@Test public void testContains() throws Exception {

  // given
  prefser.clear();
  String givenValue = GIVEN_STRING_VALUE;
  String givenKey = GIVEN_KEY;
  // when
  prefser.put(givenKey, givenValue);
  // then
  assertThat(prefser.contains(givenKey)).isTrue();
  prefser.remove(givenKey);
  assertThat(prefser.contains(givenKey)).isFalse();
}