About Question enthuware.ocpjp.v8.2.1810 :

[Martyjee]

The code will not compile because the compiler cannot find 'books'.
I think 'books' should be replaced with 'friends'.

Furthermore, the explanation contains: ...second, it maps each Person element to an int using the mapping function b.getAge().
I think the mapping function should be f->f.getAge().

Kind regards,

Martijn

[admin]

You are right. Fixed.
thank you for your feedback!