如何将可选项映射到原始可选项？

Question

I know I can map an Optional into an another wrapper type optional.

Optional<Long> millis(Date date) {
    return ofNullable(date).map(Date::getTime);
}

How can I map or flatMap into an OptionalLong ?

OptionalLong millis(Date date) {
}

I tried but had no luck.

ofNullable(value).flatMap(v -> { // javac has never liked me
    return OptionalLong.of(v.getTime());
});

Answer 1

You can use map to get an Optional<OptionalLong> then orElse to remove the outer Optional like this:

OptionalLong millis(Date date) {
    return Optional.ofNullable(date).map(Date::getTime)
            .map(OptionalLong::of).orElse(OptionalLong.empty());
}

Another (shorter in this case) way is to use the ternary operator instead:

OptionalLong millis(Date date) {
    return date == null ? OptionalLong.empty() : OptionalLong.of(date.getTime());
}

Or if you already have the Optional<Long> :

OptionalLong toOptionalLong(Optional<Long> o) {
    return o.map(OptionalLong::of).orElse(OptionalLong.empty());
}
OptionalLong toOptionalLong(Optional<Long> o) {
    return o.isPresent() ? OptionalLong.of(o.get()) : OptionalLong.empty();
}

Answer 2

Keep in mind that you will take a performance hit in this scenario if you involve Optional<Long> in any way. From Joshua Bloch's Effective Java, 3rd Edition :

"Returning an optional that contains a boxed primitive type is prohibitively expensive compared to returning the primitive type because the optional has two levels of boxing instead of zero. [...] Therefore you should never return an optional of a boxed primitive type , with the possible exception of the "minor primitive types," Boolean , Byte , Character , Short , and Float "

There is no reason to involve Optional here. The best solution is to do the null check yourself, and then return an OptionalLong , eg

OptionalLong millis(Date date) {
    return date == null ? OptionalLong.empty() : OptionalLong.of(date.getTime());
}

Answer 3

Using the StreamEx library

OptionalLong millis(final Date date) {  
   return StreamEx.ofNullable(date).mapToLong(Date::getTime).findAny();
}

or

OptionalLong toLong(Optional<Long> o) {
    return  StreamEx.of(o).mapToLong(Long::longValue).findAny();
}

or

OptionalLong toLong(final Optional<Date> o) {
    return StreamEx.of(o).mapToLong(Date::getTime).findAny();
}

Answer 4

I would implement it as follows:

OptionalLong toOptionalLong(Optional<Long> optionalValue) {
    return optionalValue.map(OptionalLong::of).orElseGet(OptionalLong::empty);
}

The function OptionalLong::empty is called if and only if the optionalValue is empty.

But I'm not sure I would cast an Optional<Long> into an OptionalLong unless I'm really obliged to. As said in former answers, using a ternary operator is probably a better approach in this case.