Lesson 37 • Advanced

Unit Testing with JUnit & Mockito

Tests aren't just "nice to have" — they're your safety net. Every time you refactor, add features, or fix bugs, tests tell you instantly if something broke. JUnit 5 + Mockito is the gold standard for Java testing, and mastering this combo makes you a reliable developer that teams trust.

Before You Start

You should know OOP (classes, interfaces for mocking) and Annotations (JUnit is annotation-driven). Familiarity with Exception Handling helps for assertThrows testing.

What You'll Learn

✅ JUnit 5 lifecycle: @Test, @BeforeEach, @AfterEach
✅ Assertions: assertEquals, assertThrows, assertAll
✅ Parameterized tests with @ParameterizedTest
✅ Mocking dependencies with Mockito
✅ Test-Driven Development (TDD) workflow
✅ Code coverage and testing best practices

1️⃣ The Testing Pyramid

Analogy: Unit tests are like spell-checking individual words — fast, cheap, catch most errors. Integration tests check that sentences make sense together. E2E tests verify the whole essay reads correctly. You want a LOT of unit tests, some integration tests, and few E2E tests.

Level	Speed	Scope	Target
Unit	⚡ ms	Single method/class	70% of tests
Integration	🔄 seconds	Multiple components + DB	20% of tests
E2E	🐢 minutes	Full application	10% of tests

Try It: JUnit Test Suite

Try it Yourself »

JavaScript

// 💡 Try modifying this code and see what happens!
// Build a complete test suite with assertions

console.log("=== JUnit Test Suite ===\n");

// Mini test framework
let passed = 0, failed = 0;
function assertEquals(expected, actual, msg) {
  if (expected === actual) { passed++; console.log("  ✅ " + msg); }
  else { failed++; console.log("  ❌ " + msg + " (expected " + expected + ", got " + actual + ")"); }
}
function assertTrue(condition, msg) {
  if (condition) { passed++; console.log("  ✅ " +
...

2️⃣ Arrange → Act → Assert

Every well-structured test follows this three-step pattern. It keeps your tests readable and focused:

1. Arrange: Set up test data and dependencies

2. Act: Call the method you're testing

3. Assert: Verify the result is what you expected

Try It: Mockito — Isolating Dependencies

Try it Yourself »

JavaScript

// 💡 Try modifying this code and see what happens!
// Simulating Mockito — mocking external dependencies

console.log("=== Mockito — Mocking Dependencies ===\n");

// The problem: UserService depends on UserRepository (database)
// We can't test UserService without a real database... or CAN we?

// Mock UserRepository (simulating @Mock)
class MockUserRepository {
  constructor() { this.calls = []; this.returnValues = {}; }
  
  // when(mock.method(args)).thenReturn(value)
  whenFindById(id, ret
...

Common Mistakes

⚠️ Testing implementation, not behavior: Don't assert that a specific private method was called. Assert the outcome
⚠️ Testing too much per test: One logical concept per test. If it fails, you should immediately know why
⚠️ Not testing edge cases: Test null inputs, empty lists, boundary values, error conditions
⚠️ Mocking everything: Only mock external dependencies (DB, API). Test real logic with real objects
⚠️ Ignoring flaky tests: A test that sometimes passes is worse than no test — it erodes trust

Pro Tips

💡 @DisplayName — give tests human-readable names: @DisplayName("should throw when email is blank")
💡 @ParameterizedTest — run the same test with 20 different inputs in one declaration
💡 assertAll() — check multiple assertions at once. All failures are reported, not just the first
💡 TDD workflow: 🔴 Write failing test → 🟢 Write minimal code to pass → 🔵 Refactor while tests stay green

Try It: Parameterized Tests & TDD

Try it Yourself »

JavaScript

// 💡 Try modifying this code and see what happens!
// Parameterized tests and TDD workflow

console.log("=== Parameterized Tests & TDD ===\n");

// TDD Example: Build a password validator step by step

// 🔴 RED: Write failing tests first
console.log("1. TDD STEP 1 — 🔴 RED (write tests first):");
console.log("  Tests define the requirements BEFORE writing code\n");

// 🟢 GREEN: Implement to make tests pass
class PasswordValidator {
  validate(password) {
    let errors = [];
    if (!password
...

📋 Quick Reference

Annotation	Purpose	Example
@Test	Mark test method	Required on each test
@BeforeEach	Setup before each test	Initialize fresh objects
@ParameterizedTest	Run with multiple inputs	@CsvSource, @ValueSource
@Mock	Create mock object	Mockito annotation
verify()	Check mock was called	verify(mock).method()

🎉 Lesson Complete!

You can now write professional unit tests that catch bugs before users do! Next: Maven & Gradle — build tools and dependency management.

Unit Testing with JUnit & Mockito

Before You Start

What You'll Learn

1️⃣ The Testing Pyramid

Try It: JUnit Test Suite

2️⃣ Arrange → Act → Assert

Try It: Mockito — Isolating Dependencies

Common Mistakes

Pro Tips

Try It: Parameterized Tests & TDD

📋 Quick Reference

🎉 Lesson Complete!

Cookie & Privacy Settings