BankDatabase.java

import java.util.HashMap;
import java.util.Map;

/**
 * BankDatabase class - simulates a database of bank accounts.
 * Demonstrates: singleton pattern, collections framework.
 */
public class BankDatabase {
    private static BankDatabase instance;
    private Map<String, BankAccount> accounts;

    // Private constructor to implement Singleton
    private BankDatabase() {
        accounts = new HashMap<>();
        initializeAccounts();
    }

    // Singleton instance accessor
    public static synchronized BankDatabase getInstance() {
        if (instance == null) {
            instance = new BankDatabase();
        }
        return instance;
    }

    // Initialize with some dummy accounts
    private void initializeAccounts() {
        accounts.put("1234567890123456", new SavingsAccount("1234567890123456", "1234", 5000.0, 2.5));
        accounts.put("9876543210987654", new CheckingAccount("9876543210987654", "4321", 3000.0, 500.0));
    }

    // Get account by account number
    public BankAccount getAccount(String accountNumber) {
        return accounts.get(accountNumber);
    }

    // Authenticate user by matching PIN
    public boolean authenticateUser(String accountNumber, String pin) {
        BankAccount account = getAccount(accountNumber);
        return account != null && account.validatePin(pin);
    }
}