Android instrumented test no tests found

I am new to Android instrumented unit tests. I have a simple test that looks like this. This is in instrumented tests; before I really write an actual instrumented test, I just want to test a simple one:

@RunWith(AndroidJUnit4.class)
@SmallTest
public class AddressBookKeepingInstrumentedTest {


    public static final String TEST_STRING = "This is a string";
    public static final long TEST_LONG = 12345678L;

    @Test
    public void test_simple() {
        assertEquals(2,1+1);
    }
}

When I run this, I get the following error:

junit.framework.AssertionFailedError: No tests found in com.example.myfirstapp.AddressBookKeepingInstrumentedTest
at android.test.AndroidTestRunner.runTest(AndroidTestRunner.java:191)
at android.test.AndroidTestRunner.runTest(AndroidTestRunner.java:176)
at android.test.InstrumentationTestRunner.onStart(InstrumentationTestRunner.java:554)
at android.app.Instrumentation$InstrumentationThread.run(Instrumentation.java:1729)

Tests ran to completion.

Gradle build passed with success before this. Any ideas why this is happening? Do you need any more info?

Thanks, Regards