Testing Android Apps

Why Test?

Automated testing has benefits both for the overall organization and for individual developers. For example, tests help catch problems before code is shipped to users, lowering the costs of fixing. A solid set of tests that thoroughly check all functionality also let developers experiment more and take risks, safe in the knowledge that they won’t break any existing functions without a test letting them know.

Writing Tests

Types of Testing

Classifications of Tests:

  • Unit Tests: Small, fast tests that check a tiny bit of functionality. For example, a unit test for a calculator might check that 1 + 1 = 2. These are usually run in an unrealistic environment so that a small part of the product can be tested in isolation. This means that these types of tests can miss problems that arise only when different small parts of the product interact with each other. The narrowness of unit tests also make it easy to pin down a bug, since you know exactly what is tested by each test.

  • Integration and End-to-End Tests: Take longer, but test the entire product together. This catches more subtle bugs, but those bugs can be hard to catch if they aren’t caught by a unit test.

Unit Tests

Testing in Android, and in Java more generally, is usually done with JUnit. While the latest version is JUnit 5, the Android documentation refers to JUnit 4 as the most recent. The testing directory is [module*name]/src/test/java/, and it is created automatically whenever Android Studio creates a new project. The following should be included in your dependencies in build.gradle:

dependencies {
    // Required -- JUnit 4 framework
    testImplementation 'junit:junit:4.12'
    // Optional -- Mockito framework
    testImplementation 'org.mockito:mockito-core:1.10.19'
    // Optional -- Hamcrest matchers like is()
    testImplementation 'org.hamcrest:hamcrest-library:1.3'
    // Optional -- Robolectric and Android support library needed for JSON
    testImplementation 'org.robolectric:robolectric:3.8'
    testImplementation 'com.android.support.test:runner:1.0.2'
    testImplementation 'com.android.support.test:rules:1.0.2'

This will import the dependencies JUnit and Mockito.

Create one or more testing classes in the testing directory, and fill them with methods that describe your tests. Methods with tests should be prefixed with @Test and use assertThat statements to check that the expected results are obtained. Below is one example:

import org.junit.Test;
import static org.junit.Assert.assertFalse;
import static org.junit.Assert.assertTrue;

public class CalculatorTest {

    public void calculator_one_isNumber() {
        assertThat(Calculator.isNumber(1), is(true));

Note that the is(true) statement is a Hamcrest matcher that serves only to make the statement more readable.

Handling Dependencies

Many times, the functionality you want to test relies on code elsewhere in your project. You could just call that code, but then your test will also fail if that code has a bug. This destroys the value of unit tests, namely that when they fail you know exactly where the bug is. There are two tools to handle this: Mockito and Robolectric. Mockito lets you define exactly how you expect the other code to behave, but it can get cumbersome to mock complex objects like those that are part of Android. For those, Robolectric contains logic stubs that emulate how Android behaves while still running faster than test would on an emulator.

These considerations are especially important if you need to use any Android classes because Android Studio will execute your tests against a dummy version of the android.jar library that throws exceptions in response to any call. This forces you to replace all components of the Android libraries you use with either mocking from Mockito or Robolectric.

To run your unit tests, right-click the directory of tests and select Run tests.

For more information, see https://developer.android.com/training/testing/unit-testing/local-unit-tests#java


With Mockito, you have to specify how you expect the dependency code to behave. This is done with when([dependency function call]).thenReturn([expected result]) calls. For example, imagine if the calculator example from before returned a resource string that specified the boolean result like so:

public class Calculator {
    private Context context;

    public Calculator(Context inContext) {
      context = inContext;

    public boolean isNumber(int num) {
        // Any int is a number
        return context.getString(R.string.true)

We could mock the Android Context like this:

import org.junit.Test;
import static org.junit.Assert.assertFalse;
import static org.junit.Assert.assertTrue;
import org.junit.runner.RunWith;
import org.mockito.Mock;
import org.mockito.runners.MockitoJUnitRunner;
import android.content.SharedPreferences;

public class CalculatorTest {

    private static final String FAKE_TRUE = "TRUE";

    Context mockContext;

    public void calculator_one_isNumber() {
        Calculator calc = new Calculator(mockContext);
        assertThat(calc.isNumber(1), is(FAKE_TRUE));

If you forget to mock something, you will get an error saying that the method you call is not mocked. To solve this, mock the method as shown above.

Guidelines for Well-Structured Unit Tests
  • Long, descriptive names are perfect for tests since the method name is often what is displayed if the test fails. The name should include the conditions, action, and expected output for the test. For example,

    public class Tests {
      public static boolean whenLoggedOut_givenViewProfile_showGuestView() {
        // Log-out the user
        // Open up a profile
        // Check that the guest version of the profile was returned
  • Tests should be focussed on a particular piece of functionality. Leave testing of other parts to separate tests. This will help make it easy to debug when tests fail. Given any method name of a test that failed, you should know exactly where in the codebase the problem is.

Test-Driven Development

To take testing even further, you can make writing tests the first thing you do before writing any code. This forces you to think through how you want your product to behave before you even start coding. This makes it clearer what the code needs to do. Developers usually have to do this anyway, but by doing it through tests instead of while coding, they both avoid writing code that is later discarded and save time by writing the tests and pinning down the requirements simultaneously.

Importantly, make sure that the tests you write fail at first. If they don’t, there is a bug in the tests.