### Question:
OptionalDouble { public static OptionalDouble ofNullable(Double value) { return value == null ? empty() : of(value.doubleValue()); } 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 testOfNullable() { assertThat(OptionalDouble.ofNullable(null).boxed()).isEmpty(); assertThat(OptionalDouble.ofNullable(1.0).boxed()).hasValue(1.0); }