Digital Wallet

A Java-based digital wallet application with a Swing GUI that manages user wallets, supports multi-currency balances, and handles deposits, withdrawals, and transfers between wallets. Built with a functional error-handling style using Vavr, CSV-based persistence, and thread-safe transfer operations.

Features

• Multi-user login — username-based session login with Switch User support; each user sees only their own wallets.
• Wallet management — create wallets with an owner name and currency (USD, EUR, GBP).
• Deposits & withdrawals — add or remove funds with validation (positive amounts, sufficient balance).
• Transfers — move funds between wallets safely, with checks for:
  • Self-transfers
  • Currency mismatch
  • Insufficient funds
  • Non-existent wallets
• Transaction history — every operation is recorded and displayed per wallet in the GUI.
• Export to CSV — user-triggered export of all transactions for the logged-in user via a file save dialog.
• CSV persistence — wallets and transactions are loaded from CSV on startup and saved on shutdown automatically.
• Functional error handling — operations return Either<DomainError, T> instead of throwing exceptions, making failure cases explicit and chainable.
• Concurrency-safe transfers — per-wallet ReentrantLocks with consistent UUID-ordered lock acquisition to prevent deadlocks during concurrent transfers.
• Logging — SLF4J + Logback integration for info/warn-level operational logs.

Tech Stack

• Java 21
• Maven (build tool)
• Swing — built-in Java GUI framework
• Vavr 0.10.4 — functional types (Either, Option, immutable List)
• Lombok 1.18.30 — boilerplate reduction (@Value, @Getter, @AllArgsConstructor)
• SLF4J 2.0.16 + Logback 1.5.12 — logging
• JUnit 5 & AssertJ — unit and concurrency testing

Project Structure

src/main/java/com/wallet/
├── Main.java                      # Entry point: service wiring, CSV load/save, login, GUI launch
├── common/
│   └── DomainError.java           # Enum of domain-level error codes & messages
├── domain/
│   ├── Wallet.java                # Immutable wallet aggregate (balance, owner, currency)
│   ├── Transaction.java           # Immutable transaction record
│   ├── TransactionType.java       # DEPOSIT, WITHDRAW, TRANSFER
│   ├── TransactionStatus.java     # SUCCESS, FAILED
│   └── Currency.java              # USD, EUR, GBP
├── service/
│   ├── WalletService.java         # Create wallets, deposit, withdraw
│   ├── TransactionService.java    # Save & query transactions
│   ├── TransferService.java       # Lock-based wallet-to-wallet transfers
│   └── PersistenceService.java    # CSV read/write for wallets and transactions
└── gui/
    └── WalletGui.java             # Swing GUI: wallet controls, transaction table, export

Domain Model

Wallet

An immutable record (@Value) holding walletId, ownerName, balance, currency, and createdAt. New balances produce a new Wallet instance via withBalance(...) rather than mutating in place. The updated instance is stored back into the ConcurrentHashMap.

Transaction

An immutable record describing a single operation. Uses Vavr Option<UUID> for fromWalletId and toWalletId because deposits have no source and withdrawals have no destination. Factory methods (createDeposit, createWithdrawal, createTransfer) validate the amount and return Either<DomainError, Transaction>.

DomainError

Enumerates all failure modes: INSUFFICIENT_FUNDS, INVALID_AMOUNT, WALLET_NOT_FOUND, CURRENCY_MISMATCH, NEGATIVE_BALANCE, SELF_TRANSFER, INVALID_INPUT, TRANSACTION_NOT_FOUND.

How It Works

Every service operation returns Either<DomainError, T>:

• Right — the success value (a Wallet or Transaction).
• Left — a DomainError describing why it failed.

This lets callers chain operations with .flatMap(...) / .map(...) and handle side effects (logging, persistence) with .peek(...) / .peekLeft(...) without throwing or catching exceptions.

For transfers, TransferService acquires ReentrantLocks for both wallets in a deterministic order (lower UUID string first) before performing the withdrawal-then-deposit sequence, preventing deadlocks when two transfers between the same pair of wallets run concurrently.

Persistence

On startup, Main loads wallets.csv and transactions.csv into memory via PersistenceService. A JVM shutdown hook saves all current state back to those files when the application exits. The CSV format is flat and comma-separated; owner names containing commas are a known limitation.

Getting Started

Prerequisites

• JDK 21+
• Maven 3.6+

Build

mvn clean compile

Run

mvn exec:java -Dexec.mainClass="com.wallet.Main"

Or run Main.java directly from your IDE. A login dialog will appear on startup.

Testing

JUnit 5 tests cover:

• Successful deposits, withdrawals, and transfers (happy paths)
• Edge cases: null inputs, invalid amounts, insufficient funds, currency mismatches, self-transfers
• Concurrency: bidirectional concurrent transfer test to verify deadlock prevention

Notes & Limitations

• CSV fragility — owner names or fields containing commas will break CSV parsing. A quoted-field CSV parser would fix this.
• No authentication — login is username-only; there are no passwords or access controls.
• DailyLimitTracker is a stub — the class exists as a placeholder; per-wallet daily transaction limits are not yet implemented.
• No REST or CLI layer — the application is GUI-only; there is no API exposed.