Java Identifiers

// Rule 1: Must start with a letter, underscore (_), or dollar sign ($)
//         Cannot start with a digit.
int age = 25;        // valid — starts with letter
int _count = 0;      // valid — starts with underscore
int $price = 100;    // valid — starts with $ (uncommon, avoid in practice)
int 2fast = 10;      // INVALID — starts with digit (compile error)

// Rule 2: After the first character, can contain letters, digits, _, $
int userName1 = 0;   // valid — digit after first character is fine
int user_name = 0;   // valid — underscore mid-name (avoid by convention)
int user-name = 0;   // INVALID — hyphen not allowed

// Rule 3: Cannot be a Java keyword or reserved word
int class = 5;       // INVALID — 'class' is a keyword
int for = 10;        // INVALID — 'for' is a keyword
int static = 0;      // INVALID — 'static' is a keyword

// Rule 4: No length limit — but keep them readable
int x = 0;                                    // valid (but too short — meaningless)
int thisIsAVeryLongVariableNameThatIsStillValid = 0;  // valid (but too long)

// Unicode is fully supported:
int café = 10;       // valid — é is a Unicode letter
int π = 3;           // valid — π is a Unicode letter
int 数 = 5;          // valid — Chinese character, Unicode letter

// ── VALID IDENTIFIERS ──────────────────────────────────────────
int age;
String firstName;
double accountBalance;
boolean _isActive;
int $count;
float MAX_VALUE;
String userName2;
int i;               // single letter — fine in loops

// ── INVALID IDENTIFIERS ─────────────────────────────────────────
int 1stPlace;        // starts with digit
String my-name;      // contains hyphen
double my name;      // contains space
int class;           // Java keyword
boolean true;        // reserved literal
float @rate;         // @ is not allowed
String null;         // reserved literal

// ── TECHNICALLY VALID BUT AVOID ────────────────────────────────
int _;               // underscore alone — legal but meaningless
int $;               // dollar sign alone — legal but meaningless
int $_;              // legal, meaningless
// Note: in Java 9+, _ alone as an identifier is deprecated and
// produces a warning. In Java 21, it's been repurposed as the
// unnamed variable placeholder in pattern matching.

// These are four completely separate variables:
int age = 25;
int Age = 30;
int AGE = 35;
int aGe = 40;

// The JDK itself uses case-sensitivity deliberately:
String string1 = "hello";   // String = class (uppercase)
// string is not a keyword — String is the class name

// Common mistake — wrong case on class names:
arraylist<String> list = new arraylist<>();  // INVALID — ArrayList, not arraylist
system.out.println("hi");                    // INVALID — System, not system
math.sqrt(4);                                // INVALID — Math, not math

// Correct:
ArrayList<String> list = new ArrayList<>();
System.out.println("hi");
Math.sqrt(4);

// Java fully supports Unicode identifiers:
String naïve = "simple";        // valid — î is a Unicode letter
double göße = 1.8;              // valid — ö and ß are Unicode letters
int счёт = 0;                   // valid — Cyrillic letters

// Unicode escape sequences also work in identifiers:
// A is 'A' in Unicode
int Age = 25;              // same as:  int Age = 25;

// In practice: always use ASCII for identifiers
// Reason: consistency, readability, tool compatibility, team collaboration
// Exception: if your domain vocabulary is inherently non-ASCII (rare)

// You CAN use Unicode in string literals freely:
String greeting = "こんにちは";       // Japanese — totally fine in strings
String currency = "₹1,000";          // Rupee symbol — fine in strings

// ── BE DESCRIPTIVE ─────────────────────────────────────────────
// DON'T:
int d;
String s;
double x;

// DO:
int daysSinceLastLogin;
String customerEmail;
double annualInterestRate;

// ── EXCEPTION: short names in tight scopes ──────────────────────
for (int i = 0; i < list.size(); i++) { }   // i is fine here
list.forEach(s -> System.out.println(s));    // s is fine in a lambda

// ── AVOID ABBREVIATIONS UNLESS UNIVERSAL ────────────────────────
// DON'T:
int usrCnt;
String cstmrNm;
double accBal;

// DO:
int userCount;
String customerName;
double accountBalance;

// Universal abbreviations that are fine:
int id;        // 'id' is universally understood
String url;    // 'url' is universally understood
int min, max;  // universally understood
HttpRequest req; // 'req' is widely understood in web context

// ── DON'T ENCODE TYPE IN THE NAME ───────────────────────────────
// DON'T (Hungarian notation — not idiomatic in Java):
String strName;
int intAge;
boolean bIsActive;

// DO:
String name;
int age;
boolean isActive;