To give an example: if the lines() method returned instances of a Line class, the string buffer and it's append() method would be aware and still create a collection of lines instead of one long string. Not necessarily nice and would probably create issues in many other interfaces, but problems like this _can_ be solved with careful language and API design. Note that his end goal was to 'read lines' and not use any particular string abstractions. Nothing to get worked up about here.