🏦 Fintech Bank — JUnit 5 Learning Project

A simple Java banking library built as a hands-on introduction to unit testing with JUnit 5.

The focus is on learning testing concepts — not building a full app. The codebase is intentionally small so you can understand every line.

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🎯 What This Project Covers

Concept	Where to find it
@Test, @BeforeEach, @AfterEach	BankAccountTest — top of file
assertEquals, assertTrue	DepositTests, TransactionHistoryTests
assertThrows — testing exceptions	WithdrawalTests, TransferTests
Inspecting exception fields	withdraw_moreThanBalance_shouldThrowInsufficientFunds
@ParameterizedTest + @ValueSource	DepositTests, WithdrawalTests
@Nested for grouping tests	All sections in BankAccountTest
@DisplayName for readable output	Every test
Testing state doesn't change on failure	transfer_onFailure_shouldNotChangeAnyBalance
Testing immutability	transactionHistory_shouldBeUnmodifiable

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📁 Project Structure

fintech-bank/
├── src/
│   ├── main/java/com/fintech/
│   │   ├── exception/
│   │   │   └── InsufficientFundsException.java
│   │   └── model/
│   │       ├── BankAccount.java
│   │       └── Transaction.java
│   └── test/java/com/fintech/
│       ├── exception/
│       │   └── InsufficientFundsExceptionTest.java
│       └── model/
│           └── BankAccountTest.java        ← Start here
├── pom.xml
└── README.md

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🚀 Getting Started

Prerequisites

• Java 11+
• Maven 3.6+

Run the tests

mvn test

Run a single test class

mvn test -Dtest=BankAccountTest

Run a single test method

mvn test -Dtest=BankAccountTest#deposit_validAmount_shouldIncreaseBalance

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🔑 Key JUnit 5 Concepts — Quick Reference

Test lifecycle annotations

@BeforeEach   // runs before every @Test — use for fresh setup
@AfterEach    // runs after every @Test — use for cleanup
@BeforeAll    // runs once before all tests in the class (must be static)
@AfterAll     // runs once after all tests in the class (must be static)

Common assertions

assertEquals(expected, actual)         // values are equal
assertNotEquals(unexpected, actual)    // values are not equal
assertTrue(condition)                  // condition is true
assertFalse(condition)                 // condition is false
assertNull(object)                     // object is null
assertNotNull(object)                  // object is not null
assertThrows(Exception.class, () -> {  // code throws the right exception
    // code that should throw
});

Parameterized tests

// Run the same test with multiple inputs
@ParameterizedTest
@ValueSource(doubles = {0, -1, -999.99})
void myTest(double value) {
    // runs 3 times, once per value
}

Grouping with @Nested

@Nested
@DisplayName("Deposit tests")
class DepositTests {
    @Test void someTest() { ... }
}

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💡 Extension Ideas

When you have time to continue, here are natural next steps:

• SavingsAccount — extends BankAccount, adds interest rate, test applyInterest()
• AccountService — handles multiple accounts, test findByAccountNumber()
• Transaction filtering — filter history by type or date range, test edge cases
• Overdraft protection — allow a configurable negative balance limit, update tests
• @ParameterizedTest with @CsvSource — test deposit + expected balance pairs
• Mockito — mock dependencies when you're ready to move past pure unit tests

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📚 Resources

• JUnit 5 User Guide
• JUnit 5 Annotations Cheat Sheet
• Parameterized Tests in JUnit 5