The problem is that you have to write the interface not only separately grom the implementation, but together with the implementation as well, which leads to duplication of information;
So? Yes you need some duplicatin but it forces you to put information useful to api users where they won't have to wade through pages of information not of interest to them. eventually they may need to read the source but a lot of common questions are answered by the header and so the small cost to make them is worth it.
of course javadoc can answer the same questions but then you have to run it.
of course javadoc can answer the same questions but then you have to run it.