Step 3: An account that protects its rules

import java.util.*;

public class Main {
  record Money(long cents) {
    Money plus(Money o) { return new Money(cents + o.cents); }
    Money minus(Money o) { return new Money(cents - o.cents); }
    public String toString() { return String.format("$%d.%02d", cents / 100, cents % 100); }
  }

  record Entry(String kind, Money amount, Money balanceAfter) {}

  static class InsufficientFundsException extends RuntimeException {
    InsufficientFundsException(String message) { super(message); }
  }

  static class Account {
    private final String owner;
    private Money balance = new Money(0);
    private final List<Entry> history = new ArrayList<>();

    Account(String owner) { this.owner = owner; }

    void deposit(Money amount) {
      if (amount.cents() <= 0) throw new IllegalArgumentException("deposit must be positive");
      balance = balance.plus(amount);
      history.add(new Entry("deposit", amount, balance));
    }

    void withdraw(Money amount) {
      if (amount.cents() <= 0) throw new IllegalArgumentException("withdrawal must be positive");
      if (amount.cents() > balance.cents()) {
        throw new InsufficientFundsException(owner + " has " + balance + " but needs " + amount);
      }
      balance = balance.minus(amount);
      history.add(new Entry("withdraw", amount, balance));
    }

    Money balance() { return balance; }
    List<Entry> history() { return Collections.unmodifiableList(history); }   // callers cannot change it
  }

  public static void main(String[] args) {
    Account ada = new Account("Ada");
    ada.deposit(new Money(10000));
    ada.withdraw(new Money(2550));

    String[] attempts = { "withdraw 900.00", "deposit -5.00" };
    for (String attempt : attempts) {
      try {
        if (attempt.startsWith("withdraw")) ada.withdraw(new Money(90000));
        else ada.deposit(new Money(-500));
      } catch (InsufficientFundsException | IllegalArgumentException e) {
        System.out.println("refused: " + e.getMessage());
      }
    }

    for (Entry e : ada.history()) System.out.println(e.kind() + " " + e.amount() + " -> " + e.balanceAfter());
    System.out.println("balance " + ada.balance());
  }
}

refused: Ada has $74.50 but needs $900.00
refused: deposit must be positive
deposit $100.00 -> $100.00
withdraw $25.50 -> $74.50
balance $74.50