GsonPreference extends Preference<T> { @Override public boolean exists() { return mStringPreference.exists(); } GsonPreference(final String name, final String key, final Type type,
            @Nullable final T defaultValue); @Override boolean exists(); @Override T get(); @NonNull final Gson getGson();  }
@Test public void testExists() throws Exception { assertFalse(mNullPref.exists()); assertTrue(mSheikPref.exists()); }