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Student: ex04

Adding JUnit tests

Part of a series of tutorial articles about a Student class.

Code examples referred to on this page can be found here:


In this example we introduce JUnit 5 tests.

JUnit tests allow us to automate the testing of our code.

Rather than having to run a driver class and manually inspect whether the output is correct, we can write code that does this for us.

This involves three steps:

  • Setting up a directory for test code, following the Maven directory conventions.
  • Adding a dependencies for JUnit to our pom.xml
    • This tells Maven to download the .jar files for JUnit from and include them in the files for the project.
  • Setting up the actual test code.

Setting up the directory

To our existing Maven directory structure, we add the directory src/test/java as shown in the diagram below.

Maven separates test code from regular code so that:

  • Test code can be compiled in a different environment that includes extra dependencies (e.g. JUnit)
  • Test code can be excluded when the code is packaged to send to a customer.

We can create this directory with:

  • mkdir -p src/test/java

Adding the dependency

The dependency goes into the pom.xml in a section called dependencies. Here’s what the entire pom.xml looks like with the new dependency added.



         <!-- -->


You may wonder: how do we know what to put into the pom.xml for any given dependency? The answer is that the documentation for the package will usually tell you. If you don’t know, you can look up the code at the website:

The ` test means that these jar files will only be in the CLASSPATH when we are working with testing; this is one of the reasons that Maven requires us to have separate directory trees for src/main/java and src/test/java`.

Adding Tests

We can now add tests using JUnit.

Here is an example of what writing tests looks like with JUnit.

We put the tests in a class called StudentTest, in a file called under src/test/java. The full listing is followed by an explanation of the content:

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

public class StudentTest {

    public void test_getName1() {
        Student s = new Student();
        assertEquals("Sample Student", s.getName());

    public void test_getName2() {
        Student s = new Student("Chris Gaucho", 1234567);
        assertEquals("Chris Gaucho", s.getName());

    public void test_getPerm1() {
        Student s = new Student();
        assertEquals(9999999, s.getPerm());


The first two lines are imports.

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

The first of these imports indicate that we will be using assertEquals, a method that comes from the class org.junit.jupiter.api.Assertions.

  • It is important to note that we do not have to use an import statement if we are willing to type out the full name org.junit.jupiter.api.Assertions.assertEquals every time we refer to this method. However, it is much simpler to just be able to type assertEquals.
  • The first import is an import static because it refers to importing a method rather than a class.
  • The second import, ` org.junit.jupiter.api.Test imports a class, and allows us to use the @Test annotation to mark which methods are test methods. Marking these works with Maven and JUnit so that these method are invoked when we type mvn test`

We then have public class StudentTest { which names the class. It is a very typical naming convention to use the name of the class followed by Test in camel case to name classes that run tests on another class.

  • As a rule, put tests for in

Finally, we have the test methods. It is traditional to test only one unit at a time when writing unit tests (hence the name). However, to create an object, we have to invoke the constructor, and then to inspect the object we typically have to invoke at least one other method.

Accordingly, since we have two different constructors, we create two different tests for the getName method. Arguably, each of these tests is a test both of one of the constructors, and the getName method.

We have only written one of the tests that is needed for the getPerm method; this is intentional. Througout the tutorial, we are going to leave a few things out so that (a) you see what happens when we do, and (b) you have some work to try for yourself.

Running the tests

To run the tests, we can use mvn test as shown here:

pconrad@Phillips-MacBook-Pro student-tutorial % mvn test
[INFO] Scanning for projects...
[INFO] -----------------------< edu.ucsb.cs156:student >-----------------------
[INFO] Building student 1.0.0
[INFO] --------------------------------[ jar ]---------------------------------
[INFO] --- maven-resources-plugin:2.6:resources (default-resources) @ student ---
[WARNING] Using platform encoding (UTF-8 actually) to copy filtered resources, i.e. build is platform dependent!
[INFO] skip non existing resourceDirectory /Users/pconrad/github/ucsb-cs156/student-tutorial/src/main/resources
[INFO] --- maven-compiler-plugin:3.8.0:compile (default-compile) @ student ---
[INFO] Nothing to compile - all classes are up to date
[INFO] --- maven-resources-plugin:2.6:testResources (default-testResources) @ student ---
[WARNING] Using platform encoding (UTF-8 actually) to copy filtered resources, i.e. build is platform dependent!
[INFO] skip non existing resourceDirectory /Users/pconrad/github/ucsb-cs156/student-tutorial/src/test/resources
[INFO] --- maven-compiler-plugin:3.8.0:testCompile (default-testCompile) @ student ---
[INFO] Nothing to compile - all classes are up to date
[INFO] --- maven-surefire-plugin:2.12.4:test (default-test) @ student ---
[INFO] Surefire report directory: /Users/pconrad/github/ucsb-cs156/student-tutorial/target/surefire-reports

 T E S T S
Running StudentTest
Tests run: 3, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.038 sec

Results :

Tests run: 3, Failures: 0, Errors: 0, Skipped: 0

[INFO] ------------------------------------------------------------------------
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  0.822 s
[INFO] Finished at: 2020-10-12T14:07:01-07:00
[INFO] ------------------------------------------------------------------------
pconrad@Phillips-MacBook-Pro student-tutorial % 

The part of this output that we really want to pay attention to is this part:

Tests run: 3, Failures: 0, Errors: 0, Skipped: 0

To see what it would look like if there were a test failure, we’ll temporarily change our default consructor by putting in a different value from the one the test expects:

  public Student() {
        name = "Eleanor Shelstrop";
        perm = 9999999;

Now when we test, we get this output. You can see that the part that is different is in square brackets ([])

Results :

Failed tests:   test_getName1(StudentTest): expected:<[Sample Student]> but was:<[Eleanor Shelstrop]>

Tests run: 3, Failures: 1, Errors: 0, Skipped: 0

Changing it back restores the tests output to zero failures:

Tests run: 3, Failures: 0, Errors: 0, Skipped: 0

Now, this is good, but this still relies on us to remember to run the tests. In the next tutorial, we’ll see that we can use GitHub Actions to automate this process for us.

(updated for JUnit 5/Java 17, 01/05/22)

The directory listing after adding tests

Note that we add a src/test subdirectory to put the tests in. This isn’t just a “suggestion”; maven is set up to expect this directory to exist. If you put tests under the src/main, then you may find that they don’t compile.

Maven typically only puts the JUnit software in the classpath when it is compiling code under the src/test directory, and it does not put it in the classpath when compiling code under the src/main directory. This helps to keep the size of the code smaller when we deliver a final tested application; we typically need the test code (and the test libraries) only while doing development.

Here’s what the new directory structure looks like: