Nowadays, people are more dependent on mobile than desktop. If we consider Android, millions of applications are on the Play Store and other app stores. So, it is very important to test the UI to make a UI bug free and stable product in the app store. You need to be very careful during testing as there are myriad devices with various display sizes. For the backend, you can test with the JUnit and the system is same. But now our test will be UI-based and so we will use Espresso. This is the most popular framework for UI testing.