How to Write a Unit Test in Java

A unit test in Java gives the developer the opportunity to test an object. For example, you might want to verify that a Person object contains the expected person name or perhaps the correct email address. In addition, you may want to verify that a property is not null.

The most common way to write unit tests in Java is to use JUnit, a free, open source unit-testing Java application. To learn how to write a unit test in Java, follow these seven steps.

  1. First, you need to download JUnit 4.8.2. In your browser, go to this page on and click junit.jar. Another web page will be displayed.
    JUnit Download Web Page 1
  2. Click the jar option for junit 4.8.2. This action will start the download of junit-4.8.2.jar to your computer.
    JUnit Download Web Page 2
  3. Now you will write the program that contains the JUnit tests. Open your text editor and type in the following statements:
    JUnit Tests Source
    The @BeforeClass annotation marks a method that will be called by JUnit one time before the tests are performed. The @AfterClass annotation marks a method that will be called once after the tests have been run. The @Test annotations mark the tests that will be executed by JUnit. Two tests are included in the program. The first test will verify that the first name has been successfully set to "Stephen". This test will succeed at runtime. The second test will assert that the last name is not null. This test will fail at runtime because although the Person object has been instantiated, the last name has not been set.
  4. Save your file as
  5. Copy junit-4.8.2.jar to the directory that contains your Java program.
  6. Open a command prompt and navigate to the directory containing your Java program. Then type in the command to compile the source using the classpath compiler option and hit Enter.
    Compile JUnit Tests Source
  7. Type in the command to run the JUnit test runner using your program and hit Enter.
    Run JUnit Tests
    The output indicates one test failed with an AssertionError exception. The name of the failing method and the program name are displayed in the output.
Author: Stephen Withrow

Stephen has over 30 years of experience in training, development, and consulting in a variety of technology areas including Python, Java, C, C++, XML, JavaScript, Tomcat, JBoss, Oracle, and DB2. His background includes design and implementation of business solutions on client/server, Web, and enterprise platforms. Stephen has a degree in Computer Science and Physics from Florida State University.

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