Comments, keywords and identifiers

Why this matters

Good names and helpful comments are what separate readable code from code nobody (including future you) can understand.

Example

// single line comment

/* multi line
   comment */

public class Student {          // Student is an identifier
    int rollNumber = 12;        // rollNumber is an identifier, int is a keyword
    // int class = 5;           // ERROR: 'class' is a reserved keyword
}

Step by step

1. 1Anything after `//` on a line is ignored by the compiler.
2. 2Anything between `/*` and `*/` is ignored, even across many lines.
3. 3`public`, `class`, `int`, `static` are keywords — you cannot use them as names.
4. 4Identifiers may contain letters, digits, `_` and `$`, but cannot start with a digit.

Key points

• Class names use PascalCase: BankAccount
• Variable and method names use camelCase: accountBalance
• Constants use UPPER_CASE: MAX_MARKS
• Comments never affect how the program runs

Common mistakes

• Naming a variable `2ndMark` — identifiers cannot start with a digit
• Using a keyword such as `new` or `class` as a variable name
• Writing comments that repeat the code instead of explaining the reason

Try it yourself

• Rename badly named variables a, b, c in a program to meaningful names.
• Add a comment above main explaining what your program does.