12. జావా మెథడ్స్, Pass-by-Value & కాల్ స్టాక్ ఆర్కిటెక్చర్ (Methods & Call Stack)

public class PassByValueProofDemo {

    static class CustomerAccount {
        String name;
        double balance;

        CustomerAccount(String name, double balance) {
            this.name = name;
            this.balance = balance;
        }

        @Override
        public String toString() {
            return "[" + name + " | ₹" + balance + "]";
        }
    }

    // 1. Primitive Modification Attempt
    public static void tryModifyPrimitive(int num) {
        num = num + 100;
        System.out.println("   -> Inside tryModifyPrimitive: num = " + num);
    }

    // 2. Object State Mutation Attempt
    public static void mutateAccountState(CustomerAccount acc) {
        acc.balance += 5000.0;
        System.out.println("   -> Inside mutateAccountState: acc = " + acc);
    }

    // 3. Object Reference Reassignment Attempt
    public static void reassignAccountReference(CustomerAccount acc) {
        // Assigning to a completely new heap object
        acc = new CustomerAccount("Hacker_Account", 0.0);
        System.out.println("   -> Inside reassignAccountReference: acc = " + acc);
    }

    public static void main(String[] args) {
        System.out.println("=================================================");
        System.out.println("🧪 INCONTROVERTIBLE PROOF: JAVA IS PASS-BY-VALUE");
        System.out.println("=================================================
");

        // TEST 1: Primitives
        int score = 50;
        System.out.println("1. PRIMITIVE TEST:");
        System.out.println("   Before method call: score = " + score);
        tryModifyPrimitive(score);
        System.out.println("   After method call:  score = " + score);
        System.out.println("   ✅ తీర్పు: ప్రిమిటివ్ విలువ ఏమాత్రం మారలేదు!
");

        // TEST 2: Object Internal Mutation
        CustomerAccount myAccount = new CustomerAccount("Balaji", 10000.0);
        System.out.println("2. OBJECT INTERNAL MUTATION TEST:");
        System.out.println("   Before method call: " + myAccount);
        mutateAccountState(myAccount);
        System.out.println("   After method call:  " + myAccount);
        System.out.println("   ✅ తీర్పు: రిఫరెన్స్ కాపీ ద్వారా హీప్ ఆబ్జెక్ట్ స్టేట్ మారింది!
");

        // TEST 3: Object Reference Reassignment
        System.out.println("3. OBJECT REFERENCE REASSIGNMENT TEST:");
        System.out.println("   Before method call: " + myAccount);
        reassignAccountReference(myAccount);
        System.out.println("   After method call:  " + myAccount);
        System.out.println("   ✅ తీర్పు: మెథడ్ లోపల రిఫరెన్స్‌ను కొత్త ఆబ్జెక్ట్‌కు రీఅసైన్ చేసినా,");
        System.out.println("             కాలర్ చేతిలో ఉన్న ఒరిజినల్ రిఫరెన్స్ మారలేదు!");
        System.out.println("=================================================");
    }
}

public class CallStackAndRecursionTracer {

    public static void methodC() {
        System.out.println("   -> Inside methodC() [Deepest Stack Frame]:");
        StackTraceElement[] stack = Thread.currentThread().getStackTrace();
        for (int i = 1; i < Math.min(5, stack.length); i++) {
            System.out.printf("      Frame [%d]: %s.%s() (Line: %d)%n",
                    i, stack[i].getClassName(), stack[i].getMethodName(), stack[i].getLineNumber());
        }
    }

    public static void methodB() {
        System.out.println("  -> Inside methodB()");
        methodC();
    }

    public static void methodA() {
        System.out.println(" -> Inside methodA()");
        methodB();
    }

    // Recursive Factorial with visual call stack indent
    public static long factorial(int n, int depth) {
        String indent = "  ".repeat(depth);
        System.out.println(indent + "📥 factorial(" + n + ") called -> Frame pushed");

        if (n <= 1) {
            System.out.println(indent + "🎯 Base Case reached for n = " + n + " -> returning 1");
            return 1;
        }

        long result = n * factorial(n - 1, depth + 1);
        System.out.println(indent + "📤 factorial(" + n + ") returning " + result + " -> Frame popped");
        return result;
    }

    public static void main(String[] args) {
        System.out.println("=================================================");
        System.out.println("📚 JVM CALL STACK TRACER & RECURSION VISUALIZER");
        System.out.println("=================================================
");

        System.out.println("1. NESTED METHOD CALL STACK INSPECTION:");
        methodA();

        System.out.println("
2. RECURSIVE CALL STACK VISUALIZATION (Factorial 4):");
        long fact4 = factorial(4, 0);
        System.out.println("
🎉 Final Factorial(4) = " + fact4);
        System.out.println("=================================================");
    }
}

public class EcommercePricingEngine {

    // Cart Item Model
    public static class CartItem {
        String name;
        double unitPrice;
        int quantity;

        public CartItem(String name, double unitPrice, int quantity) {
            this.name = name;
            this.unitPrice = unitPrice;
            this.quantity = quantity;
        }

        public double getItemSubtotal() {
            return unitPrice * quantity;
        }
    }

    // Modular Business Methods
    public static double calculateRawSubtotal(CartItem[] items) {
        double subtotal = 0;
        for (CartItem item : items) {
            subtotal += item.getItemSubtotal();
        }
        return subtotal;
    }

    public static double applyCouponDiscount(double subtotal, String couponCode) {
        if (couponCode == null) return 0.0;

        return switch (couponCode.toUpperCase()) {
            case "WELCOME20" -> subtotal * 0.20; // 20% off
            case "FESTIVE10" -> subtotal * 0.10; // 10% off
            case "FLAT500"   -> Math.min(500.0, subtotal);
            default          -> 0.0;
        };
    }

    public static double calculateTax(double taxableAmount, double gstRate) {
        if (taxableAmount <= 0) return 0.0;
        return taxableAmount * (gstRate / 100.0);
    }

    public static double calculateShipping(double netTotal) {
        return (netTotal >= 2000.0) ? 0.0 : 150.0; // Free shipping over ₹2000
    }

    public static void printInvoice(String customer, CartItem[] items, String coupon, double gstRate) {
        double rawSubtotal = calculateRawSubtotal(items);
        double discount = applyCouponDiscount(rawSubtotal, coupon);
        double taxableAmount = rawSubtotal - discount;
        double tax = calculateTax(taxableAmount, gstRate);
        double shipping = calculateShipping(taxableAmount);
        double finalPayable = taxableAmount + tax + shipping;

        System.out.println("=================================================");
        System.out.println("🧾 E-COMMERCE TAX INVOICE & PAYMENT ENVELOPE");
        System.out.println("=================================================");
        System.out.println("Customer Name: " + customer);
        System.out.println("-------------------------------------------------");
        System.out.printf("%-18s %8s %5s %12s%n", "Item", "Price", "Qty", "Total");
        System.out.println("-------------------------------------------------");
        for (CartItem item : items) {
            System.out.printf("%-18s ₹%7.2f %4d   ₹%9.2f%n",
                    item.name, item.unitPrice, item.quantity, item.getItemSubtotal());
        }
        System.out.println("-------------------------------------------------");
        System.out.printf("Gross Subtotal:                       ₹%10.2f%n", rawSubtotal);
        System.out.printf("Coupon Discount (%-9s):            -₹%10.2f%n", coupon, discount);
        System.out.printf("GST Tax (%4.1f%%):                      ₹%10.2f%n", gstRate, tax);
        System.out.printf("Shipping Charges:                     ₹%10.2f (%s)%n",
                shipping, (shipping == 0.0 ? "FREE" : "Standard"));
        System.out.println("=================================================");
        System.out.printf("💰 NET PAYABLE AMOUNT:                 ₹%10.2f%n", finalPayable);
        System.out.println("=================================================");
    }

    public static void main(String[] args) {
        CartItem[] cart = {
            new CartItem("Wireless Keyboard", 1899.00, 1),
            new CartItem("Ergonomic Mouse",   1250.00, 2),
            new CartItem("USB-C Fast Cable",   499.00, 3)
        };

        printInvoice("Balaji", cart, "WELCOME20", 18.0);
    }
}