Code blocks are just lambdas though? It's pretty much mainstream these days, even Java and C++ have them.

    // java equivalent
    int numberOfAuthors = arr.stream().map(BlogPost::getAuthor).distinct().collect(Collectors.toList()).size();

I happen to think for loops are usually a better choice in languages like Java, but the option is there.

EDIT: actually there's a better way, no need to create a list just to count its elements:

    int numberOfAuthors = arr.stream().map(BlogPost::getAuthor).distinct().collect(Collectors.counting());