浅析Java 9 Optional API 新增方法（浅析旅游业的发展对经济的影响）

@Test
public void givenOptional_whenPresent_thenShouldTakeAValueFromIt() {
  //given
  String expected = "properValue";
  Optional<String> value = Optional.of(expected);
  Optional<String> defaultValue = Optional.of("default");
 
  //when
  Optional<String> result = value.or(() -> defaultValue);
 
  //then
  assertThat(result.get()).isEqualTo(expected);
}

@Test
public void givenOptional_whenEmpty_thenShouldTakeAValueFromOr() {
  // given
  String defaultString = "default";
  Optional<String> value = Optional.empty();
  Optional<String> defaultValue = Optional.of(defaultString);
 
  // when
  Optional<String> result = value.or(() -> defaultValue);
 
  // then
  assertThat(result.get()).isEqualTo(defaultString);
}

@Test
public void givenOptional_whenPresent_thenShouldExecuteProperCallback() {
  // given
  Optional<String> value = Optional.of("properValue");
  AtomicInteger successCounter = new AtomicInteger(0);
  AtomicInteger onEmptyOptionalCounter = new AtomicInteger(0);
 
  // when
  value.ifPresentOrElse(
   v -> successCounter.incrementAndGet(), 
   onEmptyOptionalCounter::incrementAndGet);
 
  // then
  assertThat(successCounter.get()).isEqualTo(1);
  assertThat(onEmptyOptionalCounter.get()).isEqualTo(0);
}

@Test
public void givenOptional_whenNotPresent_thenShouldExecuteProperCallback() {
  // given
  Optional<String> value = Optional.empty();
  AtomicInteger successCounter = new AtomicInteger(0);
  AtomicInteger onEmptyOptionalCounter = new AtomicInteger(0);
 
  // when
  value.ifPresentOrElse(
   v -> successCounter.incrementAndGet(), 
   onEmptyOptionalCounter::incrementAndGet);
 
  // then
  assertThat(successCounter.get()).isEqualTo(0);
  assertThat(onEmptyOptionalCounter.get()).isEqualTo(1);
}

@Test
public void givenOptionalOfSome_whenToStream_thenShouldTreatItAsOneElementStream() {
  // given
  Optional<String> value = Optional.of("a");
 
  // when
  List<String> collect = value.stream().map(String::toUpperCase).collect(Collectors.toList());
 
  // then
  assertThat(collect).hasSameElementsAs(List.of("A"));
}

@Test
public void givenOptionalOfNone_whenToStream_thenShouldTreatItAsZeroElementStream() {
  // given
  Optional<String> value = Optional.empty();
 
  // when
  List<String> collect = value.stream()
   .map(String::toUpperCase)
   .collect(Collectors.toList());
 
  // then
  assertThat(collect).isEmpty();
}