Collaboration is one way of modeling relationship between different objets. In a collaboration, there is a has-a type of relationship reffered to as association (see Inheritance vs. Association). Collaborator objects can be of any time.
class Library
def initialize
@book = []
end
def add_book(book)
@books << book
end
end
class Book
end
my_library = Library.new
my_book = Book.new
my_library.add_book(my_book)
In the code above, there is a collaborative relationship between Library and Book classes within the class definition of Library. However, my_book doesn’t become associated with my_library until the last line when we call the Library#add_book method.
In general, collaboration is view in terms of modeling relationships, i.e. even if we never call Library#add_book (and thus never associate my_book with my_library), it is safe to say Book class is a collaborator for Library because of the design decision in our code.