### Question:
OptionalDouble { public boolean isPresent() { return optional.isPresent(); } private  OptionalDouble(); private  OptionalDouble(double value); static OptionalDouble empty(); static OptionalDouble of(double value); static OptionalDouble ofNullable(Double value); Optional<Double> boxed(); OptionalDouble filter(@NonNull DoublePredicate predicate); double getAsDouble(); void ifPresent(DoubleConsumer consumer); boolean isPresent(); Optional<T> map(@NonNull DoubleFunction<T> mapper); double orElse(double other);  }### Answer:
@Test public void testIsPresent() { assertThat(OptionalDouble.empty().isPresent()).isFalse(); assertThat(OptionalDouble.of(1.0).isPresent()).isTrue(); }