What to test in Android

What you should test depends on factors such as the type of app, the development team, the amount of legacy code, and the architecture used. The following sections outline what a beginner might want to consider when planning what to test in their app.

Organization of test directories

A typical project in Android Studio contains two directories that hold tests depending on their execution environment. Organize your tests in the following directories as described:

The androidTest directory should contain the tests that run on real or virtual devices. Such tests include integration tests, end-to-end tests, and other tests where the JVM alone cannot validate your app's functionality.
The testdirectory should contain the tests that run on your local machine, such as unit tests. In contrast to the above, these can be tests that run on a local JVM.

Essential unit tests

When following best practice, you should ensure you use unit tests in the following cases:

Unit tests for ViewModels, or presenters.
Unit tests for the data layer, especially repositories. Most of the data layer should be platform-independent. Doing so enables test doubles to replace database modules and remote data sources in tests. See the guide on using test doubles in Android
Unit tests for other platform-independent layers such as the Domain layer, as with use cases and interactors.
Unit tests for utility classes such as string manipulation and math.

Testing Edge Cases

Unit tests should focus on both normal and edge cases. Edge cases are uncommon scenarios that human testers and larger tests are unlikely to catch. Examples include the following:

Math operations using negative numbers, zero, and boundary conditions.
All the possible network connection errors.
Corrupted data, such as malformed JSON.
Simulating full storage when saving to a file.
Object recreated in the middle of a process (such as an activity when the device is rotated).

Unit Tests to Avoid

Some unit tests should be avoided because of their low value:

Tests that verify the correct operation of the framework or a library, not your code.
Framework entry points such as activities, fragments, or services should not have business logic so unit testing shouldn't be a priority. Unit tests for activities have little value, because they would cover mostly framework code and they require a more involved setup. Instrumented tests such as UI tests can cover these classes.

UI tests

There are several types of UI tests you should employ:

Screen UI tests check critical user interactions in a single screen. They perform actions such as clicking on buttons, typing in forms, and checking visible states. One test class per screen is a good starting point.
User flow tests or Navigation tests, covering most common paths. These tests simulate a user moving through a navigation flow. They are simple tests, useful for checking for run-time crashes in initialization.

Other tests

There are more specialized tests such as screenshot tests, performance tests, and monkey tests. You can also categorize tests by purpose, such as regressions, accessibility, and compatibility.