JUnit and Test-Driven Development (TDD) Case Study

In this article on JUnit and Test-Driven Development (TDD) Case Study, a real-world problem is discussed.

JUnit and Test-Driven Development (TDD) Case Study: Solving a Real-World Problem

Test-Driven Development (TDD) is a software development approach that places testing at the forefront of the development process. JUnit, a widely used testing framework in the Java ecosystem, plays a pivotal role in making TDD efficient and effective. In this blog post, we will describe a real-world case study, illustrating how JUnit and TDD can be employed to solve a specific programming problem.

The Problem Statement

Imagine we are tasked with developing a simple utility class for calculating the area of various geometric shapes: circles, rectangles, and triangles. The initial requirements are straightforward. For example.

  1. Calculate the area of a circle given its radius.
  2. Also, calculate the area of a rectangle given its length and width.
  3. Then, calculate the area of a triangle given its base and height.

Our goal is to implement this utility class using TDD principles, with a primary focus on JUnit for writing and running tests.

Step 1: Writing the First Test

In TDD, we start by writing a failing test case that represents the desired functionality. This test case should be minimal and specific. For our case, let’s begin with calculating the area of a circle.

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

public class GeometryCalculatorTest {

    public void testCircleAreaCalculation() {
        double expectedArea = 78.54; // Pre-calculated for radius = 5.0
        double actualArea = GeometryCalculator.calculateCircleArea(5.0);
        assertEquals(expectedArea, actualArea, 0.01); // Allowing for a small delta

Step 2: Implement the Code

Now that we have our failing test, it’s time to implement the functionality to make it pass. So, we create a GeometryCalculator class with a method calculateCircleArea(double radius) and run the test. Of course, it fails initially, but we refine the code until the test passes.

Step 3: Repeat for Other Shapes

While, following the same pattern, we proceed to write tests and implement functionality for calculating the area of rectangles and triangles.

public void testRectangleAreaCalculation() {
    double expectedArea = 20.0; // Pre-calculated for length = 4.0 and width = 5.0
    double actualArea = GeometryCalculator.calculateRectangleArea(4.0, 5.0);
    assertEquals(expectedArea, actualArea, 0.01);

public void testTriangleAreaCalculation() {
    double expectedArea = 10.0; // Pre-calculated for base = 4.0 and height = 5.0
    double actualArea = GeometryCalculator.calculateTriangleArea(4.0, 5.0);
    assertEquals(expectedArea, actualArea, 0.01);

Step 4: Refactor and Optimize

With all tests passing, we can refactor the code for clarity and efficiency, confident that we haven’t introduced any regressions due to our comprehensive test suite.


In this case study, we’ve demonstrated the power of JUnit and TDD by systematically solving a real-world problem. By focusing on writing tests before code, we ensure that our software functions as expected, is maintainable, and resilient to future changes.

JUnit’s role in this process is crucial, as it provides a simple yet robust framework for writing and running tests, making it an indispensable tool for any Java developer practicing TDD. By following these principles and harnessing the capabilities of JUnit, we can build reliable and high-quality software solutions.

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