ATM Simulator

Introduction

Simulate an ATM machine with account balance, deposit, withdraw, transfer and PIN validation. Practice OOP (Account, ATM, Transaction), concurrency (synchronized withdrawals) and file-based persistence.

Informative example

Account with thread-safe withdrawal:

class Account {
    private final String id;
    private BigDecimal balance;

    synchronized Optional<Receipt> withdraw(BigDecimal amount) {
        if (balance.compareTo(amount) < 0) return Optional.empty();
        balance = balance.subtract(amount);
        return Optional.of(new Receipt(id, amount, balance));
    }
}

class ATM {
    boolean authenticate(String pin) { ... }
    void menu() { /* deposit / withdraw / balance / exit */ }
}

Best practices

• Use BigDecimal for money — never double.
• Log every transaction to an audit file.
• Model invalid PIN attempts with lockout after 3 tries.

Purpose of this lesson

Simulate an ATM with OOP, synchronized withdrawals and file persistence.

Step-by-step explanation

1. Understand the core idea behind ATM Simulator.
2. Walk through the runnable example and tweak it in the playground.
3. Apply the pattern in a small Spring Boot or CLI exercise of your own.
4. Re-read the common mistakes and interview Q&A to lock the concept in.

Interactive workflow diagram

Debugging tips

• Read the full stack trace — Java's exception messages name the offending class and line.
• Reproduce in the smallest possible main() method before fixing in the real app.
• Use IntelliJ's debugger breakpoints and 'Evaluate Expression' rather than scattering System.out.

Optimization strategies

• Profile before optimizing — JFR (Java Flight Recorder) and async-profiler reveal real hotspots.
• Prefer immutable data and stream pipelines over hand-rolled loops when readability matters.
• Reach for the right JDK collection (ArrayList vs LinkedList vs ArrayDeque) before writing custom data structures.

Enterprise example

Banking simulators are common in OOP interviews — focus on thread-safe balance updates.

Interview questions & answers

Summary

In this lesson you learned ATM Simulator — the concept, syntax, a runnable example, and the production pitfalls to avoid. Apply it in the playground before moving on.