAX_LIMIT, PI_VALUE
Interface	PascalCase	Runnable, Serializable
Package	lowercase with dots	com.company.project

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🔁 Comparison Summary

Feature	Comments	Identifiers
Purpose	Explain or document code	Name programming elements like variables
Types	Single-line, Multi-line, Javadoc	Variable, Method, Class, Interface names
Compiler	Ignored	Used in compilation
Syntax	//, /* */, /** */	Custom names following rules
Visibility	Not part of code logic	Directly affects program behavior