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Unit Tests in Java

By David Burnham

What are Unit Tests?

In this tutorial, we are going to look at how we can use unit tests to verify that our program is working correctly. The first questions you may have is what is unit testing and why are we doing it? Unit testing is where we test specific parts of our code against specific requirements. Tests are small in nature and focused on one specific aspect of the programs functionality. Unit test often make up the first part of testing when it comes to testing a program.


There are several important reasons to test code. Proper test cases are written against user requirements, independent of the code that is developed. With this in mind, we can use unit tests to verify that our code is actually working properly. If you are working on a larger project for a customer, unit tests can be helpful on several fronts. First, they can be used to verify that each part of the code works before integration. During integration, code may get changed or refactored and the unit tests can then be rerun to very that the code is still producing the expected output. Additionally, if you have a deliverable to a customer, unit tests can help provide documentation that your product is working as intended.

Using the Test Class


For our testing, we are going to use a basic test class. This class is similar to other testing classes such as JUnit testing, but is simplified for this tutorial.


The Test class contains one overloaded method, assertEqual. We will use the first variation where we will pass it a test name, followed by the actual results and the expected results. Like our ArrayList class, this class can take any type of object and compare them.


Take a minute to play around with the Test class before we get into writing our test cases.

Executing Our Test Cases

When it comes to executing our test case, there are a couple of guiding principles that we want to follow. First, each test should only test one thing at a time. If we try and test more than one thing, we may not know what fails.


As we set up our tests, we want to follow the 3 A's: Arrange, Act, and Assert. Let's take a look at each of these steps:


Arrange - In the first step, we are setting up our test cases. For example, if we are going to test adding an object to the middle of our ArrayList, we want to start with an ArrayList


Act - The act step is where we actually perform the action we are going to test. For example, if we want to test adding to the middle of our ArrayList, our act step will be to add to the middle.


Assert - This is where we will test our results and make sure we are getting the expected results.


Let's take a look at this for one of our test cases below.

What's Next?

From here, we can add more of our test cases. More tests have been added below, but see if you can take time to finish them. Notice how each test case builds off of the others. This makes can make it easier so that you don't need to recreate the set up each time, but sometime you will still need to arrange before you act.