IntelliJ IDEA 2021.3 Help

Tutorial: Test-driven development with Kotlin

Whether you like to write your tests before writing production code, or like to create the tests afterwards, IntelliJ IDEA makes it easy to create and run unit tests. In this tutorial we’re going to show how to use IntelliJ IDEA to write tests first (Test Driven Development or TDD).

Creating Your First Test

Given that we’re writing our tests first without necessarily having the code we’re testing available to us yet, we’ll create our first test via the project panel.

  1. Right-click the package you want to create the test in and select New | Kotlin File/Class.

  2. Enter the test name - given we’re testing using JUnit, this will likely be [Something]Test, for example CalculatorTest.

    Test class created
  3. With the cursor placed somewhere inside the class’s curly braces, press Alt+Insert.

  4. Select Test Function | JUnit 4 from the menu. This will create a test function with the default template. Fill in a name for the test, press Enter, and the cursor will end up in the function body.

    Creating your first test

    You can alter the default test function template - for example, if you wish to change the start of the function name from test to should.

Writing the Test Body

It may seem counter-intuitive to write test code for classes and functions that don’t exist, but IntelliJ IDEA makes this straightforward while keeping the compiler happy. IntelliJ IDEA can create classes and functions for you if they don’t already exist.

  1. Write your tests describing what you want to achieve, pressing Alt+Enter on any classes that don’t exist and selecting Create class. This will give you a minimum implementation that keeps the compiler happy.

    Creating a new class
  2. Continue writing the test body, including names of functions that you need that don’t exist, again you can use Alt+Enter and select Create function to have IntelliJ IDEA create a bare skeleton function.

    Creating a function
  3. Implement the tested function so that it returns the required type and compiles without issues. The correctness of the result is not important now.

    Implementing the function in the tested class
  4. Add an assertion to the test function. This statement will compare the actual return of the function with the expected one and thus determine the outcome of the test.

    assertEquals() checks if the parameters are equal and fails the test if
            they are not

Running the Tests

When following a TDD approach, typically you go through a cycle of Red-Green-Refactor. You’ll run a test, see it fail (go red), implement the simplest code to make the test pass (go green), and then refactor the code so your test stays green and your code is sufficiently clean.

The first step in this cycle is to run the test and see it fail.

Given that we’ve used IntelliJ IDEA’s features to create the simplest empty implementation of the method we’re testing, we do not expect our test to pass.

  • From inside the test, press Ctrl+Shift+F10 to run this individual test.

    The results will be shown in the run dialog. The test name will have an icon next to it - either red for an exception, or yellow for an assertion that fails. For either type of failure, a message stating what went wrong is also shown.

    Running the test

Implementing the Code

The next step is to make the tests pass, which means implementing the simplest thing that works.

  1. You can navigate to the code being tested using the usual functions - clicking through on the function name, pressing Ctrl+Alt+B while the cursor is on the function name, or pressing Ctrl+Shift+T to switch between the test and the production code.

  2. Make the change to the function to make the test pass. Often with TDD, the simplest thing that works might be hard-coding your expected value. We will see later how iterating over this process will lead to more realistic production code.

    Fixing the code of the method to satisfy test conditions
  3. Re-run the test, using Shift+F10 to re-run the last test.

  4. See the test pass - the icon next to the test function should go green. If it does not, make the required changes until the test passes.

    Rerunning tests


Developing code is an iterative process. When following a TDD-style approach, this is even more true. In order to drive out more complex behaviour, we add tests for other cases.

  1. In your test class, use Alt+Insert again to create a new test function.

  2. Pick a second test case that specifies more requirements of the function you’re testing. Remember that you can use IntelliJ IDEA’s features to create classes and functions to keep the compiler happy.

    Creating a new test
  3. Run this second test case, showing that it fails for the correct reason.

  4. Change the code in the function being tested to make this test pass.

    Improving the code so that the second test passes
  5. Re-run both the tests by pressing Ctrl+Shift+F10 inside the test class, not inside a single function, and see that both tests now pass. If either test fails, make the changes needed to the code to ensure the tests pass.

    The second test passes


Writing your first test in a test-first style takes a small amount of setup - creating the test class, creating the test methods, and then creating empty implementations of the code that will eventually become production code. IntelliJ IDEA automates a lot of this initial setup.

As you iterate through the process, creating tests and then making the changes required to get those tests to pass, you build up a comprehensive suite of tests for your required functionality, and the simplest solution that will meet these requirements.

Last modified: 01 August 2022