Errors to previously functioning code due to new changes are called Regression errors.

Let’s add a new functionality to upload images related to an Ad. We’ll use an interface to support more file types like PDF, Doc etc. in the future. Create files Scanner.java and ImageScanner.java with the following code.

// File: Scanner.java

package project;

public interface Scanner {

    public String uploadData();

}

// File: ImageScanner.java

package project;

public class ImageScanner implements Scanner {

    public String uploadData() {

        return "Uploaded Ad image";

    }

}

Make these changes to the AdHandler.java file

• Add these imports to the top of the file (after the package project; line)

import project.Scanner;

import project.ImageScanner;

• Add a new method, addImageToAd() which uses the ImageScanner class

public static String addImageToAd(String imageURI) {

        Scanner imageScanner = new ImageScanner();


        return imageScanner.uploadData();

}

Uncomment the testAddImageToAd() method in AdHandlerTest.java which tests the new addImageToAd() method. Run the tests now. Select Proceed if you get the below pop-up.

The Test Report will look like this. (If you used ./gradlew test to run the tests, the tests won’t be run due to the error)

All the tests that had passed earlier are now failing. As you’d have found out by now, there’s a conflict between the in-built Scanner class used for reading data and the new Scanner interface added. Without unit tests in place, we may skip the testing for older methods and test only the newly added methods. Scenarios like these are pretty common when multiple developers work together on a project as not everyone knows about all the classes.

With unit tests, we can implement new functionality or refactor code without worrying about breaking existing functionality unintentionally. This is because the existing functionality will get tested and any regression errors will be caught. We can fix the errors before delivering the refactored code.