Prerequisite: The project is configured to use Gradle already. If you have not done that yet, follow the Gradle Tutorial to add Gradle support to the project first.

1. Update the build.gradle file to include JUnit as a dependency. Here are the relevant lines that needs to be in the build.gradle (change the version number as necessary):

test {
    useJUnitPlatform()
}

dependencies {
    testImplementation group: 'org.junit.jupiter', name: 'junit-jupiter-api', version: '5.5.0'
    testRuntimeOnly group: 'org.junit.jupiter', name: 'junit-jupiter-engine', version: '5.5.0'
}

2. Add a test class, while following the conventions given earlier in this page. If you don't follow those conventions, Gradle will not be able to find your test class. For example, if you have a class src\main\java\seedu\duke\Todo.java, you can add a test class src\test\java\seedu\duke\TodoTest.java. Here's some sample code:

package seedu.duke;

import org.junit.jupiter.api.Test;

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

public class DukeTest {
    @Test
    public void dummyTest(){
        assertEquals(2, 2);
    }

    @Test
    public void anotherDummyTest(){
        assertEquals(4, 4);
    }
}

Later, you can add more tests and test classes as necessary.

3. Run tests, either using the Intellij UI (preferred -- this makes debugging failed test cases easier) or using Gradle itself, as explained in the section below.

1. Add a folder named [project root]\src\test\java\

2. Go to Intellij and add a new module to the project as follows.

   1. File → New → Module From Existing Sources ...
   2. Choose the [project root]\src\test\ (not the java) folder.
   3. In the next screen, select Create module from existing sources
   4. Keep clicking Next until the process is complete

3. In the Project panel of Intellij, expand the newly-created test module, right-click on the java folder inside it, and choose Mark Directory as → Test Source Root (that will make the folder turn to green color).

4. Now, create a class inside the java folder and type @Test inside it. A code example given below.
   Note: If you are using packages, create this class in a matching package (to test duke.Duke class, create a duke.DukeTest i.e., in src\test\java\duke\DukeTest.java).

   public class DukeTest {
       @Test
   }
   

5. Note how the @Test turn to red because Intellij (not having JUnit support yet) does not understand it. But it will pop up a hint, asking if you want to add support for JUnit. Select Add JUnit 5.* to classpath.

6. In the dialog that pops up, you can optionally tick the Sources, JavaDocs and Annotations boxes. After that, click OK to add the JUnit 5 to the project dependencies.
   

   When using the dialog shown below, if the version auto-selected by Intellij is not available (i.e., it shows Found: 0), change the version number to a slightly lower number (e.g., 5.6.0) and click the button to try again.
   

7. To check if JUnit integration is working as expected,
   7a. Add a dummy test method to the class e.g.,

   import org.junit.jupiter.api.Test;
   
   import static org.junit.jupiter.api.Assertions.assertEquals;
   
   public class DukeTest {
       @Test
       public void dummyTest(){
           assertEquals(2, 2);
       }
   }
   

   7b. Run the test (right-click on the class and choose Run DukeTest).

8. To be able to refer to Duke from DukeTest class, you need to add main module as a dependency of the test module you just created.
   8a. First, ensure there is a main module in your project, as follows:

   1. Check if your project explorer look like this (note how the main and test modules show up).
      
      
   To see other project sub-folders (e.g., docs) not currently shown (refer to the screenshot above), you can change the dropdown (shown at the top of the screenshot) from Project to Project files

   1. If there is no module named main, Intellij may have failed to recognize the main folder as the source folder and auto-created a module to include the entire project folder, usually given the same name as project folder. If there is such an extraneous module, delete that first (Go to File -> Project Structure, click on Modules, select the module you want to delete, and click on the - button), followed by OK/Apply.
      
   2. After that, create the main module the same way you created the test module in an earlier step above.
      Tip: You may have to close the project and open it again after creating the main module, before you can see the main module in the project explorer view.

   8b. Now that you have the modules main and test, you can set up the dependency using one of these options:

   • Option 1: When you add a reference to the Duke inside the DukeTest, Intellij will flag it as an error and will give you an option (i.e., in the bulb icon that pops up) to add the main module as a dependency.
     
   • Option 2: Follow the info here to add the dependency yourself.