Identifiers in Java

Compiler Rules for Valid Identifiers

These rules are enforced by the compiler; violating any of them prevents the code from compiling at all.

• An identifier can contain letters, digits, underscores (_), and dollar signs ($) — no other characters (spaces, hyphens, @ symbols, etc.) are allowed.
• It cannot start with a digit, though it can contain digits after the first character (e.g. score2 is valid, 2ndScore is not).
• It cannot be a reserved keyword (such as class, int, or static) or a reserved literal (true, false, null).
• Java identifiers are case-sensitive, so total, Total, and TOTAL are three completely different identifiers.

Naming Conventions (Best Practice, Not Enforced)

Conventions are not checked by the compiler, but ignoring them makes code confusing to other Java developers and to tools like IDEs that assume standard style. Classes and interfaces use UpperCamelCase (StudentRecord), while variables and methods use lowerCamelCase (studentName, calculateTotal()). Constants (fields declared static final) are written in all uppercase with underscores separating words (MAX_DISCOUNT). Package names are written entirely in lowercase (com.webnest.utils). The underscore and dollar sign are technically legal in any identifier, but by convention they are avoided in ordinary variable names — the dollar sign is mostly reserved for tool-generated or compiler-generated code.

Example

public class IdentifierDemo {
    public static void main(String[] args) {
        int _count = 10;
        double $price = 19.99;
        String studentName = "Asha";
        int number2 = 5;

        System.out.println(_count);
        System.out.println($price);
        System.out.println(studentName);
        System.out.println(number2);
    }
}

10
19.99
Asha
5

Exercise

This program does not compile because three names break the identifier rules, and one more breaks the convention for constants. Rename them so the program compiles and follows Java conventions.

Expected output: Total: 250

public class Checkout {
    static final int maxitems = 10;       // convention problem

    public static void main(String[] args) {
        int 2ndPrice = 150;               // rule problem
        int first-price = 100;            // rule problem
        int class = 2ndPrice + first-price; // rule problem
        System.out.println("Total: " + class);
    }
}

public class Checkout {
    static final int MAX_ITEMS = 10;

    public static void main(String[] args) {
        int secondPrice = 150;
        int firstPrice = 100;
        int total = secondPrice + firstPrice;
        System.out.println("Total: " + total);
    }
}

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