How to android unit test and mock a static method

Static methods aren't related to any object - your helper.fetchUsernameFromInternet(...) is the same (but a bit confusing) as HelperUtils.fetchUsernameFromInternet(...) - you should even get a compiler warning due to this helper.fetchUsernameFromInternet.

What's more, instead of Mockito.mock to mock static methods you have to use: @RunWith(...), @PrepareForTest(...) and then PowerMockito.mockStatic(...) - complete example is here: PowerMockito mock single static method and return object

In other words - mocking static methods (and also constructors) is a bit tricky. Better solution is:

• if you can change HelperUtils, make that method non-static and now you can mock HelperUtils with the usual Mockito.mock

• if you can't change HelperUtils, create a wrapper class which delegates to the original HelperUtils, but doesn't have static methods, and then also use usual Mockito.mock (this idea is sometimes called "don't mock types you don't own")

I did this way using PowerMockito.

I am using AppUtils.class, it contains multiple static methods and functions.

Static function:

public static boolean isValidEmail(CharSequence target) {
    return target != null && EMAIL_PATTERN.matcher(target).matches();
}

Test case:

@RunWith(PowerMockRunner.class)
@PrepareForTest({AppUtils.class})
public class AppUtilsTest {

    @Before
    public void setUp() throws Exception {
        MockitoAnnotations.initMocks(this);
        PowerMockito.mockStatic(AppUtils.class);

        PowerMockito.when(AppUtils.isValidEmail(anyString())).thenCallRealMethod();
    }

    @Test
    public void testValidEmail() {
        assertTrue(AppUtils.isValidEmail("[email protected]"));
    }

    @Test
    public void testInvalidEmail1() {
        assertFalse(AppUtils.isValidEmail("[email protected]"));
    }

    @Test
    public void testInvalidEmail2() {
        assertFalse(AppUtils.isValidEmail("@email.com"));
    }
}

Edit 1:

Add following imports:

import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertFalse;
import static org.junit.Assert.assertNotNull;
import static org.junit.Assert.assertTrue;

Hope this would help you.