banking_system.java

// Banking system using Multi-Threading
import java.util.*;
import java.io.*;

public class Demo {

    static synchronized void write(String msg) {
        try {
            FileWriter fw = new FileWriter("transactions.txt", true);
            fw.write(msg + "\n");
            fw.close();
        } catch (Exception e) {
            System.out.println("File error!");
        }
    }

    static void saveAccount(String accNo, double balance) {
        try {
            File file = new File("account.txt");
            List<String> lines = new ArrayList<>();
            boolean found = false;

            if (file.exists()) {
                BufferedReader br = new BufferedReader(new FileReader(file));
                String line;

                while ((line = br.readLine()) != null) {
                    String[] parts = line.split(",");

                    if (parts.length < 2) {
                        continue;
                    }

                    if (parts[0].equals(accNo)) {
                        lines.add(accNo + "," + balance);
                        found = true;
                    } else {
                        lines.add(line);
                    }
                }
                br.close();
            }

            if (!found) {
                lines.add(accNo + "," + balance);
            }

            FileWriter fw = new FileWriter(file);

            for (String l : lines) {
                fw.write(l + "\n");
            }

            fw.close();

        } catch (Exception e) {
            System.out.println("Error saving account!");
        }
    }

    static double loadAccount(String accNo) {
        try {
            BufferedReader br = new BufferedReader(new FileReader("account.txt"));
            String line;

            while ((line = br.readLine()) != null) {
                String[] parts = line.split(",");

                if (parts.length < 2) {
                    continue;
                }

                if (parts[0].equals(accNo)) {
                    double balance = Double.parseDouble(parts[1]);
                    br.close();
                    return balance;
                }
            }

            br.close();

        } catch (Exception e) {
        }

        return -1;
    }

    static class BankAccount {
        String accNo;
        double balance;

        BankAccount(String accNo, double bal) {
            this.accNo = accNo;
            this.balance = bal;
        }

        synchronized void deposit(double amt, String user) {
            if (amt <= 0) {
                System.out.println(user + " Invalid deposit amount!");
                return;
            }

            balance += amt;

            String msg = user + " deposited Rs " + amt +
                    " | Balance: Rs " + balance;

            System.out.println(msg);
            write(msg);
        }

        synchronized void withdraw(double amt, String user) {
            if (amt <= 0) {
                System.out.println(user + " Invalid withdrawal amount!");
                return;
            }

            if (amt > balance) {
                String msg = user + " FAILED to withdraw Rs " + amt +
                        " | Balance: Rs " + balance;

                System.out.println(msg);
                write(msg);

            } else {
                balance -= amt;

                String msg = user + " withdrew Rs " + amt +
                        " | Balance: Rs " + balance;

                System.out.println(msg);
                write(msg);
            }
        }
    }

    //Thread

    static class UserThread extends Thread {
        String name;
        BankAccount acc;
        String type;
        double amount;

        UserThread(String name, BankAccount acc, String type, double amt) {
            this.name = name;
            this.acc = acc;
            this.type = type;
            this.amount = amt;
        }

        public void run() {
            if (type.equals("D")) {
                acc.deposit(amount, name);

            } else if (type.equals("W")) {
                acc.withdraw(amount, name);

            } else {
                System.out.println(name + " Invalid transaction type!");
            }
        }
    }



    public static void main(String[] args) throws InterruptedException {

        Scanner sc = new Scanner(System.in);

        System.out.println("----------------------------------------------------------");
        System.out.println("                    Banking System                        ");
        System.out.println("----------------------------------------------------------");

        // Account number validation
        String accNo;

        while (true) {
            System.out.print("\nEnter Account Number: ");
            accNo = sc.nextLine().trim();

            if (accNo.matches("\\d+") && !accNo.startsWith("0")) {
                break;
            } else {
                System.out.println(
                    "Invalid Account Number! Enter positive digits only."
                );
            }
        }

        double balance;

        double loaded = loadAccount(accNo);

        if (loaded != -1) {
            balance = loaded;
            System.out.println("Account loaded!");
            System.out.println("Current Balance: Rs " + balance);

        } else {

            do {
                System.out.print("Enter Initial Balance: ");
                balance = sc.nextDouble();

                if (balance < 0) {
                    System.out.println("Balance cannot be negative!");
                }

            } while (balance < 0);

            sc.nextLine();
        }

        BankAccount account = new BankAccount(accNo, balance);

        // Number of users validation
        int n;

        do {
            System.out.print("\nHow many users: ");
            n = sc.nextInt();

            if (n <= 0) {
                System.out.println("Enter at least 1 user!");
            }

        } while (n <= 0);

        sc.nextLine();

        UserThread[] threads = new UserThread[n];

        for (int i = 0; i < n; i++) {

            System.out.println("\nUser " + (i + 1));

            System.out.print("Name: ");
            String name = sc.nextLine();

            System.out.print("Transaction (D/W): ");
            String type = sc.nextLine().toUpperCase();

            System.out.print("Amount: ");
            double amt = sc.nextDouble();
            sc.nextLine();

            threads[i] = new UserThread(name, account, type, amt);
        }

        System.out.println("\nTransactions running...\n");

        // Start all threads
        for (UserThread t : threads) {
            t.start();
        }

        // Wait for all threads to finish
        for (UserThread t : threads) {
            t.join();
        }

        System.out.println("\nFinal Balance: Rs " + account.balance);

        saveAccount(accNo, account.balance);

        System.out.println("Data saved to file");

        sc.close();
    }
}