How knows Stream.toArray(Book[]::new); how many elements the array has and where is the implementation of “Book[]::new”

问题

I'm triying to understand method references and I don't know how with this "Book[]::new" it can create an array with the right number of elements.

Book[] arrayBook = stBooks.toArray(Book[]::new);

In order to create the array when I use the second and third option I have to specify a Functional Interface that Receive an Int and return a new Array.

But, in the first option I don't know where is the implementation and where you specify the number or elements that is going to have the array. I figure that it create a new array element for each Stream element but if I understand well you have to provide the implementation and "Book[]::new" doesn't have it.

Book book1 = new Book("ESDLA");
Book book2 = new Book("Harry Potter");
Stream<Book> stBooks = Stream.of(book1, book2);


1. Method References
Book[] arrayBook = stBooks.toArray(Book[]::new);

2. With Lambda Implementation
Book[] arrayBook1 = stBooks.toArray( (numElements) -> new Book[numElements] );

3. With Implementation
Book[] arrayBook2 = stBooks.toArray( new IntFunction<Book[]>() {

    @Override
    public Book[] apply(int numElements) {
        return new Book[numElements];
    }
});

回答1:

This is a special case in the language specification:

If the method reference expression has the form ArrayType::new, a single notional method is considered. The method has a single parameter of type int, returns the ArrayType, and has no throws clause. If n = 1, this is the only potentially applicable method; otherwise, there are no potentially applicable methods.

And also:

If the form is Type[]^k :: new (k ≥ 1), then the body of the invocation method has the same effect as an array creation expression of the form new Type [ size ] []^k-1, where size is the invocation method's single parameter. (The notation []^k indicates a sequence of k bracket pairs.)

So the "implementation" that you want to find is hidden somewhere in the Java compiler, as it's all "compiler magic".

来源：https://stackoverflow.com/questions/65123253/how-knows-stream-toarraybooknew-how-many-elements-the-array-has-and-where