Static Keyword
& Static Class

public class StaticMethodBasics {

    static int  count = 0;   // static — class level
    String      name;         // instance — object level

    StaticMethodBasics(String name) {
        this.name = name;
        count++;
    }

    // ── Static Method ──
    static void showCount() {
        System.out.println("Objects created: " + count);  // ✅ static access
        // name = "test"; ← COMPILE ERROR! instance var nahi
        // greet();       ← COMPILE ERROR! instance method nahi
    }

    static int add(int a, int b)       { return a + b; }
    static boolean isPositive(int n)   { return n > 0; }
    static String repeat(String s, int n) {
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < n; i++) sb.append(s);
        return sb.toString();
    }

    // ── Instance Method ── (object chahiye)
    void greet() {
        // instance method: dono access kar sakta hai
        System.out.println("Hello from " + name + " | Total: " + count);
        showCount();  // ✅ instance method static ko call kar sakta hai
    }

    public static void main(String[] args) {
        // Object banaye bina static call!
        StaticMethodBasics.showCount();
        System.out.println("add(10,5) = " + add(10, 5));
        System.out.println("isPositive(-3) = " + isPositive(-3));
        System.out.println("repeat(\"Ha\",3) = " + repeat("Ha", 3));

        // Objects banao
        StaticMethodBasics s1 = new StaticMethodBasics("Rahul");
        StaticMethodBasics s2 = new StaticMethodBasics("Priya");
        s1.greet();
        s2.greet();
        showCount(); // 2
    }
}

Objects created: 0
add(10,5) = 15
isPositive(-3) = false
repeat("Ha",3) = HaHaHa
Hello from Rahul | Total: 1
Objects created: 1
Hello from Priya | Total: 2
Objects created: 2
Objects created: 2

class MathUtils {

    // Private constructor — object banana allowed nahi!
    // Ye pure utility class hai, sirf static methods ke liye
    private MathUtils() {
        throw new UnsupportedOperationException("Utility class!");
    }

    static int    abs(int n)            { return n < 0 ? -n : n; }
    static double power(double b, int e) {
        double r = 1;
        for (int i = 0; i < e; i++) r *= b;
        return r;
    }
    static boolean isPrime(int n) {
        if (n < 2) return false;
        for (int i = 2; i * i <= n; i++)
            if (n % i == 0) return false;
        return true;
    }
    static int factorial(int n) {
        if (n <= 1) return 1;
        return n * factorial(n - 1);  // recursive static!
    }
    static int gcd(int a, int b)        { return b == 0 ? a : gcd(b, a % b); }
    static int lcm(int a, int b)        { return a / gcd(a, b) * b; }
    static double percentage(double val, double total) { return (val / total) * 100; }
    static int clamp(int val, int min, int max) {
        return Math.max(min, Math.min(max, val));
    }

    public static void main(String[] args) {
        // Koi object nahi! Seedha class name se:
        System.out.println("abs(-42)         = " + MathUtils.abs(-42));
        System.out.println("power(2,10)      = " + MathUtils.power(2, 10));
        System.out.println("factorial(7)     = " + MathUtils.factorial(7));
        System.out.println("isPrime(97)      = " + MathUtils.isPrime(97));
        System.out.println("isPrime(100)     = " + MathUtils.isPrime(100));
        System.out.println("gcd(48,18)       = " + MathUtils.gcd(48, 18));
        System.out.println("lcm(4,6)         = " + MathUtils.lcm(4, 6));
        System.out.println("percentage(45,60)= " + MathUtils.percentage(45, 60) + "%");
        System.out.println("clamp(150,0,100) = " + MathUtils.clamp(150, 0, 100));
    }
}

abs(-42)         = 42
power(2,10)      = 1024.0
factorial(7)     = 5040
isPrime(97)      = true
isPrime(100)     = false
gcd(48,18)       = 6
lcm(4,6)         = 12
percentage(45,60)= 75.0%
clamp(150,0,100) = 100

class User {

    private String username;
    private String email;
    private String role;
    private boolean active;

    // Private constructor — sirf class ke andar se call hoga
    private User(String username, String email, String role) {
        this.username = username;
        this.email    = email;
        this.role     = role;
        this.active   = true;
        System.out.println("✔ Created [" + role + "] user: " + username);
    }

    // ── Static Factory Methods ──
    // Naam se clear hai kya ban raha hai — constructor se better!
    static User createAdmin(String username, String email) {
        return new User(username, email, "ADMIN");
    }
    static User createCustomer(String username, String email) {
        return new User(username, email, "CUSTOMER");
    }
    static User createModerator(String username, String email) {
        return new User(username, email, "MODERATOR");
    }
    static User createGuest() {
        String tempName = "guest_" + System.currentTimeMillis() % 10000;
        return new User(tempName, tempName + "@temp.com", "GUEST");
    }

    // Static validation method
    static boolean isValidEmail(String email) {
        return email != null
            && email.contains("@")
            && email.indexOf("@") > 0
            && email.contains(".");
    }
    static boolean isValidUsername(String u) {
        return u != null && u.length() >= 4 && u.matches("[a-zA-Z0-9_]+");
    }

    void display() {
        System.out.printf("  User: %-15s | Role: %-10s | Active: %b%n",
                           username, role, active);
    }

    public static void main(String[] args) {
        System.out.println("── Validation First ──");
        String email = "rahul@company.com";
        System.out.println("Email valid? " + isValidEmail(email));
        System.out.println("Username valid? " + isValidUsername("rk"));   // false (too short)
        System.out.println("Username valid? " + isValidUsername("rahul")); // true

        System.out.println("\n── Creating Users ──");
        User admin = User.createAdmin("superadmin", "admin@company.com");
        User cust  = User.createCustomer("rahul_k", "rahul@gmail.com");
        User mod   = User.createModerator("mod_priya", "priya@company.com");
        User guest = User.createGuest();

        System.out.println("\n── User List ──");
        admin.display(); cust.display(); mod.display(); guest.display();
    }
}

── Validation First ──
Email valid? true
Username valid? false
Username valid? true

── Creating Users ──
✔ Created [ADMIN] user: superadmin
✔ Created [CUSTOMER] user: rahul_k
✔ Created [MODERATOR] user: mod_priya
✔ Created [GUEST] user: guest_7342

── User List ──
  User: superadmin      | Role: ADMIN       | Active: true
  User: rahul_k         | Role: CUSTOMER    | Active: true
  User: mod_priya       | Role: MODERATOR   | Active: true
  User: guest_7342      | Role: GUEST       | Active: true

class StringUtils {
    private StringUtils() {}  // utility class — no instances

    // Reverse a string
    static String reverse(String s) {
        return new StringBuilder(s).reverse().toString();
    }

    // Check palindrome
    static boolean isPalindrome(String s) {
        s = s.toLowerCase().replaceAll("[^a-z0-9]", "");
        return s.equals(reverse(s));
    }

    // Count occurrences of a character
    static int countChar(String s, char c) {
        int count = 0;
        for (char ch : s.toCharArray()) if (ch == c) count++;
        return count;
    }

    // Capitalize first letter of each word
    static String titleCase(String s) {
        String[] words = s.toLowerCase().split(" ");
        StringBuilder sb = new StringBuilder();
        for (String w : words)
            sb.append(Character.toUpperCase(w.charAt(0))).append(w.substring(1)).append(" ");
        return sb.toString().trim();
    }

    // Mask sensitive data (e.g., email, phone)
    static String maskEmail(String email) {
        int at = email.indexOf('@');
        if (at < 2) return "***@" + email.substring(at + 1);
        return email.charAt(0) + "*".repeat(at - 2) + email.charAt(at-1) + email.substring(at);
    }

    static String maskPhone(String phone) {
        return phone.substring(0, 2) + "****" + phone.substring(6);
    }

    // Truncate with ellipsis
    static String truncate(String s, int maxLen) {
        return s.length() <= maxLen ? s : s.substring(0, maxLen - 3) + "...";
    }

    public static void main(String[] args) {
        System.out.println("reverse('Hello')     = " + StringUtils.reverse("Hello"));
        System.out.println("isPalindrome('racecar') = " + StringUtils.isPalindrome("racecar"));
        System.out.println("isPalindrome('A man a plan a canal Panama') = "
                          + StringUtils.isPalindrome("A man a plan a canal Panama"));
        System.out.println("countChar('banana','a') = " + StringUtils.countChar("banana", 'a'));
        System.out.println("titleCase('hello world java') = "
                          + StringUtils.titleCase("hello world java"));
        System.out.println("maskEmail('rahul@gmail.com') = "
                          + StringUtils.maskEmail("rahul@gmail.com"));
        System.out.println("maskPhone('9876543210') = "
                          + StringUtils.maskPhone("9876543210"));
        System.out.println("truncate('Hello World Java',10) = "
                          + StringUtils.truncate("Hello World Java", 10));
    }
}

reverse('Hello')     = olleH
isPalindrome('racecar') = true
isPalindrome('A man a plan a canal Panama') = true
countChar('banana','a') = 3
titleCase('hello world java') = Hello World Java
maskEmail('rahul@gmail.com') = r***l@gmail.com
maskPhone('9876543210') = 98****3210
truncate('Hello World Java',10) = Hello W...

public class WhyMainIsStatic {

    static String appName = "JavaApp";
    int instanceId;
    static int nextId = 0;

    WhyMainIsStatic() {
        instanceId = ++nextId;
        showBanner();  // constructor mein static method call kar sakte hain ✅
    }

    // Static method calling another static
    static void showBanner() {
        System.out.println("╔══════════════════════════╗");
        System.out.println("║  " + appName + " - Starting Up   ║");
        System.out.println("╚══════════════════════════╝");
    }

    // Static method — calculations
    static int doubleIt(int n)    { return multiply(n, 2); }  // static → static ✅
    static int multiply(int a, int b) { return a * b; }

    // Instance method
    void showInstanceInfo() {
        // Instance method can access both
        System.out.println("Instance #" + instanceId + " of " + appName);
        showBanner();  // instance method → static method ✅
    }

    public static void main(String[] args) {
        /*
         * JVM karta hai: WhyMainIsStatic.main(args)
         * JVM ne koi object nahi banaya!
         * Isliye main() MUST be static — object ke bina call hona chahiye.
         *
         * Agar main() non-static hoti:
         * JVM ko pehle WhyMainIsStatic obj = new WhyMainIsStatic() banana padta
         * Lekin fir constructor call hota, aur object kaun banata? — Chicken-egg problem!
         */

        System.out.println("JVM ne main() directly call kiya — no object created yet!");
        System.out.println("appName = " + appName);  // static variable ✅

        // Instance methods call karna ho toh object banana padega
        WhyMainIsStatic obj1 = new WhyMainIsStatic();
        WhyMainIsStatic obj2 = new WhyMainIsStatic();
        obj1.showInstanceInfo();
        obj2.showInstanceInfo();

        System.out.println("\ndoubleIt(7) = " + doubleIt(7));   // static → static ✅
        System.out.println("nextId = " + nextId);              // static var ✅
    }
}

JVM ne main() directly call kiya — no object created yet!
appName = JavaApp
╔══════════════════════════╗
║  JavaApp - Starting Up   ║
╚══════════════════════════╝
╔══════════════════════════╗
║  JavaApp - Starting Up   ║
╚══════════════════════════╝
Instance #1 of JavaApp
JavaApp banner...
Instance #2 of JavaApp
JavaApp banner...

doubleIt(7) = 14
nextId = 2