使用不可为空值启动可选调用链的优缺点

Question

I had a discussion with my colleague about whether you should start an Optional chain of calls from the first nullable value or from the variable at hand, and we couldn't agree on what is the way to go.

So, to be explicit in my question, what are the pros and cons of the two following snippets? Is there an agreement in the community as to which is better?

    public String findAuthorName(Book book) {
        return Optional.of(book)
                .map(Book::getAuthorId)
                .flatMap(repository::findById)
                .map(Author::getName)
                .orElse("Hello world!");
    }

or

    public String findAuthorName(Book book) {
        return Optional.ofNullable(repository.findById(book.getAuthorId()))
                .map(Author::getName)
                .orElse("Hello world!");
    }

Answer 1

There are no advantages of the first approach. Optional should be used for objects which may be not present, but here:

public String findAuthorName(Book book) {
    return Optional.of(book)
                   .map(Book::getAuthorId)

you are using Optional on Book object which looks like not null, only to invoke the getter method. book.getAuthorId()) is sufficient in this case.

The second approach clearly tells us where to expect non-present values and what we would like to do with them.

Answer 2

I do believe the second code snippet actually looks like (you are doing .flatMap(repository::findById) in the first one, that means repository#findById() returns optional):

    public String findAuthorName(Book book) {
        return repository.findById(book.getAuthorId())
                .map(Author::getName)
                .orElse("Hello world!");
    }

and those code snippets behave differently: if Book#getAuthorId() returns null the first one will return "Hello world " whilst the second one may fail (depends on the implementation of repository ), so, the first one looks safer, however both those patters have a very limited usage, the main problem is there are three possible situations:

• Book#getAuthorId() is null
• repository.findById(book.getAuthorId()) returns Optional#empty() - there is no author in repository
• Author#getName() is null

and for all those cases the result will be Hello world - that might be acceptable when we are returning data to end-user, but there is no place for such patters in business logic.