Create a simple bank with accounts in Java

Question

I have two interfaces Bank and Account which have some functions:

public interface Bank {
    String createAccount(String owner) throws IOException;
    boolean closeAccount(String number) throws IOException;
    Set<String> getAccountNumbers() throws IOException;
    Account getAccount(String number) throws IOException;
    void transfer(Account a, Account b, double amount) throws IOException, IllegalArgumentException, OverdrawException, InactiveException;
}
public interface Account {
    String getNumber() throws IOException;
    String getOwner() throws IOException;
    boolean isActive() throws IOException;
    void deposit(double amount) throws IOException, IllegalArgumentException, InactiveException;
    void withdraw(double amount) throws IOException, IllegalArgumentException, OverdrawException, InactiveException;
    double getBalance() throws IOException;
}

I have a class Driver which has two inner classes Bank and Account which implements the interfaces above, the Bank has a HashMap as accounts of customers and a customer can have multiple accounts with different account numbers. The problem that i couldn't solve it, is the generating of these account numbers even multiple for each customer!!! :

    public class Driver {
        private Bank bank = null;
        static class Bank implements Bank {
            private final Map<String, Account> accounts = new HashMap<>();
            @Override
            public Set<String> getAccountNumbers() {
                return accounts.keySet();
            }
            @Override
            public String createAccount(String owner) {
                //TODO create new Account and return the account number
            }

            @Override
            public boolean closeAccount(String number) {
                //TODO if the account isActive and balance is zero then set it as inactive and return true.
                return false;
            }

            @Override
            public bank.Account getAccount(String number) {
                return accounts.get(number);
            }

            @Override
            public void transfer(bank.Account from, bank.Account to, double amount)
                    throws IOException, InactiveException, OverdrawException {
                if (amount <= from.getBalance()) {
                    from.withdraw(amount);
                    to.deposit(amount);
                }
        }

        static class Account implements Account {
            private String number;
            private String owner;
            private double balance;
            private boolean active = true;

            Account(String owner) {
                this.owner = owner;
                //TODO set the account number ??? 
            }

            @Override
            public double getBalance() {
                return balance;
            }

            @Override
            public String getOwner() {
                return owner;
            }

            @Override
            public String getNumber() {
                return number;
            }

            @Override
            public boolean isActive() {
                return active;
            }

            @Override
            public void deposit(double amount) throws InactiveException {
                if (!isActive())
                    throw new InactiveException();
                if (amount < 0)
                    throw new IllegalArgumentException();
                balance += amount;
            }

            @Override
            public void withdraw(double amount) throws InactiveException, OverdrawException {
                if (!isActive())
                    throw new InactiveException();
                if (amount > balance)
                    throw new OverdrawException();
                if (amount < 0)
                    throw new IllegalArgumentException();
                balance -= amount;
            }
        }
}

How to generate account numbers? There is a HashMap with all existing Accounts into the Bank. A customer can have multiple accounts with different account numbers! How does it work like this!?

Answer 1

Your Bank class could provide this feature by internally having a counter. It will work as all accounts are only maintained by this one bank object, and not shared with others.

If a user requests an account the bank will give it the number the counter currently displays and then increment the counter. The next request will thus get a different number.

Could look like

public class Bank {
    private int nextAccountId = 1;

    public int createAccount(String owner) {
        // Get account ID
        int accountId = getUniqueAccountId();

        // Create account
        ...

        return accountID;
    }

    private int getUniqueAccountId() {
        int accountId = nextAccountId;
        // Increment ID for next request
        nextAccountId++;

        return accountId;

        // Method can be made compact by just using
        // return nextAccountId++;
    }
}

Obviously this has some drawbacks. It is limited to the range of int (about 4 Mrd accounts, can of course be extended by using long). Users can also see how many accounts the bank already has. And it is easy to guess a valid ID of another users account.

For a more random approach you could use

String uniqueID = UUID.randomUUID().toString();

see here for more information.