14. జావా OOP మూలాలు — క్లాసులు, ఆబ్జెక్టులు & హీప్ మెమరీ (Classes, Objects & Heap)

public class InstanceVsStaticMemoryDemo {

    public static class BankBranch {
        // Static variable: Shared by all branches of the bank
        public static String bankName = "State Financial Bank";
        public static int totalBranchesCount = 0;

        // Instance variables: Unique to each specific branch object
        public String branchCity;
        public double localVaultCash;

        public BankBranch(String city, double initialCash) {
            this.branchCity = city;
            this.localVaultCash = initialCash;
            totalBranchesCount++; // Increment global shared counter
        }

        public void printBranchStatus() {
            System.out.printf("🏦 [%s - %-10s] Local Vault: ₹%,10.2f | Total Bank Branches: %d%n",
                    bankName, branchCity, localVaultCash, totalBranchesCount);
        }
    }

    public static void main(String[] args) {
        System.out.println("=================================================");
        System.out.println("🏛️ INSTANCE VS STATIC MEMORY ARCHITECTURE DEMO");
        System.out.println("=================================================
");

        System.out.println("1. Creating 3 distinct branch objects:");
        BankBranch b1 = new BankBranch("Hyderabad", 500000.0);
        BankBranch b2 = new BankBranch("Bengaluru", 750000.0);
        BankBranch b3 = new BankBranch("Visakhapatnam", 350000.0);

        b1.printBranchStatus();
        b2.printBranchStatus();
        b3.printBranchStatus();

        System.out.println("
2. Modifying static variable BankBranch.bankName:");
        BankBranch.bankName = "NextGen Unified Bank";

        System.out.println("   Modifying b1 local vault cash only (+ ₹1,00,000):");
        b1.localVaultCash += 100000.0;

        System.out.println("
3. Status after modifications:");
        b1.printBranchStatus();
        b2.printBranchStatus();
        b3.printBranchStatus();

        System.out.println("
=================================================");
        System.out.println("✅ తీర్పు: Static వేరియబుల్ మారితే అన్ని ఆబ్జెక్టులలో మారింది,");
        System.out.println("          కానీ Instance వేరియబుల్ కేవలం b1 లో మాత్రమే మారింది!");
        System.out.println("=================================================");
    }
}

public class ObjectLifecycleAndGcSimulation {

    static class DataNode {
        String payloadId;
        byte[] heavyBuffer = new byte[1024 * 512]; // 512 KB payload

        DataNode(String id) {
            this.payloadId = id;
        }

        @Override
        public String toString() {
            return "DataNode[" + payloadId + "]";
        }
    }

    public static void main(String[] args) {
        System.out.println("=================================================");
        System.out.println("♻️ OBJECT LIFECYCLE & HEAP REALLOCATION DEMO");
        System.out.println("=================================================
");

        Runtime rt = Runtime.getRuntime();

        // Initial free memory
        long initialFree = rt.freeMemory();
        System.out.printf("1. Initial Heap Free Memory: %,d KB%n", initialFree / 1024);

        // Step 1: Allocating first object
        DataNode node1 = new DataNode("Session_Alpha");
        System.out.println("2. Created active reference: " + node1);
        System.out.printf("   Heap Free after node1:    %,d KB%n", rt.freeMemory() / 1024);

        // Step 2: Reassigning reference to a new object (making Session_Alpha orphan)
        System.out.println("
3. Reassigning node1 = new DataNode('Session_Beta')...");
        node1 = new DataNode("Session_Beta");
        System.out.println("   Current active reference: " + node1);
        System.out.println("   ⚠️ 'Session_Alpha' is now UNREACHABLE (Orphan in Heap)!");

        // Step 3: Nullifying reference (making Session_Beta orphan too)
        System.out.println("
4. Nullifying node1 = null...");
        node1 = null;
        System.out.println("   ⚠️ Both objects are now 100% eligible for Garbage Collection!");

        // Triggering GC hint
        System.gc();
        System.out.printf("5. Post-GC Request Free Memory: %,d KB%n", rt.freeMemory() / 1024);

        System.out.println("=================================================");
    }
}

public class BankingDomainCoreEngine {

    public static class BankAccount {
        // Global Bank Configuration (Static - Shared across all accounts)
        private static final String IFSC_CODE = "APGB0004921";
        private static double annualInterestRate = 6.5; // 6.5% APY
        private static long accountNumberGenerator = 1001002000L;
        private static int activeAccountsTotal = 0;

        // Instance State (Unique to each customer account)
        private final long accountNumber;
        private final String accountHolder;
        private double balance;
        private boolean isFrozen;

        public BankAccount(String holderName, double initialDeposit) {
            this.accountNumber = ++accountNumberGenerator;
            this.accountHolder = holderName;
            this.balance = Math.max(0.0, initialDeposit);
            this.isFrozen = false;
            activeAccountsTotal++;
        }

        // Behavior: Deposit
        public boolean deposit(double amount) {
            if (isFrozen || amount <= 0) {
                System.out.printf("❌ Deposit failed for Acc #%d: Invalid amount or account frozen.%n", accountNumber);
                return false;
            }
            this.balance += amount;
            System.out.printf("✅ Deposited ₹%,.2f into Acc #%d (%s). New Balance: ₹%,.2f%n",
                    amount, accountNumber, accountHolder, balance);
            return true;
        }

        // Behavior: Withdraw with overdraft check
        public boolean withdraw(double amount) {
            if (isFrozen) {
                System.out.printf("❌ Withdrawal denied: Account #%d is frozen.%n", accountNumber);
                return false;
            }
            if (amount <= 0 || amount > this.balance) {
                System.out.printf("❌ Withdrawal failed for Acc #%d: Insufficient funds (Avail: ₹%,.2f, Req: ₹%,.2f)%n",
                        accountNumber, balance, amount);
                return false;
            }
            this.balance -= amount;
            System.out.printf("💸 Withdrawn ₹%,.2f from Acc #%d (%s). Remaining Balance: ₹%,.2f%n",
                    amount, accountNumber, accountHolder, balance);
            return true;
        }

        public void printStatement() {
            System.out.println("-------------------------------------------------");
            System.out.println("📄 APGB OFFICIAL ACCOUNT AUDIT STATEMENT");
            System.out.printf("Account Number: %d | IFSC: %s%n", accountNumber, IFSC_CODE);
            System.out.printf("Holder:         %s%n", accountHolder);
            System.out.printf("Current Status: %s | Annual Interest: %.1f%%%n",
                    (isFrozen ? "FROZEN ❄️" : "ACTIVE 🟢"), annualInterestRate);
            System.out.printf("Liquid Balance: ₹%,.2f%n", balance);
            System.out.println("-------------------------------------------------");
        }

        public static int getActiveAccountsTotal() {
            return activeAccountsTotal;
        }

        public static void updateGlobalInterestRate(double newRate) {
            annualInterestRate = newRate;
            System.out.printf("📢 Global Bank Interest Rate updated to %.2f%% APY for all accounts.%n", newRate);
        }
    }

    public static void main(String[] args) {
        System.out.println("=================================================");
        System.out.println("🏦 ENTERPRISE BANKING CORE DOMAIN ENGINE");
        System.out.println("=================================================
");

        BankAccount acc1 = new BankAccount("Balaji Nemani", 75000.0);
        BankAccount acc2 = new BankAccount("Karthik Verma", 15000.0);

        System.out.println("Total Active Customer Accounts: " + BankAccount.getActiveAccountsTotal() + "
");

        // Performing banking transactions
        acc1.deposit(25000.0);
        acc1.withdraw(15000.0);

        acc2.withdraw(20000.0); // Should fail due to insufficient funds

        // Bank updates interest rate globally
        System.out.println();
        BankAccount.updateGlobalInterestRate(7.25);

        // Printing audit statements
        acc1.printStatement();
        acc2.printStatement();

        System.out.println("=================================================");
    }
}