Java: Test-Driven Development (TDD) – A Beginners Guide

A Beginner’s Tutorial on TDD Featuring Hands-On Examples

Test-Driven Development (TDD) is a software development methodology where you write tests for your code before you write the actual code.

In large and complex projects, Test-Driven Development (TDD) becomes even more crucial due to the intricate interdependencies, the number of developers involved, and the need for maintainable and bug-free code.

Let’s first see how it works using a simple example using Java and the JUnit framework

Step 1: Write a Failing Test

Suppose we want to implement a method that adds two numbers. First, we’ll write a test for this method, even though we haven’t implemented the method yet.

import static org.junit.jupiter.api.Assertions.assertEquals;
import org.junit.jupiter.api.Test;

public class CalculatorTest {

    public void testAddition() {
        Calculator calculator = new Calculator();
        int result = calculator.add(2, 3);
        assertEquals(5, result);

Running this test will fail because we haven’t created the Calculator class or the add method yet.

Step 2: Write Minimal Code to Pass the Test

Now, let’s write the minimal amount of code needed to make the test pass.

public class Calculator {
    public int add(int a, int b) {
        return a + b;

Now, if we run our test again, it should pass.

Step 3: Refactor (if necessary)

In this case, our addition function is straightforward, and there isn’t really any refactoring needed. But if there were any improvements or optimizations to be made, we would make them here and then rerun the test to ensure it still passes.

The above steps demonstrate the TDD cycle. The key idea is to let tests drive your development. This ensures that:

  1. You’re writing code that meets the requirements (since you’re writing tests based on requirements).
  2. You have tests for all your code, ensuring it works as expected.
  3. Your design might end up more modular and clean, as writing tests often requires modular design for testability.

Remember, in TDD, you’ll go through these steps many times, in small increments, as you build up the functionality of your system.

How TDD Assists in Managing Big and Complex Projects

Let’s dive deeper into some of the points with real-world examples and code snippets to illustrate the concepts:

Absolutely! Let’s dive deeper into some of the points with real-world examples and code snippets to illustrate the concepts:

1. Breakdown Features:

  • Scenario: Imagine a large project like an e-commerce platform. One of the features might be “User Registration.”
  • TDD Approach: Before building the entire feature, start with a single requirement, e.g., “A user should be able to input their email.”
// Test
public void userCanInputEmail() {
    RegistrationPage regPage = new RegistrationPage();
    assertEquals("", regPage.getEmail());

2. Incremental Development:

  • Scenario: Building on the e-commerce platform, after email input, the next task might be password input.
  • TDD Approach: Write a test for password input, then implement just that.
// Test
public void userCanInputPassword() {
    RegistrationPage regPage = new RegistrationPage();
    assertEquals("secure123", regPage.getPassword());

3. Continuous Integration:

  • Scenario: Developers A and B are working on the checkout and payment features, respectively, of the e-commerce platform.
  • TDD Approach: As Developer A completes the checkout feature tests and implementation, they push their code. The CI system automatically runs all tests, including the ones Developer B is working on for payments.
  • Code: N/A (This would involve CI configuration with tools like Jenkins, Travis CI, or GitHub Actions.)

4. Collaboration and Code Reviews:

  • Scenario: Developer C is reviewing Developer A’s code for the checkout feature.
  • TDD Approach: Developer C can understand the intent and functionality by looking at Developer A’s tests. They can also run the tests to ensure everything works as intended.
// Test written by Developer A
public void checkoutProcessCalculatesTotalCorrectly() {
    Cart cart = new Cart();
    cart.addItem(new Item("book", 10.00));
    cart.addItem(new Item("pen", 0.50));
    CheckoutProcess checkout = new CheckoutProcess(cart);
    double total = checkout.calculateTotal();
    assertEquals(10.50, total);

5. Refactoring and Architecture Evolution:

  • Scenario: The e-commerce platform initially calculates tax as a flat rate but needs to change to support different tax rates based on location.
  • TDD Approach: Before refactoring, tests ensure the tax calculation works. After refactoring to support location-based tax, the same tests ensure no regressions.
// Initial Test
public void taxIsCalculatedAtFlatRate() {
    CheckoutProcess checkout = new CheckoutProcess(new Cart());
    double tax = checkout.calculateTax();
    assertEquals(5.00, tax); // Flat rate

// After Refactoring
public void taxIsCalculatedBasedOnLocation() {
    CheckoutProcess checkout = new CheckoutProcess(new Cart(), "NY");
    double tax = checkout.calculateTax();
    assertEquals(6.00, tax); // NY tax rate

6. Dependency Management:

  • Scenario: The payment module depends on an external credit card processing service.
  • TDD Approach: Use mock objects to simulate the external service, allowing the payment module to be tested in isolation.
// Using a mock tool like Mockito
public void paymentIsProcessed() {
    CreditCardProcessor mockProcessor = mock(CreditCardProcessor.class);
    PaymentModule payment = new PaymentModule(mockProcessor);
    boolean success = payment.processPayment(new CreditCard("1234-5678-9012-3456"));

These examples demonstrate how TDD integrates into the development of large, real-world projects. While the code snippets are simplified, they illustrate the TDD principles in action within the context of a broader system.

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Thanks for reading 🙂