Test-Driven Development

Introduction

TDD is red → green → refactor: write a failing test, write the minimum code to pass, then improve the design. In Java/Spring projects it produces better APIs, higher coverage and fewer regressions.

Informative example

TDD cycle for a discount calculator:

// RED — test fails (class doesn't exist yet)
@Test
void noDiscountUnder100() {
    assertThat(calc.apply(50)).isEqualByComparingTo("50.00");
}

// GREEN — minimum implementation
public BigDecimal apply(int amount) {
    return BigDecimal.valueOf(amount);
}

// RED — next requirement
@Test
void tenPercentOffOver100() {
    assertThat(calc.apply(200)).isEqualByComparingTo("180.00");
}

// GREEN → REFACTOR — extract constants, add edge cases

Best practices

• Write the test first — if you can't, your API isn't clear yet.
• Keep the green step minimal — don't over-engineer.
• Refactor only when tests are green.

Purpose of this lesson

Master Test-Driven Development so you can apply it confidently in production Java code, technical interviews, and code reviews.

Step-by-step explanation

1. Understand the core idea behind Test-Driven Development.
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

Teams at Netflix, Uber and Goldman Sachs apply Test-Driven Development daily — usually wrapped behind Spring Boot services with observability hooks (Micrometer + OpenTelemetry).

Interview questions & answers

Summary

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