Java 可选 - 如何将一种类型的列表转换为另一种类型

Question

I have an optional Optional<TypeA> from which I can do a map function and get Optional<List<TypeB>> .

But I would like to now map it to Optional<List<TypeC>> and then orElse() it. So putting in pseudo code

Optional.ofNullable(fooReturnsTypeA()).map(TypeA::barReturnsListTypeB()).{?}.orElse(new ArrayList<TypeC>());

I know how to use map function, but it's been a while dealing with Optional with map . So, I might be just staring at the answer. Could someone help me with {?} part?

Regards,

Answer 1

What about this?

Optional.ofNullable(fooReturnsTypeA())
    .map(TypeA::barReturnsListTypeB())
    .map(typeBList->typeBList
        .stream()
        .map(TypeB::barReturnsTypeC)
        .collect(Collectors.toList())
    )
    .orElse(new ArrayList<TypeC>())

This basically creates a new Stream in .map and uses it to convert all elements of TypeB to TypeC (assuming TypeB has a method barReturnsTypeC without parameters that returns an instance of TypeC ).

Answer 2

You can use another map and write method to convert List<TypeB> to List<TypeC> .

You're code will look like this:

    Optional.ofNullable(fooReturnsTypeA())
    .map(TypeA::barReturnsListTypeB())
    .map(this::convert)
    .orElse(new ArrayList<TypeC>());

And your method to convert:

    private List<TypeC> convert(List<TypeB> list) {
    // conversion logic
    // return List<TypeC>
}

Answer 3

First, I would replace the orElse with orElseGet . Depending on your code, using orElseGet instead of orElse can improve your performance. The parameter of orElse is evaluated even when the Optional is non-empty. On the other hand, the parameter of orElseGet is evaluated only when the Optional is empty. In your specific case, you may be instantiating unnecessary ArrayList s. From the Javadoc:

Supplier method passed as an argument is only executed when an Optional value is not present.

You can also dismiss the {?} by using a stream in the map .

Optional.ofNullable(fooReturnsTypeA())
    .map(typeA -> typeA.barReturnsListTypeB().stream()
            .map(TypeB::barReturnsTypeC).collect(Collectors.toList()))
    .orElseGet(() -> new ArrayList<TypeC>());

Note: see this for more information about the difference between orElse and orElseGet .