User-specific types

Now that users can log in, let’s add some code that is specific to the logged in user. The following Database class keeps track of each user and their account balance. It can keep track of multiple users and different balances.

class Database {
  private final Map<String, Account> accounts = new HashMap<>();

  Database() {}

  Account getAccount(String username) {
    return accounts.computeIfAbsent(username, Account::new);

  static final class Account {
    private final String username;
    private BigDecimal balance = BigDecimal.ZERO;

    Account(String username) {
      this.username = username;

    String username() {
      return username;

    BigDecimal balance() {
      return balance;

    void deposit(BigDecimal amount) {
      balance = balance.add(amount);

Now let’s incorporate the database into LoginCommand to print the current balance when a user logs in:

final class LoginCommand extends SingleArgCommand {
  private final Database database;

  LoginCommand(Database database, Outputter outputter) {
    this.database = database;
    this.outputter = outputter;

  public Result handleArg(String username) {
    Account account = database.getAccount(username);
        username + " is logged in with balance: " + account.balance());
    return Result.handled();

Try logging in with your name, and then again with jesse. Ignore for now that you can run login multiple times in a row without logging out. We’ll address that later.

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