java - create a list from subproperties of another list -

i have 3 classes defined getters , setters follows orm:

class author {   integer id;   string name; }  class bookauthor {   integer id_book;   author author; }  class book {   string title;   list<bookauthor> authors; } 

i'd create list of id_author class book. i've found way using streams. i've tried this:

list<integer> result = authors.stream().map(bookauthor::getauthor::getid).collect(collectors.tolist()); 

but not seem work. can access "id" property in author class?

edit: maybe way be:

list<author> authorlist = authors.stream().map(bookauthor::getauthor).collect(collectors.tolist()); list<integer> result = authorlist.stream().map(author::getid).collect(collectors.tolist()); 

thank you.

i assume authors variable list (or collection) of bookauthor, not author (that seems based on code).

i think have right idea, dont think can chain :: operators.

so try lambda:

authors.stream().      map(ba -> ba.getauthor().getid()).      collect(collectors.tolist());