Notation of the Dynamic Querying Associations

Question

In the latest documentation of Grails, we can read:

Querying Associations. Associations can also be used within queries:

def author = Author.findByName("Stephen King")

def books = author ? Book.findAllByAuthor(author) : []

I'd like to know what is the meaning of ? and : []

Answer 1

Shorthand if statement in Groovy (and Java, see the first comment).

def books = author ? Book.findAllByAuthor(author) : []

is equivalent of:

def books
if (author) {
    books = Book.findAllByAuthor(author)
}
else {
    books = []
}

See the elvis operator (Groovy only, not Java) here.